Clipper System to Bring Snow on Tuesday

One storm system has moved through and another is arriving as a strong clipper system is set to bring a period of light to moderate snowfall during the day on Tuesday to southern New England.  We aren't looking at a major snow event here, but we are looking at plowable snows.

  A trough will be working into the upper Mid-west region over the next few days and begin to amplify (sharpen) during the morning hours of Tuesday.  As this trough, with an associated Arctic cold front near the region, a piece of rather strong shortwave energy will rotate around the base of the trough.  With the trough amplifying, this will allow the shortwave energy to strengthen even further.  The trough amplifying will also allow for moisture to be drawn into the system from the Atlantic and this system will also have moisture associated with it.  Below is a 4-panel chart from today's 18z run of the GFS valid for 7:00 AM EST Tuesday morning.  The graphic on the top left displays the 500mb pattern (roughly 18,000ft AGL).  Circled is the piece of shortwave energy discussed above.  The graphic on the bottom left displays relative humidity values at 700mb (roughly 10,000ft AGL).  The darker the green shading, the more moist the airmass is at that level.  As you can see we are plenty moist.  The graphic on the bottom right displays 6-HR precipitation values and you can see the area of precipitation working towards the region.

  

Right now we are expecting light snows to break out very early Tuesday morning and lasting through late afternoon.  As stated above, we are not expecting a major snow producing event here given this is a clipper system which are typically very fast moving and they aren't usually associated with a great deal of moisture.
The airmass in place across the region will be cold enough to support snow for the entire region, with perhaps some mixing across the outer Cape.  Snowgrowth looks rather decent as well and snowfall ratios may be as high as 20:1!  This will help us in the snow accumulation department.  Mid-level lapse rates also look to steepen and this coupled with some strong lift which is suggested by stronger vertical velocities moving across the region suggest, there could be a very brief period of heavy snowfall somewhere across the region.  Given what we have presented to us now, this is what to expect right now.

Updated snowfall forecast for southern New England

Given the latest trends in the computer forecast models during the overnight period, snowfall totals have been increased slightly, although hesitantly.  A few negative factors for higher end totals is 1) The speed of the storm 2) The duration of the heaviest snowfall rates and 3) No real established mid-level circulation.  However, computer forecast models are continuing to indicate a tremendous amount of atmospheric lift coupled with a very saturated atmosphere and excellent snowgrowth, at least until some warmer air begins to work in aloft.  During the height of the storm, we could see snowfall rates exceed 2'' per hour at times, thus making some of the higher end totals quite possible.  Given the computer forecast models are projecting the strongest lift to occur across portions of extreme NE CT/ NW RI/portions of interior eastern MA, this is where the highest snowfall totals are expected.  

What to Expect

• Light snows to being breaking out, possibly as early as 2-3 PM
• Heaviest snowfall expected to occur between about 7-8 PM and 3-4 AM.  
• Snowfall rates at the height of the storm may approach 2'' per hour for a period
• Coastal areas will experience a mix/changeover to sleet keeping snowfall totals down across these areas
• As the storm winds down, many may see a period of mixing with sleet or changing over to sleet with the exception of western MA and extreme NW CT
• Storm is gone early AM Sunday with light snows lingering across NE MA until late morning

Accumulating snowfall Saturday into Sunday across southern New England

A winter storm is set to impact southern New England Saturday into Sunday as a quick hitting system is set to bring accumulating and plow-able snowfall to the entire region.  We have been closely monitoring this potential over the course of the past several days and as we draw closer to the weekend, the details being presented to us have become much clearer and we have a better idea of what to expect.

Meteorological Insight

Below are images of the 500mb pattern (images on the left) and sea-level pressure (images on the right) from today's 12z runs of the GFS and the Euro.  For the 500mb level pattern there are 2 highlighted areas.  The 1st highlighted area is the southern stream energy which will spawn the area of low pressure which will produce our storm.  The 2nd highlighted area is the northern piece of energy.  Initially we were watching as some model guidance did suggest that the possibility was there for these two systems to phase with each other, or in other words, combine.  In this type of solution we would be looking at a more power and potent storm system and a bit slower moving system.  However, it's now begging to appear that these two pieces of energy will not phase (at least over our area) so the southern stream energy will remain the dominant feature.

The image on the right, as mentioned above, is showing sea level pressure are there are two highlighted features here, with the exception of the Euro as the Euro has yet to develop our surface low yet.  The main feature here is the area of high pressure which is circled just to the north of New England.  This Arctic High pressure is expected to slightly strengthen this weekend and this will really help to keep colder air in the lower levels of the atmosphere and at the surface locked in place for a longer period of time and this colder air will also really help to make for fluffier snow, especially the further north you go.

Below we fast forward the GFS/Euro models to 72 hours out, with each projecting how the atmosphere will look come 7:00 AM EST Sunday.  What we can see is both models show a low pressure track very close to the coast and with that Arctic High Pressure eventually departing off to the northeast, this will eventually allow for some warmer air to work in towards the surface and aloft in the atmosphere.  This will be associated with a warm front which will work close to parts of the region.

As the area of low pressure develops and strengthens, this will allow for moisture to increase across the region and with a very cold thermal profile throughout the atmosphere, this will lead to some excellent snowgrowth and pretty high snowfall ratios, perhaps as high as 15:1 to 17:1.

What to Expect

This is not going to be a classic snowstorm by any means.  This isn't a storm forming from the phasing of two energies, as described above, and the mid-levels of the atmosphere aren't presenting us with that look which makes us go...WOW.  While light snows may being falling as early as early Saturday afternoon with snows lingering into perhaps late Sunday morning, the bulk of the heaviest snowfall is actually only expected to occur in a 4-6 hour window, likely from late Saturday evening into the overnight hours.  This is where much of the snowfall accumulations will occur.  The burst of the heaviest snowfall will be associated with the nose of the increasing warm-air advection aloft and a swath of very strong atmospheric lift.  During this 4-6 hour window, we could see snowfall rates potentially exceed 2''/HR.  As the Arctic high does begin to depart, allowing for some warmer air to work in, this will decrease snowfall ratios some and also may allow a changeover or a mix of sleet and perhaps some freezing rain, especially for portions of CT/RI/far SE MA.  Towards the end of the event though, all areas may experience some brief periods of sleet as well.  Below is current thinking of what to expect for snowfall totals.

Weekend Storm Potential for southern New England

By now I'm sure just about everyone has heard about the potential for a winter storm at some point this weekend with the time frame of impact somewhere between Saturday and Sunday.  At this stage, not only is trying to pin down the exact timing of the onset of any storm, but we are faced with many questions such as, will there be a storm and if so, how strong of a storm are we looking at?  When answering these questions we rely heavily on the many different computer forecast guidance models we have available and try to piece the puzzle together.  When there are differences among the computer forecast guidance models and not much consistency, it lowers the confidence of any forecast.  When looking at what potential this weekend may hold, we will look at the key pieces within the atmosphere and then see how different computer forecast guidance models are handling them.

Below we are looking at the 500mb level of the atmosphere (images on the left), or about 18,000ft AGL.  This level of the atmosphere is of major importance as the jet stream at this level, along with the upper-air pattern is the driver of our weather and we are also looking at 1000-500mb thicknesses along with sea-level pressure (images on the right)  The top image is from the European model (Euro) and the last two images are from the GFS model and the NAM model, two of the American models.  

With the images on the left, each model output graphic has two circled areas of interest.  These two features are what will eventually develop a storm off the East coast.  Each of these graphics are what each model is forecasting what the 500mb pattern will look like come 7:00 AM EST Saturday morning.  As you can see each model is handling the two features differently and that is one factor leading to a low confidence call right now, especially with regards to potential strength.

What each of these models do have in common is a rather strong northern branch of energy (the area near the US/Canadian border).  With each model in agreement on this, all 3 of the selected models will continue to lead to amplification of this feature (strengthening) and lead to a digging trough working into and through the central-tier of the US.  What we also notice is, the two American models, have another piece of very weak energy ahead of the more stronger energy.  The Euro has just the one piece of potent energy.

When looking into the southern stream energy (circled area across the southern-tier of the US), we can see various differences between each of these models.  The Euro and NAM models, however, keep the southern stream energies intact, with the NAM being stronger than the Euro with this energy.  The GFS, however, has the southern stream energy separate into two pieces.

The key for this weekend will be how each of these two streams interact with each other; when do they phase (combine), where do they phase, and how strong will each feature be?  As we go through the next few days, computer forecast guidance should begin to encounter a consensus of how the pattern will unfold and once this occurs, we will have a much better idea of exactly what to expect and when to expect it.

Going back to each model, we see what their projections are for 7:00 AM EST Sunday for the Euro/GFS models and for 1:00 AM EST Sunday from the NAM.  The NAM computer only runs out to 84 hours from it's initialized time.

From going by the latest data from this afternoon, what we see right now is the Euro/NAM computer forecast models are very close to the development of what would be a pretty strong storm system with the potential to produce plow-able snowfall across the entire region.  The GFS, however, wants to phase the two streams later rather than earlier, and that solution would reduce the risk for a more potent system.

Given how the GFS ensembles (a collection of different members associated with a particular model run) vary widely with it's solutions, this tells us that the GFS right now is really all over the place with this system and seeing this, certainly keeps the possibility in hand of a major coastal storm with the potential for plow-able snowfall throughout the region.

As new data continues to come in, over the course of the next 24-36 hours, the situation should become much more clear and details can start to be ironed out.

Plains like airmass to overspread New England Wednesday, September 11th, 2013 along with threat for storms

Over the past few days I've discussed the possibility of strong to severe thunderstorms across New England from the Tuesday through Thursday timeframes as a very favorable environment was moving into place for strong to severe t'storms.  While Tuesday's potential was focused more across central/upstate NY and into VT/western NH, this potential was not realized today.

The warm front moving through the region has been advancing northeastward much slower than what computer models have suggested so it took longer to get the hot/humid airmass to work into that region.  Also, a morning MCS, which actually also tracked further southwest than what models suggested, which actually is not uncommon, and left a great deal of cloud debris across that region.  While by mid to late afternoon, the atmosphere recovered across the mentioned areas of New York and the airmass became violently unstable, the air was capped thanks to height rises and subsidence in wake of the MCS.  While the atmosphere became very prime for severe weather including the possibility of tornadoes, a capped atmosphere prevented any development from occurring.

As we move into Wednesday get ready for a mid-July type airmass across New England.  Temperatures will be ranging from the mid-80's across central/northern New England to mid-90's across much of CT/RI and including the Boston metro area.  Combine this with dewpoints into the lower 70's and it will feel absolutely miserable outside, and in fact very dangerous for anyone working outdoors.  It's extremely important to drink lots, and lots of water!  Remember to check on the elderly and the young as well.

(Image from today's 18z NAM showing 2M temps for 2 PM Wednesday)

Over the past few days I've also mentioned the elevated mixed-layer, a plume of very warm and dry air situation between 800-550mb or so in the atmosphere.  The core of this elevated mixed-layer will arrive overhead tomorrow.  This will be associated with extremely steep mid-level lapse rates on order of 7-7.5, perhaps even 8C/KM.  This means the temperature will be dropping that many degrees in C per KM of the atmosphere between the mentioned levels!  This combined with the extremely hot surface temperatures and very high dewpoints will lead to an extremely unstable airmass across our region.  The atmosphere will be very prime for severe thunderstorms.

(Image from today's 18z NAM showing surfaced-based Capes for 2 PM Wednesday)

Also, yesterday I had discussed how our atmosphere would remain capped tomorrow and we would likely not see any activity except across central/northern NY/VT/NH/ME as these areas would be closer to the cold front and source of lift.  Throughout the day on Tuesday as new computer model data would become available, several computer models have been indicating that this cap may weaken very late in the afternoon as mid-level temperatures actually cool as the core of the warmest mid-level temperatures at 700mb begin to slide east.  Computer models are also showing some height falls later in the day as well along with a very weak piece of shortwave energy lifting northeast from PA.

So, what can we take from all this?

At this point it's very difficult to say whether or not we will see thunderstorms.  In fact, it's probably more unlikely we see nothing as the cap may be just too strong to overcome.  However, the atmosphere will be very prime for explosive t'storms so if t'storms are able to develop or move into the area they will be quite strong to severe with damaging winds and large hail definite possibilities.  This is just a situation we will have to watch as the day unfolds.

Meanwhile, across central/upstate NY/VT/NH/ME, where strong lift/forcing will be available, we could be looking at a highly active severe weather day across these parts with damaging winds, large hail, and isolated tornadoes all a distinct possibility.

The threat for strong to severe t'storms will also be a possibility on Thursday as the cold front continues to work east through the region and once passing ushering a more fall-like airmass.

Dangerous t'storms possible across New England Tuesday-Thursday

Computer forecast models continue to indicate a very volatile airmass will advect into New England tomorrow morning lasting through Thursday as a highly anomalous late season weather pattern will be developing.  A strengthening ridge across the southeastern United States coupled with an amplifying trough working through the Pacific Northwest and northern tier of the country is allowing the advection of an elevated mixed-layer into New England.  A warm front is approaching New England now and the elevated mixed-layer will be associated with the warm front.  As the warm front moves through the region Tuesday morning, a very strong surge of low-level moisture will work into the region along with an unseasonably warm low-level airmass.  In the mid-levels of the atmosphere a pocket of very warm temperatures around 700mb of +10C to +12C will work overhead, signaling the elevated mixed-layer which will lead to very steep mid-level lapse rates on order of 7.5-8 C/KM.  The combination of rich low-level moisture, very steep mid-level lapse rates associated with the elevated mixed-layer, and strong surface heating will lead to extreme amounts of instability across the region at times over the next few days.  This combined with increasing shear/forcing aloft will lead to the potential for a significant severe weather event to occur across portions of the region.  Let's break this down day-by-day and into details.

Tuesday, September 9th, 2013

As a warm front lifts northward on Tuesday morning, a very strong surge of theta-e air will slide into the region with the highest axis occurring across central/upstate New York, western MA and into VT/western NH where the core of the low-level jet will slide through.  Currently, a cluster of t'storms is ongoing across portions of the upper-Midwest and southern Canada and this complex, or mesoscale convective system (MCS) will continue diving southeast along this boundary and should impact central/upstate NY and parts of MA/VT/NH during the morning hours.  There could be some strong to severe t'storms within this complex during the morning hours with large hail being the primary threat.  Once this complex clears the area this is when things could become very interesting.

Once the complex clears, skies are expected to rapidly clear and the surge of rich moisture will continue working into the region and mid-level lapse rates will continue to steepen as the elevated mixed-layer works overhead.  Dewpoints should climb into the mid to upper 60's and surface temperatures should reach the mid to upper 80's.  Combine these with mid-level lapse rates approaching 7.0-7.5 C/KM will lead to an extremely unstable airmass with surfaced-based Cape values exceeding 3000-4000 J/KG, mixed-layer Cape values exceeding 2000-3000 J/KG, and lifted index values as low as -9C to -12C.
With the warm front still in the vicinity, helicity values are expected to be very high with 0-3km helicity values on order of 300-400 m2s2, and 0-1km helicity values on order of 200-300 m2s2.  Since this region will also be closer to an eastward progressing cold front/upper level trough, wind shear in the mid and upper levels of the atmosphere will be quite strong with a mid-level jet around 40 knots which should yield to 30-35 knots of vertical shear.

These factors will yield to atmospheric conditions extremely favorable for severe thunderstorms including the possibility of tornadoes and the potential would exist for strong tornadoes.  While the atmosphere will be prime for these conditions, it's very unclear whether any storms will develop in the afternoon.  There will be a pretty strong cap across the region thanks to the elevated mixed-layer and there are no clear triggers to break this cap, though there could be a weak shortwave that moves through aloft or orographic lifting could be a source of lift.  If the cap can't be broken then t'storms are unlikely to develop.  However, if the cap can be broken, the potential for severe t'storms would be highly likely, though coverage may be limited, and any t'storm would have the potential to produce very large hail, possibly as big as 2'', damaging winds in excess of 65-70 mph, along with the threat for a tornado and given the high helicity values/Cape values a strong tornado would be possible.  Again, this is all dependent on whether the Cap can break.

If t'storms are able to develop tomorrow, they could converge into another MCS complex late evening and work southward impacting MA/CT/RI.  If we do see an MCS develop and work into these areas, the impacts would be overnight and again this would pose a threat for severe weather, likely with large hail being the main threat though strong winds would also be a possibility.

Wednesday, September 11th, 2013

Wednesday will be highly prime for high end significant severe weather region wide, however, across much of MA/CT/RI the airmass is expected to remain strongly capped so t'storms are more unlikely across these areas, though the potential can't be ruled out.  If any storms do develop they would quickly become severe posing a large hail/damaging wind threat.  Shear will be much weaker here so storm movement would be very slow and given that flash flooding would be a concern as well.
Across upstate NY/VT/NH/western ME it's a much different story.  Shear will be much stronger and there will be a defined forcing mechanism.  If strong surface heating can develop the atmosphere will become extremely unstable once again.  On Wednesday, we could be looking at the possibility of a high end severe weather outbreak across this region with large hail as big as 2'' in diameter, damaging winds in excess of 70 mph, and the threat for tornadoes and possibility of a strong tornado.  The tornado threat will depend on whether or not storms remain discrete or develop into a line.  Discrete would increase tornado chances while linear action would favor damaging winds.  This is  situation which will be closely monitored over the next 34-36 hours.

Thursday, September 12th, 2013

This day would favor eastern NY/southern VT/NH/MA/CT/RI for strong to severe t'storms, however, this potential is a bit more unclear.  The elevated mixed-layer should be east of the region or weakened but lapse rates could still remain steep, up near 6.5 C/KM.  If there is little cloud debris or morning showers, the lapse rates should remain fairly steep and we would see strong surface heating.  This would yield to moderate amounts of instability across the region.  With the front/trough progressing eastward, winds aloft will strengthen and become very favorable for organized t'storms.  As far as Thursday goes we will have to watch how the atmosphere responds to previous day's convection.  We will have more details over the next few days.

  

Strong/Severe T'storm Threats Tuesday, September 10th-13th, 2013

As we move through August and into September strong/severe thunderstorm possibilities across New England begin to decrease very rapidly.  This is due to the fact that as we move through this time of year we begin to transition from a summer weather pattern toward an Autumn weather pattern and it's much more difficult to get higher amounts of instability to develop across the region.  While it becomes much more difficult, it certainly isn't impossible and for it to happen you need a really special setup.  For the past several days now, medium/long range computer forecast models have been very consistent with this "special setup".  When referring to this "special setup" we are talking about the elevated-mixed layer.

When talking about the potential for severe weather and higher event significant severe weather events, having an elevated mixed-layer in place is extremely critical in order to achieve a major severe weather outbreak consisting of high end significant severe weather.  High end significant severe weather refers to widespread wind damage, typically with winds in excess of 65-70 mph, widespread large hail reports exceeding 1.5''-2'', and tornadoes, typically with at least one strong/violent tornado (EF2 or greater).

Elevated mixed-layers are areas of the atmosphere, typically between 800-600mb of extremely warm and dry air.  This lead to very steep mid-level lapse rates on order of at least 7.5-8 C/KM and can even exceed 8.5-9.5 C/KM.  This combination, especially when coupled with very high surface temperatures and lots of low-level moisture (high dewpoints) leads to extreme amounts of instability which is the main source of fuel for t'storms.  Elevated mixed-layers are located across the Inter-mountain west region across the southwestern United States and can/usually gradually shift east into the Great Plains region.  While it's very rare for elevated mixed-layers to advect all the way to New England, they can do so given special atmospheric setups.  If you happen to remember June 1st, 2011, the day of the Springfield tornado, that was associated with an elevated mixed-layer.

When looking for elevated mixed-layer advection into New England, what you want to see is a very strong 700mb ridge centered across the southeastern US with 700mb temperatures in excess of +10 to +12C over your region.  You also want ridging at 500mb and to be on the crest or top of the ridge along with an amplifying trough to your northwest.  This allows you to be closer to the cooler 500mb temperatures and also allows you to be closer to the much strong mid-level jet.  Here are some images from today's 12z GFS run for 48 hours out, meaning the following images are for 8:00 AM eastern time Tuesday morning.

During the morning hours of Tuesday a warm front will be lifting northward through the region as well, and in fact, will be the leading edge of the elevated mixed-layer air which will move into the region.  As the warm front lifts northward humidity will be on the sharp increase as dewpoints will climb to near 70F and the low-level airmass will warm as well and to the point to where surface temperatures could reach the mid-80's to even near 90F in spots if full heating potential is realized.  

On Tuesday, severe weather could be a definite possibility across northern New England first in the AM associated with the warm front and then again during the afternoon as a cold front slowly slides east and a piece of short-wave energy moves through aloft.  Late Tuesday evening/overnight then could be interesting for central/southern New England as we could be looking at the potential for strong to severe t'storms during the overnight hours.  

As we move into Wednesday and perhaps even Thursday, as a cold front slides east severe weather may be a possibility both days as well.  What we will have to see is whether we can hold the elevated-mixed layer or at least steeper lapse rates both days as if we can, both days could be extremely active with severe weather.

At this point it's a bit too early to get into specifics for Wednesday or Thursday but as far as Tuesday is concerned it could be very active across central/northern NY and then into VT/western NH/northwestern MA during the AM/PM.  We could be looking at the possibility for damaging winds, large hail, and perhaps the threat for isolated tornadoes depending on whether or not the low-level jet is strong enough and hodographs can become enlarged.  

As far as Tuesday night is concerned for the rest of MA and down into CT/RI, this will all depend on how much forcing is present.  We definitely will have to watch the Tuesday night period for some severe weather across these areas with large hail/damaging winds being more of a threat.

As we move through the next 24-48-60 hours, details should continued to be ironed out.  

Severe Weather threat Tuesday, September 3rd, 2013

Just one more day of high humidity to go as a cold front is slowly creeping towards the region.  Once the cold front pushes through the region sometime tomorrow, much more seasonable temperatures will arrive along with much lower humidity values.  In fact, a few nights later this week will give us temperatures feeling more like fall.  Before the cold front moves through though we may have to deal with more downpours and t'storms and some of these storms may be quite strong.

The million dollar question right now regarding tomorrow is exactly when the cold front will slide through the region.  Currently, the projected timing of the cold front appears to occur at the time of day when conditions could be rather ripe for strong/severe t'storms, however, this window is very critical right now.  If the front slides through 3-4 hours later that what is currently modeled than the threat for strong/severe t'storms could be higher.  If the front moves through around the time the models are suggesting, the better threat may exist across RI and eastern MA.  If the front moves through even 2 hours quicker than what is modeled, than the threat for strong to severe storms would be extremely low region wide.

If past history is any indication, typically computer models are a bit too quick to move cold fronts through the region, especially when you have an amplifying trough and/or strong ridging out in the western Atlantic.  When looking at the overall synoptic pattern and the models handling of the evolution of the synoptic pattern over the next day, we see some strong ridging in the western Atlantic along with an amplifying trough allowing from winds in the mid to upper-levels of the atmosphere to work parallel with the cold front.  Typically this scenario would actually allow for a slower eastward progression of the cold front but this does not guarantee it.  Sometimes, short wave energy associated with the cold front and surface low will de-amplify and this could cause a disconnect between the trough and the cold front.  i.e. the cold front begins progressing east quicker than the trough.  This will have to be closely monitored throughout the remainder of the night and into the morning hours tomorrow.

Another issue we have to contend with will be cloud cover and instability, which always seems to be a cause for concern here.  Today, much of the region was socked in with overcast, although there were several breaks of sun across many locations.  Upstream there was a quite a bit of convection today and cloud debris continues to move into the region.  It's quite likely tomorrow there will be lots of cloud cover which will ultimately hold back instability values.  What does seem highly possible though is there are many breaks within the clouds allowing for pockets of stronger destabilization.  The more sun and stronger the heating, especially when coincident with a slower frontal passage, the better chance for strong to severe t'storms.  This will also have to be monitored in the morning hours.

Winds aloft will also be fairly strong tomorrow with vertical shear values favorable for storm organization and longevity.  There also appears to be a decent amount of winds turning with height in the 0-3km levels with 0-3km helicity values perhaps exceeding 150-200 m2s2. 0-1km helicity values though are quite low, however, if surface winds back ahead of the cold front, these values will be much higher.  Temperatures in the mid--levels of the atmosphere will also be cooling, allowing for steepening mid-level lapse rates.

Tomorrow, besides the threat for torrential downpours and isolted flash flooding, the potential will also exist for numerous t'storms with several becoming strong to severe.  However, the extent of the threat will all be determined by the timing of the front and if we can destabilize the atmosphere enough.  If we are able to get strong/severe t'storms the potential would be large hail and damaging winds.

The potential will be monitored through the night and into the morning as the details start to become much more clear.

T'storm possible Tuesday, August 13th, 2013

After a very gorgeous weekend with little to no humidity and cooler temperatures, a warm front will begin to slowly lift through the region during the day on Monday and with the passage of the warm front we will once again see the return of higher dewpoints/humidity along with the threat for showers on Monday.  The humidity won't be around for long though as a rather strong cold front is expected to cross the region sometime either late Tuesday or overnight Tuesday into Wednesday.  Right now there are differences in some of the computer model data on the exact timing of the cold front. 

An amplifying trough moving across the Great Lakes and southeastern Canada will allow for winds aloft to increase over southern New England.  Computer model guidance suggests that winds at 700mb will increase on order of 30-40 knots with winds at 500mb increasing to as much as 40-50 knots across the region.  While the low-level jet doesn't appear all that great, we still see winds at 850mb increase to 25 to perhaps 30 knots.  The region will also be in the right entrance region of a rather potent 100-110 knot upper-level jet streak.  

As the trough approaches the region, temperatures in the mid levels of the atmosphere will also begin to cool as 500mb temperatures are modeled to drop as low as -12C to perhaps -13C.  With 700mb temperatures around +5C to begin the day (although cooling as the afternoon goes on) this should yield to some much better mid-level lapse rates than we have been dealing with as they should be on order of around 6-6.5 C/KM.  

As far as instability goes, that picture is much more unclear right now as we may be dealing with a bit of cloudiness which could hold back temperatures.  Given how 925mb temperatures are modeled to be around +20C with 850mb temperatures closer to +14C, that would yield max temps with full mixing well into the mid to upper 80's.  However, with clouds expected to be around we probably won't realize this full potential so temperatures should be closer to 80F, although with more sun these temperatures could easily be boosted higher, especially locally where there happened to be more breaks.  Dewpoint temperatures are expected to climb into the mid-60s.  

The combination of surface temperatures near 80F, dewpoints in the mid-60's, and modest mid-level lapse rates, should yield to surfaced-based Cape values around 1500-2000 J/KG with mixed-layer cape values around 1000-1500 J/KG and surfaced-based lifted index values around -3C to -4C.

When taking into account the combination of strong winds aloft and potential for some modest instability, and a cold front/trough sliding east, we will be looking at the potential for some t'storms during the day on Tuesday.  

We will have to watch the exact timing of the front, however, as if the front comes through very late on Tuesday or during the night, then the threat for t'storms will be much more decreased as instability will be minimized.  However, some computer model guidance does suggest a pre-frontal trough will setup just east of the Hudson River Valley and that would be a focus for t'storms.

Given the strong shear/cold temperatures aloft, any t'storm would have the potential to produce strong winds and perhaps some hail.  

Severe Weather Potential Friday, August 9th, 2013

Friday has the possibility to be a fairly interesting and perhaps active weather day across southern New England as the possibility will exist for showers and t'storms.  While the overall threat may be torrential downpours leading to pockets of flash flooding, the possibility of some strong to severe t'storms will also be possible, in which strong winds and even the possibility of a few isolated tornadoes will be possible.

During the overnight hours of Wednesday into early Thursday, a warm front lifted northward through southern New England and this allowed for dewpoints/humidity to rapidly increase across the region as dewpoints have risen into the upper 60's to around 70F.

On Friday, a mid-level trough sliding east passed the Great Lakes will allow for an area of surface low pressure to slide east through southern Canada along with a cold front towards southern New England.  Some computer forecast models are also showing a "secondary" warm front development just to the west of the region early tomorrow along with another weak wave of low pressure.  It is these two features which is opening eyes with regards to the possibility of severe weather tomorrow.

Some computer forecast models are developing a fairly potent low-level jet later tonight, lasting through much of tomorrow with winds in the 925-850mb layer exceeding 30-40 knots which is quite strong for this time of year.  Not only are these models developing strong winds here but these models also are indicting strong directional wind shear with winds at the surface coming from the south with winds turning towards the SW/WSW aloft with height.  Below are two hodographs taken from today's 18z GFS run for Windsor Locks, CT tomorrow afternoon.  I have circled four areas of interest.  The first two areas of interest are showing the hodograph, which measures wind speeds/direction with height.  The "longer" the hodographs, the stronger the winds aloft are, the more curved they are, the stronger the turning of winds with height are.  The second two areas of interest are the models suggesting the potential for several hundred to perhaps 1000 J/KG of Cape, which with combined with those hodographs insists the potential for severe weather/tornadoes would be a possibility tomorrow:

When it comes to the low-level jet, however, often times computer models can overdo the strength of them as computer models can sometimes suffer from what is called convective feedback.  This is one factor that makes this forecast/potential rather unclear at this time.

As far as instability goes, weak mid-level lapse rates and extensive cloud cover are expected to hold back instability values, however, with surface dewpoints near 70F and rich low-level moisture, surfaced-based instability values could exceed 1000-1500 J/KG along with mixed-layer cape values and lifted index values in the -3C to -5C range.

If this low-level jet verifies, or comes close to verifying, and we are able to manage even a few hundred J/KG of Cape, any t'storm that develops will have to be closely watched as atmospheric conditions will be quite favorable for strong winds and perhaps even some tornadoes, especially with extremely low lifted condensation levels, the level at which the cloud base is located (to put it simply).

Widespread t'storms expected Saturday, July 19th, 2013 with the threat for strong to severe t'storms

Saturday will be the final day of the high heat/humidity across New England as an amplifying trough moving through southern Canada and the Great Lakes will swing an area of surface low pressure through southeastern Canada with an attendant cold front towards and eventually through the region.  As the trough/cold front move closer to the region we will have to deal with the potential for t'storms and several of these are likely to become quite strong to severe.

Over the past few weeks, strong ridging has dominated much of the United States thanks to a very strong Bermuda High.  However, over the past several days, a trough has been developing and amplifying as it moves eastward and this is allowing for the ridge to slowly break down.  As this trough continues to amplify and the area of surface low pressure continues to deepen, the increased pressure gradient will allow for the winds aloft in the lower, mid, and upper levels of the atmosphere all to increase.  Throughout the day on Saturday we will see the low-level jet increase to 25-35 knots, with the mid-level jet increasing to 35-45 knots, even upwards of 50-60 knots across central/northern New England, and the region will be in the right entrance region of a 90-100 knot upper-level jet max which will really help to increase divergence and lift across much of New England.  With the increase in the mid-level wind fields, 0-6km shear values should increase to 30-40 knots, which is more than sufficient for organized t'storms.

While we may have more in the way of cloud cover on Saturday, temperatures are expected to reach anywhere from the mid-80's along the coast to as high as the low to mid 90's inland.  This coupled with dewpoints in the upper 60's to lower 70's will once again make for very uncomfortable conditions outside.  During the day on Friday steeper mid-level lapse rates (change of temperature with height) have advected into the region, and there is even a hint at a weak elevated mixed-layer (EML) across much of southern New England as mid-level lapse rates are approaching 7 C/KM and some forecast soundings do have an EML look.  As we move into Saturday, mid-level lapse rates are expected to remain quite favorable, in the 6.5-7 C/KM.  These stepper lapse rates coupled with surface temperatures ranging from the mid-80's to the lower to mid-90's and dewpoints in the upper 60's to lower 70's should yield to a moderate to extremely unstable atmosphere.  We could be looking at surfaced-based cape values ranging from 2500-4000 J/KG, mixed-layer cape values in the 1500-2500 J/KG range, MUcape values in the 2000-3000 J/KG range, and lifted index values as low as -6C to -9C...these numbers are indicative of an extremely unstable atmosphere and will be the fuel for the t'storms.

Out ahead of the cold front, a pre-frontal trough is expected to develop across the Hudson Valley and then slide east during/just past prime heating and this will be the focus for lift.  The combination of increasing lift along with adequate wind shear and a moderately to extremely unstable atmosphere will lead to the development of widespread showers and t'storms tomorrow afternoon and lasting through much of the evening and into the early overnight hours.  Winds aloft will be fairly unidirectional which suggests that the main storm mode tomorrow will be a linear squall line with bowing segments.  If we are to achieve the higher end of instability, then some supercells will be possible as well.  Forecast soundings also show an inverted-v signature.  Given all this, the main severe threat with thunderstorms tomorrow will be damaging winds in excess of 50-60 mph.  If we do see supercells, then the threat for large hail, possibly as large as golf balls would also be a possibility.  All storms will contain vivid cloud-to-ground lightning along with torrential downpours, perhaps leading to poor drainage flooding.

Below is a map indicting the risk area along with hazards:

***NOTE***  Not everyone in the shaded area may receive rain or even t'storms.  Not everyone will experience a strong to severe t'storm either.  The highlighted area outlines the area where severe weather may occur***

Strong to Severe t'storms possible on Saturday, July 20th, 2013

An amplifying trough will continue to strengthen as we move towards the weekend and slide a cold front east.  Once the cold front pushes east of the region we will usher in some "cooler" (in a sense of more seasonable temperatures) and drier air into the region as the pattern changes from ridge dominated to more trough dominated through much of next week.  Before this cooler and drier air works in though we will have to deal with the threat for t'storms and the potential exists for several t'storms to become quite strong to severe.

While isolated t'storms are possible both Thursday and Friday, Saturday is the day of greater concern for widespread t'storms along with an enhanced threat for some severe weather.  Saturday will not be a washout by any means, however, we may have some shower/t'storms activity around in the AM before we see a break.  This is when we see sun and lots of surface heating which will help to fuel the atmosphere for storms later on in the day.

A strong piece of energy moving through southern Canada along with the approaching trough/front will help to really tighten the pressure gradient over our region given the Bermuda High is centered to our south.  In response, the winds aloft will really begin to strengthen.  Forecast computer models show winds in the lower levels of the atmosphere to strengthen to 20-30 knots with winds in the mid levels of the atmosphere strengthening to 35-45 knots.  The region will also be in the right entrance region of a very strong upper-level jet which will really help to increase lift/forcing.  Given the strong mid-level winds, we are looking at vertical shear values to be in excess of 35-40 knots across the region.

At the surface, temperatures are expected to reach the mid 80's to around 90F with dewpoints right around 70F.  A piece of computer model guidance is showing mid-level lapse rates around 6.5 C/KM  on Saturday which is pretty decent.  The combination of high temperatures, high dewpoints, and steep mid-level lapse rates should lead to moderate amounts of instability region wide.  We are looking at the potential for SBcape values in excess of 2500-4000 J/KG, MLcape values 1500-2000 J/KG, and LI values in the -6C to -8C range.

As the cold front slides east on Saturday, there are some hints that a pre-frontal trough may develop out ahead of the cold front.  If this does come true, we could be looking at t'storms firing as early as 1-2 PM across western sections and then sliding east.  If a pre-frontal does not develop, storms may not fire until 2-3 or even 4 PM.  Typically, we almost always see pre-frontal troughs develop so I believe we will see action early on in the day along/ahead of the pre-frontal trough and then quite possibly again later on just ahead of the cold front.

Given the potential for moderate instability coupled with fairly strong winds aloft, and strong forcing from the front and/or pre-frontal trough and enhanced forcing from the right front quadrant of the upper-level jet, we are looking at numerous t'storms Saturday afternoon and quite possibly multiple lines of t'storms.  Any t'storms that develop will have the potential to become quite strong very rapidly and approach severe limits.  The main threat with any storms will be damaging winds in excess of 60 mph.  Small hail is also a possibility, however, the freezing levels appear to be rather high but if any supercells develop, which is a possibility if we can achieve 3000+ Capes, then there could be a secondary threat for large hail.  Torrential downpours are very likely with any storms so flash flooding will be a possibility.

This potential will continued to be closely monitored.  

Periods of rain/storms overnight, more threat for storms on Friday 6/27/13

This entire week has been quite unsettled as we have had showers and t'storms across the region every single afternoon.  While not everyone has seen action, several places have been hit several times this week with showers and t'storms and as we move into Friday and into the weekend we will remain in a similar pattern, in fact, this pattern should remain in place through next week.  However, the point of this blog post is to discuss the weather for Friday.  You may hear forecasters every now and then (or even so often) say, "this is a highly complex weather forecast"...well this is yet another case of that and, well b/c it is.  As much as we know and understand the weather and the atmosphere and how they work, both are still a highly complex science and sometimes there is just so much chaos going on in the atmosphere that forecasting to certainties and exacts can be extremely difficult at times.  This is why I am always in favor of explaining the situation in as much detail as possible to hopefully help the reader have an understanding of the situation.

Anyways, this past week we have been dealing with several different meteorological phenomena's which have lead to some very difficult forecasts, some turning out good, and some getting completely wrong.  As we move through the remainder of tonight and through the day tomorrow, the forecast won't necessarily become much easier, however, confidence is increasing on several weather events we will see across the region.

During Wednesday, including the overnight period, a weak backdoor cold front (cold front which moves into the region from the north, northeast, or east) slid southwestward across the region and by Thursday morning, this weak boundary had washed out across central CT.  Typically when cold fronts move through, we usually see a refreshed airmass, it's cooler and less humid.  Well, that wasn't the case this time.  This front, as mentioned was very weak, and the cooler temps/dewpoints actually lagged well behind this front and you'd have to look all the way into northern New England to find this refreshing air.  During the day today, the washed up front left a boundary in place across the region and that was the focus for some afternoon showers/t'storms that popped up across the region.

Too our west, a deepening trough has been slowly sliding eastward dragging a cold front east as well.  As the trough/front slowly slide eastward, a surface area of low pressure has been developing and this will slide to our northwest during the day tomorrow.  As this occurs, the washed up backdoor cold front over the area will actually retreat north as a warm front, in fact this process is already occurring.  With the system strengthening to our west we have seen stronger winds aloft move into the region, especially in the lower levels of the atmosphere, these factors will lead to the development of showers and t'storms during the overnight hours.  While instability is very marginal, winds aloft are increasing so we can't completely rule out a strong storm overnight with some gusty winds.  The main threat though will be torrential downpours leading to pockets of flash flooding.

Now, as we move into Friday morning, there still should be numerous showers and perhaps some t'storms around.  After this activity moves out, this is where the forecast becomes much more complex and challenging.  Winds aloft will be quite strong tomorrow, also, with the possibility of the warm front being close-by, this will lead to some very high helicity values (change of wind direction with height), across the region.  There should be lots of clouds present, however, some computer model guidance is showing that there may be breaks of sun late tomorrow morning, especially across western CT/MA.  If we are able to get any sunlight, given how dewpoints will be into the upper 60's to lower 70's, and the potential for temperatures to get into the lower 80's, this will lead to a fairly unstable airmass tomorrow.  If we are able to achieve sufficient destabilization, the threat for more t'storms in the afternoon will vastly increase.  Given the presence of very strong wind shear and a very strong system to our west, the threat for strong to severe t'storms would exist.  Now, conditions right now don't appear to favor widespread severe weather, in fact, we are probably only looking at an isolated threat for a few of the storms to become strong to severe.  Any stronger storm will have to be closely watched though as it would have the potential to acquire rotation and the threat for an isolated tornado would be in the cards.  Again, the confidence in this right now is low b/c there are major questions with how unstable the airmass will become tomorrow.  Basically, the more sun we see tomorrow, the higher the threat for t'storms and the higher the threat for some of these storms to become strong to severe and be capable of damaging winds, large hail, and perhaps a tornado.  If we see little sun, the threat for t'storms will be much lower and the threat for severe storms will be extremely low, although still would have to closely monitor any storm that would develop.  Besides instability, the tornado potential would also depend on where the front front positions itself.  If the warm front is able to get well into northern New England, well then we would have less helicity values as the directional component to the wind would decrease and winds aloft would be blowing from the same general direction.

Once again, a severe weather outbreak is not expected at this time, however, the threat for t'storms will exist and if we see enough sunshine to really destabilize the atmosphere, the threat for t'storms and threat for some of these to become strong to severe increase.  Less or no sun, low threat for storms and even lower threat for any of these to become strong to severe.

More t'storms in the forecast for Wednesday, JUne 25th, 2013 with increased potential for strong to severe t'storms

We are fully engaged in a summer-like pattern as temperatures the past few days have been in the upper 80's to lower 90's across the region and with dewpoints into the upper 60's to lower 70's it has felt very uncomfortable outside and of course, we've had our daily t'storms, although not everyone has seen rain or storms.  This pattern of heat/humidity and storms will continue through the week and into the first part of the weekend and each day there will be a threat for strong to severe storms.  However, on this post we will just focus on Wednesday and what tomorrow may bring.

We are looking at yet another hot and humid day across the region tomorrow as temperatures away from the immediate coastline will once again get well into the mid to upper 80's to perhaps even 90F, in fact, places like Windsor Locks and Hartford could be looking at a 4th consecutive of 90F+ temperatures!  Dewpoints will once again remain in the mid to upper 60's so it will feel very humid outside.  

The big difference between tomorrow and the past two days is we are looking at more of a widespread threat for showers/t'storms, along with an elevated risk for stronger to severe t'storms.  Remember, when talking about strong t'storms, we are referring to t'storms which are capable of producing winds 40 mph or greater and/or small hail and when talking about severe t'storms, we are referring to t'storms capable of producing winds of 58 mph or greater, and/or hail 1'' in diameter or greater.  Also, not every thunderstorm becomes severe and the majority of people don't usually experience "severe" t'storms.

With that said, tomorrow, computer forecast models are showing a fairly strong piece of shortwave energy, embedded in the mid-level wind flow moving east-northeast from PA to NY right through southern New England during the afternoon hours.  As this shortwave energy approaches the region, winds well up in the atmosphere will begin to increase and provide us with sufficient wind shear, something we have lacked the past few days.  This wind shear will really help for storm to become better organized and sustain themselves much longer.  The shortwave energy will provide a focus for strong lift, something we've also lacked and that will allow for t'storms to be more widespread than they have been the past few days.  

With very little in the way of a cap tomorrow we should begin to see cumulus clouds beginning during the mid to late morning hours and this will really prevent temperatures from reaching into the lower 90's and may prevent anyone from seeing 90F tomorrow.  The combination of heat/humidity will once again provide us with quite an unstable airmass tomorrow, although probably not as unstable as the past few days b/c we will be looking at weaker mid-level lapse rates tomorrow.  This instability will provide the fuel storms will need to develop and strengthen.  

The combination of stronger wind shear and an unstable airmass will lead to the development of scattered t'storms during the early afternoon hours and we should see storms last well into the evening hours as the energy will be slow to move through.  The increased wind shear, along with instability will also increase the threat for stronger to severe t'storms tomorrow across the state.  Some of the storms that develop tomorrow will have the potential to produce winds of 40-50 mph and possibly as high as 60 mph.  While hail may not be a huge threat due to weaker lapse rates, the strong wind shear, along with some directional wind shear (change of wind direction with height), we could see some isolated supercell t'storms tomorrow, storms which have stronger rotation.  If any supercells do form, the threat for hail would locally increase and we could be looking at the potential for hail up to 1.5'' in diameter.

While confidence in the threat for t'storms is very high, confidence in how strong/severe the storms become or how many of these storms become strong to severe is lower.  This is due to the fact that the lapse rates will be so weak.  When mid-level lapse rates are weak, below 6 C/KM, (this means that for every km you rise in the atmosphere, the temperature on average decreases by 6C), air parcels aren't really rising all that rapidly.  Steeper mid-level lapse rates (higher rate of temperature change) allow for much more vigorous upward parcel acceleration and this translates to a much greater severe weather risk and t'storms have the ability to grow very tall, tapping into the much colder air in the mid/upper troposphere and stronger winds.  

All in all, we will once again see t'storms tomorrow afternoon/evening with activity being more widespread than the past two days.  The threat for stronger/severe storms will also be a bit more elevated than the past few days.  If outdoors, keep a close eye to the sky and up to the minute weather forecasts and prepare to seek shelter inside to avoid lightning.  

Threat for t'storms later today; some potentially becoming strong to severe

As highlighted yesterday, the potential exists later on for showers and t'storms across the region as a weak secondary cold front slowly shifts eastward.  For the past few days, computer model guidance has suggested that much of the morning hours would be filled with clouds, and this would be one factor limiting how unstable the atmosphere would become.  However, late last evening, cloud cover which blanked the area for much of Sunday drifted off to the east/northeast leaving us with clear skies overnight as well as now.  With much more sunshine than expected, this will allow the atmosphere to become a bit more unstable than previously thought, thus increasing the threat for storms and stronger storms later on.  It is unclear though as to how widespread t'storm activity will be along with how strong or severe the storms could become as there are some question marks which we'll highlight below:

1) Currently dewponts are into the lower 60's south of the MA Pike, lower north of the Pike.  Computer model guidance the past few days have indicated that these dewpoints will mix out...or in other words, decrease, which is beginning to occur now.  This is occurring due to the fact that above the surface, just a few thousand feet ASL, there is some drier air in place, dewpoint temperatures at 925mb (~2500') and 850mb (~5000') are only in the mid to upper 40's.  Typically you'd like to see them into the lower to mid 50's.  As we're moving through the morning hours, and the sun really heats up the atmosphere, allowing for stronger atmospheric mixing, that drier air will begin to mix down to the surface, thus lowering the dewpoints some.  The question is, how much will the dewpoints lower?  This will have major implications as well with regards to how unstable the atmosphere will become.  
Even if dewpoints do lower, there is potential for them to actually increase later on this afternoon.  We are noting some higher theta-e air working into the state, which would allow for dewpoints to increase.  A sea-breeze developing later on may help to increase them as well along with surface winds perhaps backing more to the south or southeast.  
2) As mentioned yesterday, the cold front is not particularly strong, so there isn't a great deal of forcing along it.  What this means is, that could limit how widespread the activity becomes and how much action we see develop.  On the other hand, since this would mean storms are more isolated in nature, this could allow them to become even stronger/more severe given they will have much more energy to tap into.
3) Forecast soundings show some funky warm spots in the mid-levels of the atmosphere. While some colder air will work in aloft, that doesn't necessarily mean these warm layers will dissipate.  These warm layers could affect storm updrafts in that it will be a bit more difficult for storms to grow very large, disallowing them to really tap into the wind energy and much colder air aloft, decreasing the threat for stronger winds and larger hail. 
With those factors mentioned, today, we are looking at temperatures reaching well into the mid 80's away from the immediate coast with dewpoints hovering around the 60F mark.  The combination of temps/dews, along with colder air working into the mid-levels of the atmosphere, will allow for a fairly unstable airmass with Cape values perhaps reaching 1500-2000 J/KG, especially if dewpoints increase later on and lifted index values as low as -3C to -4C.  Winds aloft are also quite strong, evident by vertical shear values in excess of 35-45 knots.  The combination of instability/shear will lead to the development of showers/t'storms later on and given the mentioned parameters, some of these storms will become quite strong to severe with the potential to produce damaging winds and large hail.  We will also have to keep an eye on for any rotating t'storms, increasing the hail threat.  Flash flooding will be a concern too, especially in low lying areas given amount of rain we have had.  The timeframe for t'storms will be from about 3-10 PM tonight.  Originally, it was thought storms would die as the sun would begin to set, however, with colder air working in aloft through the evening, this will help to keep instability around a bit longer.  
The front is expected to stall either over the state or just west and this will once again allow the potential for t'storms tomorrow with a renewed threat for strong/severe storms.  

Threat for t'storms; Monday, June 17th, 2013

A weakening storm system moving west to east across Quebec Provence in southeastern Canada will push a series of two cold fronts through southern England over the next two days.  The first cold front, is weakening as it approaches southern New England and as the front nears CT during the overnight hours this front is expected to wash out.  With the front approaching, we can expect some isolated to scattered showers through the overnight hours along with extensive cloud cover.  The combination of the front washing out along with extensive cloud cover will allow for overnight temperatures to remain around 60F with dewpoints hovering the 60F mark as well. 

As we move into and through the first half of Monday, one of the biggest questions is how much clearing and how much sunshine will we see?  There is decent agreement within the computer models, that there will be breaks of sun, along with clearing, moving from west to east during the late morning and early afternoon hours.  The greatest likelihood of seeing breaks of sun and sunshine will be along and just west of the CT River Valley.  In fact, we may see enough sunshine to where temperatures could reach the mid-80's.  

With temperatures, potentially reaching the mid-80's, dewpoints around 60F, and mid-level lapse rates between 6-6.5 C/KM, computer models are indicating we could see Cape values reach anywhere from 500 J/KG to as much as 1500 J/KG along with lifted index values getting to around -2C to even -3C.  This is all indicative of an atmosphere that will be marginally unstable tomorrow afternoon.  We will have to watch trends tomorrow with sun as any more sun than forecasted could lead to higher values.

A secondary cold front will slide towards CT tomorrow and with the potential for a marginally unstable airmass we will be looking at the threat for showers and t'storms.  The lift associated with the cold front is not very strong and the system moving through southern Canada will be weakening so we will not be looking at widespread showers/t'storms but more of the scattered nature.  

There will be some pretty impressive wind shear aloft as well with forecast models indicating 0-6km shear values as high as 35-45 knots.  These values are supportive for updrafts which can organize and also supportive of updrafts which could slightly rotate.  

Given the potential combination of instability/strong wind shear aloft, and a cold front sliding east, we should see some showers/t'storms develop during the early to mid afternoon hours.  While activity may not be all widespread, the presence of 35-45 knots of 0-6km shear will allow any activity to potentially organize into a squall line.  We will also have to watch out for a few strong to potentially severe t'storms with a threat for strong gusty winds and perhaps even some hail, especially if stronger cores can develop and acquire some weak rotation.  While not a major threat, given how much rainfall has occurred the past few weeks, there will be a flash flooding risk with any storms given there will likely be torrential downpours.  

The best timeframe for storms should be from 2-8 PM, although activity could persist until past 9 PM or so, however, as the sun goes down, the atmosphere will begin to stabilize. 

60th anniversary of the Worcester, MA tornado: The "Granddaddy" of Southern New England tornadoes.

When one thinks of southern New England one does not think of strong/violent or killer tornadoes here, and for great reason, as they are not very common, though not unheard of.  In fact, just recently, on June 1st, 2011 we had a strong/killer tornado rip through New England.  This tornado was on the ground for 39 miles, beginning in a section of Westfield, MA and finally lifting back up in a section of Charlton.  During it's 39 miles, 70 minute journey, at the peak the tornado was rated as a high end EF3 tornado with maximum winds of 160 mph, just shy of EF4 status and killed 3 people and injuring 72.  Prior to this 2011 tornado you have to go back to May 29th, 1995 when an F4 tornado struck the town of Great Barrington, MA.  In the 10 minutes this tornado was on the ground, 3 people were killed.  Prior to 1995, there was another F4 tornado (although there are major uncertainties regarding whether or not this should have been rated so high) which struck Hamden, CT on July 10th, 1989.  While only on the ground for several minutes, it produced major damage and injured over 40 people.  Then we'll go back to 1979, on October 3rd an extremely rare October tornado struck Bradley Airfield in Windsor Locks, CT.  While this was yet another short-lived tornado, it produced extreme damage along with killing 3 and injuring over 500 people.  This was yet another F4...even borderline F5 tornado.  While there were a few other tornadoes with a rating of F3 or higher we'll get right to be the big one, the Worcester, MA tornado of June 9th, 1953.

Below is a projected surface map on the day of June 9th, 1953.  An area of surface low pressure (circled in black) moving through Quebec Provence in southeastern Canada is pushing a cold front (highlighted by the blue line) to slide eastward towards southern New England and allowing for a warm front (highlighted by the red line) to lift northward through southern New England.  As the warm front would lift through CT and MA into northern New England, this allowed much warmer and humid air to work into the region, becoming two contributors on what was going to make for quite the unstable airmass later that afternoon.

Below we will take a look into the 700mb and 500mb levels of the atmosphere, which is roughly anywhere from around 10,000ft above sea-level to around 18,000ft above sea-level.  First off, is an image of the 700mb pattern over the Continental United States and much of Canada.  The red arrows indicate where the weather pattern is deriving from.  As you can see by the arrows, our airmass at 700mb originated from the desert Southwest.  This is actually pretty significant as research done over he past decade has shown that an atmospheric setup like this is conducive into allowing an elevated mixed-layer to sustain itself from the southwestern United States all the way to southern New England.  We could go into lengthy discussion into what an elevated mixed-layer is, but too make matters simple, elevated mixed-layers just lead to extremely unstable atmospheric conditions which is a major ingredient towards severe thunderstorms.  We also take note of the wind direction/strength depicted by the wind barbs.  We notice that the winds at this level are coming more from the northwest 25-35 knots, with even 40 knots over Ohio.  As the area of low pressure continued moving northeastward and then eastward into and through Canada, the system continued to actually strengthen and this allowed the wind fields to increase at this level on the day of the event.  

  

Below is a reanalysis map of the 500mb pattern along with wind strength/direction for roughly 8:00 AM eastern time on the morning of the 9th.  Just like at 700mb, we notice a trajectory of the pattern with the airmass originating from the southwestern United States and we also note a westerly flow with 40-45 knows over southern New England, however, just further off to the west, winds increase to 50-60 knots, and this would arrive over southern New England later that afternoon.

With the surface pattern, 700mb pattern, and 500mb patterns looked into we can now put all of this together and explain why such a violent tornado occurred in a region where they are oh so rare.  

The 500/700mb pattern which had been in place across the region for several days allowed an elevated mixed-layer plume to eject from the southwestern United States and survive it's journey to southern New England.  When the approaching storm system from the west allowed for a warm front to push northward through southern New England.  When the warm front pushed through this allowed dewpoints to soar into the 80's across spots and dewpoints to climb into the 60's.  The combination of temps into the 80's, dewpoints and into the 60's along with the elevated mixed-layer lead to extreme instability across the region. The warmfront, also in close proximity to the region, also allowed for winds at the surface to likely be from a more southerly/southeasterly direction.  With winds at the surface from the south/southeast, turning to the west/northwest aloft in the mid/upper levels of the atmosphere this created large amounts of helicity, or directional wind shear across the region...an extremely important ingredient for supercells and tornadoes.  

As the afternoon went on and the cold front and energy aloft associated with the system progressed eastward, thunderstorms began to develop.  One thunderstorm in particular became extremely severe and reaching supercell status, a supercell is a thunderstorm with very strong rotation.  At the height of the storm, the supercell thunderstorm produced a tornado which too this day is known as the granddaddy of southern New England tornadoes.  Along it's path, the tornado was on the ground for 90 minutes, traveling 48 miles tracking through eight different towns in central MA.  The tornado produced F4 damage across six of the eight towns affected, and even producing borderline F5 damage in spots.  When all said and done, 94 people were killed and over 1,000 people were injured.  Thousands of buildings/structures were heavily damaged with many completely destroyed.  

This was not the only killer tornado this system would produce, however.  Just the day before, the same system produced an F5 tornado in the town of Flint, MI.  That tornado killed 116 people and injured over 800 people.  This storm system, which began producing severe weather across NE/IA/WI would end up producing a total of 46 tornadoes, with 13 of them F3 or higher.  To this day, this series of outbreaks remains one of the most devastating and deadly in recorded history.  

Below is a slideshow, created by the National Weather Service in Taunton, MA, which also explains more regarding the setup, as well as showing numerous photos of the damage produced by the tornado, and even some images of the tornado itself.

http://www.erh.noaa.gov/box/papers/WorcesterTornado53_files/WorcesterTornado53.html

*Data was obtained through various National Weather Service Products and the images were obtained from the reanalysis site from Plymouth State and the Daily Weather Map Archive's ran by NOAA.*

Potential for strong to severe t'storms Wednesday, May 29th, 2013

Memorial Day is usually considered the "unofficial" beginning of summer, however, this past Memorial Day felt far from summer as temperatures hung into the 40's and 50's for just about everyone across southern New England with cool and damp conditions.  However, as we move through the next 12-18 hours we will begin seeing major changes in the overall weather pattern, giving many a taste of summer.  

While it is rather cool and cloudy outside with periods of showers, all of this is in advance of a very strong warm front which is slowly approaching southern New England.  As we move through the overnight hours and morning hours tomorrow, the warm front is expected to blow through CT and  into central/northern New England where it will become hung up.  South of the warm front, hot and humid air will begin working into the region.  

As of south it appears that all of CT, and much of western/central MA, including extreme southern VT and southwestern NH will end up in the warm sector with the strongest warmth tomorrow occurring down across southwestern CT, northward into western MA...perhaps even into central CT.  Across these areas temperatures are expected to surge well into the upper 70's...perhaps even into the lower 80's if there is enough sunshine and dewpoints should climb into the mid 60's.  

As the warm front lifts northward there will be showers and perhaps some embedded t'storms along the warm front.  As the morning goes on, this activity is expected to clear through CT/western MA/southern VT and southwestern NH.  Once this morning activity clears the areas, the million dollar question becomes, how much clearing do we see?  The amount of clearing we see will be extremely critical to t'storm development/strength later on in the afternoon.  

Computer model guidance is in agreement that during the mid to late afternoon, a s/w trough will slide southwest from Canada, in response we will see some weak height falls from 700-500mb along with some slight cooling at the 500mb level.  We will also see an increase in the wind fields aloft with bulk shear in the 0-6km level nearing 35-40 knots which is sufficient for organized storms along with the possibility of rotating updrafts.  With the warm front also just to the north, there will be fairly high amounts of 0-1km and 0-3km helicity values, especially in the vicinity of the warm front.  Currently, computer models have around 150 m2s2 of 0-1 km and 0-3km helicity which is noteworthy.  We will also have to watch surface winds throughout the day as if surface winds end up backing more to the south or southeast, this could vastly increase these helicity numbers, especially locally.  

As far as instability is concerned, there is some differing within the models as to how unstable the atmosphere will become.  The latest run of the GFS model is very robust, developing as much as 2000-300 J/KG of SBcape across western CT/western MA and portions of eastern NY.  How unstable the atmosphere will become all depends on two factors; 1) Amount of solar heating we receive and 2) How steep the mid-level lapse rates will be.  

As for surface heating, it's seems quite likely, there will be at least breaks in the cloud cover, especially across western sections.  Given the warmth of the low-level airmas advecting into the region, any breaks of sun would really make the temperatures shoot up.  As for lapse rates, the GFS forecast model is also fairly robust with mid-level lapse rates as 750-500mb lapse rates are just shy of 7 C/KM!  

Given all the above, scattered t'storms are expected to develop across NY/PA and then slide eastward into southern VT/western and central MA/southern NH and into western CT later in the afternoon.  Given the potential for at least weak surface-based instability, the potential for a few of these storms to become strong to severe is there.  If we are able to achieve moderate instability, we could be looking at a bit more of a stronger severe weather threat, with a higher chance for several storms to become severe.  The intensity of the storms will again depend on how much instability we see.  While the storm mode should be mainly linear, some supercells are also possible given the highly sheared environment.    

Besides the potential for torrential downpours and poor drainage flooding, the strongest storms will be capable of damaging winds in excess of 60 mph, large hail of up to 1'' in diameter (perhaps even up to 1.5'' if we see supercells) and a brief spin-up can't be ruled out either, especially if we maximize instability.  

Below is map of where the risk areas are located: