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.