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.