First Winter Storm of 2014-2015 Set to Impact Connecticut

The first winter storm of the 2014-2015 season is only several hours away from beginning across Connecticut and the timing couldn't be any more opportune as we approach the Thanksgiving holiday.  We are also looking at a timing in which the most substantial impacts will occur from mid-to-late morning through early evening.  While this will be a fairly quick hitting storm, lasting for maybe 12 hours or perhaps a little less, the timing will cause major travel problems.  When forecasting winter storms across the state, they can often times be challenging as we often deal with the possibility of mixed precipitation such as sleet, freezing rain, and rain.  This case will be no different.

Brief Technical Look

A strong piece of shortwave energy is moving through a fairly deep digging trough as it progresses eastward across the United States.  The shortwave will be responsible for a developing area of surface low pressure just off the southeast coast.  As the area of low pressure moves north/northeastward it will strengthen:

Discussion

Computer forecast guidance are all in very strong agreement that the surface area of low pressure will pass just to the southeast of southern New England.  This storm track favors heavier precipitation impacting the state of Connecticut.

Surface temperatures across the state will be confined to about the lower to mid 30's from southeastern CT to more lower 30's elsewhere, with the exception of the Litchfield Hills where temperatures will be into the upper 20's to near 30F.  This indicates the snowfall will be more wet in nature and heavy in nature.  However, across the Litchfield Hills the snow will have a bit more fluff factor to it.

The main questions we are presented with is what happens with the mid-level temperatures, particularly, between about 8,000ft and 12,000ft.  While the surface low pressure is going to track southeast of the state, the developing area of low pressure at 700mb (roughly 10,000 ft AGL) is expected to not only track over CT but intensify as it does so:

This feature presents some trouble as with warmer air now working in aloft, this will eventually lead to a transition from snow to sleet across portions of CT.  However, it's a little uncertain as to exactly where this snow/sleet line will setup.

If the 700mb low not only develops earlier and intensifies quicker, this will mean a further surge of warmer air across the mid-levels over more of the state, meaning more locations transition over to a mix of snow and sleet which would reduce snowfall totals a bit.  However, if the 700mb low develops a bit later and doesn't intensify as quickly, the warmer air won't move much further into CT than the eastern portion of the state,  This feature will also allow for a pretty big snowfall gradient across the state in terms of accumulations.  

One point of interest is the track of the 850mb low with respect to CT.  While the system will be a quick hitter, typically something that can preclude much higher snowfall totals, computer forecast guidance indicates the 850mb low will track just to the southeast of CT,  As this occurs, this will favor a period of extremely strong low-level lift moving through the state of Connecticut meaning there will be a period of very heavy precipitation rates across the state.  This is when the majority of the snowfall accumulations will occur.  

Forecast

• Precipitation breaks out statewide between 6:00 AM and 8:00 AM
• Precipitation may begin as all rain, especially southern CT with more of a mix across the rest of the state.  
• Precipitation will quickly changeover to all snow with the exception of southeastern CT
• The heaviest of the snowfall will occur between 11:00 AM and 6:00 PM.  This is when snowfall rates will approach 2'' per hour and perhaps may even briefly approach 3'' per hour.  
• The highest snowfall totals should be confined to areas along and west of I-84.  This is where precipitation will remain mostly snow, however, some mixing is possible towards the end.  
• The storm begins to wind down between about 6:00 and 8:00 PM.

Numerous Showers and T'storms Expected Saturday, September 5th, 2014 across Southern New England

Saturday is shaping up to be quite an active day across southern New England as an area of low pressure moving through Quebec will allow for a cold front to slide through southern New England during the afternoon/evening hours.  Out ahead of the cold front the combination of surface temperatures in the mid to upper 80's with surface dewpoints in the lower 70's will yield to quite an unstable airmass across the region.  While winds aloft will be on the modest side, shear should be sufficient, especially given how unstable the atmosphere will be for showers and t'storms to develop and a few of these t'storms may become strong to severe.

The day will start off predominately dry, however, there may be a few showers around during the early morning hours.  While there will be clouds around, strong surface heating is expected to occur.  This will allow for temperatures to quickly jump and soar well into the 80's, perhaps near 90F in a few locations.  As mentioned above, with dewpoints in the low 70's, this combination will yield to quite an unstable airmass:

Computer forecast models indicating we could see CAPE values approach or exceed 2000 J/KG, suggesting quite an unstable atmosphere:


The strongest winds aloft in the mid-levels of the atmosphere are expected to lag just behind the cold front, however, computer forecast models do suggest that these winds will slowly be increasing throughout the afternoon.  While shear values at this time aren't expected to be very impressive (perhaps only upwards of 25 knots of vertical shear) the degree of instability will be enough for t'storms to develop.  There are some differences within the computer forecast models with regards to the winds aloft.  One forecast model indicates winds will begin increasing earlier as opposed to later on.  If winds aloft do increase on the quicker side and ahead of the approaching t'storms, this could lead to more in the way of severe weather potential:

The NAM computer forecast model showing area of stronger winds aloft approaching southern New England late in the afternoon, a bit slower than a few other computer forecast models:


For tomorrow we will see showers and t'storms develop in the afternoon and become rather numerous.  While numerous showers and t'storms are expected only a few of these storms are expected to become severe and pose the threat for strong to damaging winds and perhaps large hail.  If winds aloft do increase earlier and ahead of the approaching storms than the risk of strong to damaging winds and hail would increase.  A few rotating storms can't be ruled out either but this all depends on the strength and intensity of the lower-level wind fields.  Besides the threat for strong to damaging winds and hail, a very moist atmosphere will lead to torrential downpours and with this we will see a risk for localized areas of flash flooding, especially for the typical flood prone areas.

Stormy Wednesday On Tap Across Southern New England

A strengthening storm system just west of southern New England is slowly allowing a warm front to push northward.  Currently, the warm front is draped across the mid-Atlantic region, however, that front will continue sliding north towards southern New England during the overnight and early morning hours.  As the warm front approaches moisture will begin to rapidly increase and this will be accompanied by periods of extremely heavy rainfall overnight tonight and through the morning hours tomorrow.  Pretty much everyone across southern New England will see at least one period of heavy rainfall between tonight and tomorrow afternoon.  In fact, rainfall amounts on average should be between 1-2'' with isolated rainfall amounts as high as 3-4'' when all said and done.  This will lead to pockets of flash flooding.  What's the cause for the heavy rainfall?

A very strong an anomalous low-level jet and anomalously high precipitable water values will be the drivers behind very heavy rainfall:

18z GFS model showing a 40+ knot low-level jet working into southern New England by 5:00 AM Wednesday morning:

18z GFS model showing precipitable water values approaching and even slightly exceeding 2'' by 5:00 AM across portions of southern New England:

While the focus for tomorrow will mainly be on the periods of heavy rainfall, another focus, albeit perhaps to a lesser extend will be the potential for a few strong to perhaps severe t'storms.  This potential all hinges upon how much sunshine occurs and how much instability develops once the back end of the heavier rainfall moves through.  If just enough instability can develop we will have to pay close attention to the development of t'storms.

Winds aloft will be quite strong thanks to the strengthening storm system off to the west and with the warm front in the vicinity of the region this will lead to winds turning with height, especially in the lowest 3km of the atmosphere.  If t'storms are able to develop and become strong enough to utilize these winds aloft, they will have the potential to produce strong to damaging wind gusts and perhaps the potential for a brief tornado.

While these setups don't always materialize and generate strong to severe t'storms or tornadoes, these type of setups have produced strong to severe t'storms and tornadoes in the past, including 3 weeks ago.  It's all dependent on how much instability can be generated and this typically is not known the night before...it's known more the next morning.

All in all we are looking at widespread heavy rainfall totals (although it will not be raining the entire time) with the potential for isolated areas of flash flooding.  Rainfall totals on average should be 1-2'' with isolated amounts as high as 3-4''.  While heavy rains are the main threat we can't rule out a few strong to severe t'storms capable of strong to damaging wind gusts or even a brief tornado.

Threat For Strong/Severe T'storms to Continue Tuesday...Strong to Damaging Winds, Large Hail, Isolated Tornadoes, Flash Flooding

On Sunday I had discussed that today (Monday) we would have the threat for strong to severe t'storms across southern New England with the hazards of strong to damaging winds, hail, flash flooding and perhaps even the threat for a few isolated tornadoes.  Well at this time of this writing (7:40 PM EDT) we have seen nothing of this variety occur.  However, there were a few strong t'storms which moved through Fairfield County in southwestern CT.  We were not able to realize this potential because one major ingredient was missing, a decent amount of surface-based instability.

One big question coming into today was how much instability we would be able to generate give the likelihood of substantial cloud cover.  While the day did begin with numerous breaks of sunshine, as the sun rose and began to heat the surface, clouds quickly began to develop.  These clouds were able to develop so quickly and rapidly because of the very tropical airmass we have in place, characterized by dewpoints into the lower 70's and an uncapped airmass.  An uncapped airmass virtually means that water vapor can easily reach the condensation temperature, therefore, beginning the cloud development phase.

There was also a boundary in place which was draped right over southern CT.  This boundary was leftover from the showers and t'storms which moved through portions of CT last evening.  This leftover boundary was just another focus for lift which further aided in quick development of cloud cover.

The vast development of clouds worked to keep surface heating to a minimum which resulted in minimal instability from developing.  While we had and have the wind shear in place for t'storms to produce strong to damaging winds and even an isolated tornado, we did not have the instability in place to generate t'storms and strong enough t'storms to tap into these stronger winds aloft and utilize them.

With this said as we move through the overnight hours we still run the risk for some isolated t'storms and if we do see any t'storms they would pose a threat for severe weather.  However, the #1 threat tonight will be flash flooding as any shower/t'storm will produce excessive rainfall.

This now brings us to Tuesday...

On Tuesday we will be dealing with the same potential weather wise which we were faced with on Monday.  However, all latest indications are pointing in the direction that the potential on Tuesday will be much greater than on Monday and we could also be dealing with the possibility for widespread strong to severe t'storms, especially across western and central southern New England.

Some computer forecast models are hinting that we will see more in the way of surface heating tomorrow as some mid-level dry air works into the region.  With this increased surface heating, computer forecast models also generate much more instability.  Below is the 15z SREF mean for mixed-layer cape tomorrow centered around Windsor Locks, CT.  The mean is around 1000-1500 J/KG of mixed-layer cape:

In addition to the potential of stronger instability tomorrow, wind fields aloft will also be a tad stronger than what we had in place on Monday.  In fact, some guidance develops a low-level jet at 850mb with winds exceeding 35-40 knots out of the southwest while surface winds may hang more southerly or even southeasterly:

Winds of this magnitude are very strong and when coupled with strong instability will mean any thunderstorm would quickly become strong to severe posing a risk for strong to damaging winds.  With the potential for surface winds to be south to southeast and 850mb winds backing to the southwest this will create a great deal of "spin" within the lowest levels of the atmosphere.  If adequate instability does develop not only will t'storms have the potential to produce strong to damaging winds but any individual cells "discrete cells", could utilize this atmospheric spin and become what are called supercell t'storms.  These t'storms would have the potential to produce strong to damaging winds, large hail, and even tornadoes.

All in all the potential on Tuesday for strong to severe t'storms appears higher than Monday's potential.  We will also be looking at the potential for widespread strong to severe t'storms which means more of the region has a higher risk of getting nastier storms.  The threats tomorrow will be strong to damaging winds, large hail, isolated tornadoes, and especially flash flooding.

As was the case Monday, the extent of the threat and potential hinges on how much sunshine and surface heating we are able to generate on Tuesday.  The more surface heating, the higher the surface temperatures, the greater the likelihood for strong to severe t'storms and the higher the tornado potential, especially if we get discrete cells to develop.

Strong to Severe T'storms Possible on Monday. Strong to Damaging Winds, Hail, Isolated Tornadoes, Flash Flooding All Possible.

While at this time the potential for a widespread significant severe weather outbreak appears on the low side the potential will exist for thunderstorms tomorrow and some of these thunderstorms will have the potential to become strong to severe posing a threat for strong to damaging winds, hail, flash flooding, and even the potential for isolated tornadoes.

Computer forecast models are generating some modest instability with surface-based cape values approaching 1500 J/KG and perhaps even upwards of 2000 J/KG depending on the degree of surface heating that we see occur:

Projected SBcape values across much of southern New England of up to 1500 J/KG

Computer forecast models also generate upwards of 750-1000 J/KG of mixed-layer cape values across the region:

Typically when becoming "excited" over the possibility for strong to severe t'storms we would like to see Cape values higher than what is forecasted.  While 1000 J/KG is usually a decent starting point, you want to start seeing values (especially mixed-layer cape) approach and exceed 1500 J/KG.  However, in the case of tomorrow we will be dealing with very strong winds throughout the atmospheric column and this will compensate for the lack of stronger instability.

Below is a point-and-click forecast sounding from northern CT.  Looking at the wind barbs on the right hand side we can see some pretty strong wind shear will reside over the region.  What really jumps out is the 30-35 knots of wind shear in the 850-925mb level.  These values are pretty high, especially when combined with the projected Cape values.  These values within this level will provide the threat for any thunderstorms to produce strong to damaging winds.

What also jumps out is the "backed" winds at the surface.  Notice how winds at the surface and just above go from SE to SW up around 1000mb.  This is pretty significant as this would lead to high amounts of helicity (a measure of atmospheric "spin"):

 
Looking at a hodograph we can see the backed wind profile.  Notice the long curvature in the hodograph.  This tells us that the potential will exist for supercells tomorrow along with the threat for isolated tornadoes so we will have to closely monitor any thunderstorm that develops:

The atmosphere will also be full of moisture, characterized by dewpoints into the lower 70's across the region.  This will lead to precipitable water values approaching 2.0'' with is quite high.  This indicates that any showers and t'storms will produce torrential downpours and this will lead to the threat for flash flooding, especially the typical flood prone areas.

For tomorrow we will have to closely monitor how much sunshine we see and how much instability develops.  The wind profiles aloft will be more than favorable for strong to severe t'storms with the threat for strong to damaging winds, hail, and even a few tornadoes.  It's all a matter of how much instability we can manage to develop.

Looking ahead into Tuesday we could once again see a threat for strong to severe t'storms with all the same hazards.

High Humidity Values to Return Along with Threat For Showers And T'storms

Humidity values will slowly begin to increase during the day on Sunday across southern New England as a warm front slowly pushes through the region.  As the warm front pushes through dewpoints are expected to climb into at least the lower 70's and that will make it feel very uncomfortable, especially when coupled with temperatures well in to the 80's.  The passage of the warm front will also set the stage for what looks to be an active 3-4 days of weather.  

Over the past several days, computer forecast models have hinted at a very anomalous (for July standards) trough developing and digging into the upper Mid-west/Great Lakes region in response to some very strong ridging developing across the western-tier of the United States.  

The deep digging trough will also for ridging to build along the eastern seaboard and western Atlantic Ocean.  This will work to lift a warm front northward through southern New England as the wind flow at the surface and lower levels of the atmosphere to become more southwesterly.  This will work to draw in ample moisture from the Gulf of Mexico.  Moisture from the Atlantic Ocean will also become en trained into the flow.  This will lead to the very high dewpoints described above.  

Sunday

Sunday will begin with drier air still in place, however, as the warm front approaches the dewpoints and humidity levels will slowly begin to rise from south to north across the region.  The one question at hand right now is, how quickly does the warm front push through?  This has some implications as some computer forecast models are more aggressive with how quickly the front pushes through and the result is a push of instability working into the region.  This would allow the potential for a few showers and t'storms to either develop across the region or move into western sections of southern New England late afternoon or during the evening.  Computer forecast models also show some weak energy moving through aloft which could be a focus for some development.  If this scenario were to unfold we would have to watch as any storm could become strong or perhaps severe.  However, this all depends on how quickly the front pushes through and how unstable the atmosphere were to become. 

Monday

By Monday everyone is southern New England will be under the influence of some very uncomfortable humidity values as dewpoints will be well into the lower 70's.  While last week we saw temperatures well into the upper 80's to even some 90's and dew points near 70F, this go around temperatures may not be so high as cloud cover is expected to hold temperatures back...more lower to mid 80's.  

A cold front will also be draped well west of southern New England, however, a pre-frontal trough is expected to develop across central NY and extend southward into PA.  This pre-frontal trough will be a focus for the development of showers and t'storms across central/eastern NY and PA.  The question here is will any portion of southern New England be close enough to the pre-frontal to see any development or will the pre-frontal be positioned east enough to where we see numerous development and activity across the region?  

On Monday computer forecast models are in agreement that with a combination of high dewpoints and some surface heating, this will lead to a fairly unstable atmosphere which would provide fuel for t'storms to develop.  Computer forecast models also have some pretty strong wind shear aloft thanks to the strong area of low pressure associated with the trough:

 

While the strongest 500mb winds will be confined to the Ohio Valley and western NY (closer to the cold front and trough), 500mb winds across southern New England will be in the 30-40 knot range with values increasing closer to 50 knots later in the day.  This will lead to vertical wind shear values of 35-45 knots across the region which is more than favorable for any thunderstorms that develop to become much better organized and for updrafts to sustain themselves.  

With high dewpoints and rich low-level moisture in place, this will lead to very high precipitable water values across the region, on the order of 2'':

On Monday, the combination of an unstable airmass and strong shear aloft ahead of a pre-frontal trough will lead to the development of showers and t'storms.  While the bulk of the activity may be confined to PA/NY, western.central sections of southern New England will also run the risk for activity, however, whether the threat begins early afternoon or later afternoon depends on timing and positioning of the pre-frontal trough.  The atmosphere will also be conducive for some of the t'storms to become strong to severe.  This would make a risk for strong to damaging winds and perhaps some hail.  We would also have to watch out for some rotating t'storms in any discrete t'storms.  

With the high precipitable water values, this means that any showers and t'storms will produce torrential downpours.  This will lead to the threat for flash flooding, especially across the typical flood prone areas and for any areas which are hit by multiple thunderstorms.  In fact, flash flooding will be a threat over the next several days.  

Tuesday

The forecast on Tuesday is much more uncertain as there are major questions with regards to timing and positioning of the cold front.  Computer forecast models also suggest a boundary may move through the region during the morning hours allowing for drier air to work into the lower levels of the atmosphere, especially across western/central southern New England.  This would mean the highest threat for t'storms on Tuesday could be across eastern RI and eastern MA.  Like Monday, any t'storms would have the potential to become strong to severe with risks for strong to damaging winds and some hail.  Flash flooding would be the #1 concern.  

All in all beginning tomorrow night lasting through Tuesday (perhaps lasting into Wednesday) we will run the risks for showers and t'storms to develop at any point during the morning, afternoon, evening, or overnight hours.  The threat for stronger or perhaps severe t'storms would be confined to afternoon/early evening hours as this is the times the atmosphere would be the most unstable.  Given the high precipitable water values mentioned above, any showers and t'storms will produce very torrential downpours and alot of rain would fall in a very short amount of time so flash flooding potential is on the higher side.  Some areas would be able to pick up a very quick 2''+ of rainfall very quickly.  

During this timeframe some areas will be hit much harder than others, as is always the case when you're dealing with thunderstorms so some areas may end up seeing as much as 3-4''+ of rain while other areas may be lucky to even see 0.50'' to 1'' of rainfall.  

We will continue to monitor the computer forecast models as new data becomes available and will continue to monitor the potential for strong to severe t'storms as well as flash flooding potential.  

Tuesday's Southern New England Thunderstorm Potential

A cold front which was responsible for producing numerous strong to severe thunderstorms across southern New England on Thursday has cleared the area and brought in much drier air to the region.  The departure of Hurricane Arthur to the east of southern New England has also worked to usher in drier air for the weekend.  However, as we begin the start of the work week we will see an increase in both the heat and humidity as another cold front approaching from the west will allow for surface winds to become more southwesterly allowing for hotter and more humid air to return.  We will feel these effects beginning on Monday.

On Tuesday the cold front will continue sliding east towards southern New England.  With the cold front approaching and a hot and humid airmass in place, this will set the stage once again for the potential for showers and thunderstorms across the region and a few of these storms will have the potential to become strong to severe.

In Depth Meteorological Analysis

Our current computer forecast models are developing a moderately unstable atmosphere on Tuesday with SBcape values potentially exceeding 1500-2000 J/KG, MLcape values approaching 1500 J/KG, and LI values approaching -4C.  All these values indicate an airmass which will be fairly unstable and indicate an airmass which will be supportive of not only thunderstorms but the possibility for a few strong to severe thunderstorms.

GFS forecast model showing SBcape values for 5:00 PM EDT Tuesday afternoon.  GFS indicates anywhere from 1000-1500 J/KG of SBcape.  The NAM forecast model has as much as 1500-2000 J/KG of SBcape.

Computer forecast models are also indicating more than adequate wind shear aloft with 500mb winds in the 40-50 knot range, 700mb winds in the 30-40 knot range, and 850mb winds in the 15-25 knot range.  500mb winds in the 30-40 knot range and 700mb winds in the 30-40 knot range should yield 0-6km shear values in the 30-40 knot range.  These values are solid enough to support organized thunderstorms.

GFS forecast model showing 500mb wind speeds for 5:00 PM EDT Tuesday afternoon.  The GFS has 30-40 knots across western portions of southern New England with as much as 40-50 knots just west over NY and PA.

At this point in time a major severe weather outbreak is not anticipated, however, thunderstorms can be expected and a few of these storms would have the potential to become strong to severe.  In addition to the main threats of torrential downpours and cloud-to-ground lightning, the strongest of storms would be capable of producing strong to damaging winds and perhaps some hail.

The extent of the threat and potential will all depend on the actual timing of the cold front, and extent of any cloud cover which could not only limit the amount of surface heating we see but could limit the degree of instability.