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.

Threat For Thunderstorms To Increase Across Southern New England Over The Next Few Days. Potential For A Few Strong To Severe Thunderstorms As Well

A rather quiet and stagnant weather pattern which has been in place for quite some time across southern New England is about to come to and end as we near the 4th of July holiday as an increase in heat/humidity and an approaching cold front will set the stage for the possibility of thunderstorms each day from Wednesday through Friday.  Tropical Storm Arthur, which is currently located just east of Florida will also play a factor into our weather as well.  Lot's to cover here so we'll break everything down on a day-to-day basis.

Wednesday

On Wednesday much of the thunderstorm activity is expected to be west of southern New England, however, we can't rule out some thunderstorms working into western areas of MA and CT late in the afternoon/evening or even having a few thunderstorms develop.  The atmosphere across southern New England will be be more than favorable for thunderstorms as the combination of surface temperatures (away from the immediate coast) nearing or even exceeding the 90F mark, surface dewpoints into the lower 70's, mid-level lapse rates around 6.5 C/KM, and cooling mid-level temperatures will lead to a highly unstable airmass across southern New England.  Shear values across southern New England are expected to be rather modest with 850mb winds (~5000 ft AGL) around 20-25 knots and 500mb winds (~18,000 ft ASL) around 25-30 knots.  These values are not very impressive, however, are adequate given presence of a highly unstable atmosphere.  Computer forecast models do indicate that towards evening these shear values may begin to increase across western portions of MA and CT.

(18z/01 GFS graphic displaying projected surface-based cape values around 5:00 PM tomorrow.  Reds and Pinks indicate highly unstable atmosphere)

While the atmosphere across southern New England is certainly supportive of thunderstorms there are a few limiting factors which could prevent t'storms from developing.  1) An atmospheric cap.  700mb temperatures (~10,000 ft AGL) are expected to be around 8-10C and may even warm to as much as 11-12C.  This pocket of rather warm air may be enough to prevent thunderstorms from developing.  2) Lack of a lifting mechanism.  All the best lift and forcing will be back west across New York as they will be closer to the cold front/trough sliding east and will be closer to the stronger winds aloft.  There are some indications, however, that a weak pre-frontal trough will develop and slide across New York and approach southern New England late tomorrow afternoon or early evening.  This will have to be watched and this feature could provide enough of a forcing mechanism to break the cap and allow for some thunderstorms to develop.  The best chance for thunderstorms both during the day tomorrow and tomorrow night will be across western and central southern New England.  Given the presence of high instability and modest shear any thunderstorm's that develop will have the potential to be strong to severe posing a threat for strong to damaging winds along with large hail.  Torrential downpours and lots of lightning are to be expected with any storm and with torrential downpours the threat for localized flash flooding may exist.    

Thursday

Thursday looks to be the more active day across southern New England in terms of thunderstorm activity which is expected to be more widespread as the cold front/trough which will be the focus for thunderstorms tomorrow to our west approaches.  There are some questions though on Thursday with regards to how unstable the atmosphere will be as cloud debris from thunderstorms tomorrow to our west could cut down on the degree of surface heating.  With this said, the latest data from the computer forecast models do indicate that we will see enough surface heating, and combined with dewpoints once again in the lower 70's will yield a fairly unstable atmosphere, although perhaps not to the degree we will see tomorrow given questions regarding how much surface heating we see and mid-level lapse rates may only be around 6 C/KM as opposed to 6.5-7 C/KM.  While instability may be a bit less, winds aloft will be stronger across the region.  The presence of stronger forcing and lift from the approaching cold front/trough and the presence of a fairly unstable airmass will set the stage for numerous showers and thunderstorms across southern New England.  Like tomorrow, the potential on Thursday will exist for some of these thunderstorms to become strong to severe posing a threat for strong to damaging winds and perhaps even some hail.  Torrential downpours and lots of lightning are to be expected with any storm along with the threat for localized flash flooding.

(18z/01 GFS graphic displaying projected surface-based cape values around 5:00 PM Thursday.  Reds and Pinks indicate highly unstable atmosphere)

Friday

Moving to the 4th of July this is where things become much more complicated.  At this time, what is now Tropical Storm Arthur, will be working up the east coast and may even potentially be a weak Hurricane.  There is a great deal of uncertainty as to the exact track Arthur will take as this will really all depend on how quickly the cold front moves through and where the front is situated as Arthur continues working up the coast.  Indications right now are that Arthur will not make it up the coast in time to have direct impacts on much of southern New England, with perhaps the exception of portions of RI and far SE MA.  The indications are the front will slide far enough east through southern New England to where it captures Arthur and pushes Arthur off to the east.  

Regardless, there could be a period late Thursday through Friday that there could be a period of extremely heavy rainfall along with some thunderstorms.  Where this axis of rain sets up or traverses over is very difficult at this time as it will all depend on the placement of certain features such as the cold front as we just mentioned.  Given Friday is the 4th of July, and many people have tons of outdoor plans it is a bit inconvenient to be so uncertain but that's typically the case when dealing with such delicate situations where the exact placement and timing of key features is extremely critical and typically when dealing with such cases it can be nearly impossible to know until as little as 24-hours out.  As new data rolls out the situation should begin to become a bit more clear.   

Quick Moving Storm To Bring Accumulating Snows To Much of Southern New England on Tuesday

The active weather pattern continues across southern New England as we are dealing with yet another storm which is set to impact the region during the late morning on Tuesday and lasting into the late evening hours.  While this storm does not look to be as major as significant as the past few systems we've had, this system does look to drop for plowable snows across the region.  The timing of this system will also coincide with morning rush hour traffic, afternoon rush hour traffic, and evening rush hour traffic and with this the largest impacts from this system will be on travel and commute.  

Our driver for the system tomorrow is currently this very strong piece of energy at 500mb (~18,000ft AGL) which is currently located out in the Ohio Valley region.  This very potent piece of energy is associated with a weak area of surface low pressure and while the surface low may be weak, the strength of the energy is really driving this system.  In fact, portions of  IL/IA/MO have had some pretty impressive snowfall rates along with thundersnow!  This energy is quickly moving through the fast moving mid-level jet stream and will be approaching our region tomorrow morning:

As stated above, this potent piece of energy is associated with an area of surface low pressure and this surface low will track through New England tomorrow and as it does so we will begin to see snow break out.  Also, as the surface low moves off the coast, it is actually expected to strengthen some.  This will lead to a jackpot of higher totals across portions of interior northeastern MA, southeastern NH extending into coastal sections of ME:

While this is expected to be a quick hitting system with precipitation lasting anywhere between 6 to at most 10 hours, there will be some pretty intense lift associated with the system.  What this will do is 1) lead to pretty intense precipitation (snowfall) rates and 2) For areas that may be a touch warm as far as surface temperatures go, this will help to offset that.  Unfortunately for areas on the outer Cape, the degree of warmth towards the surface may yield to a changeover to rain:

With such intense lift around, we could be looking at snowfall rates approaching 2'' per hour and perhaps even as high as 2.5'' per hour at times tomorrow during peak intensity.  While peak intensity may only be a few hours, this means snow will pile quickly.

Given what we looked at above, we are probably looking at two areas of higher snowfall amounts.  One area being in the NW Hills of CT extending into the Berkshires and another area being the NE Hills of CT, the Worcester Hills, and interior NE MA into portions of NH and into coastal ME.

As stated above, the major impacts from this system will be the impacts on travel as some of the heaviest snowfall rates will be occurring during times of higher traffic activity, especially across eastern MA.  Further west into CT the snow should have slowed down by 4-5 PM so at this point in time the highest travel impacts will be occurring out across eastern MA and back into portions of RI.

Below is what I'm currently expecting in terms of snowfall accumulations:

Strong Coastal Storm To Bring More Accumulating Snowfall To Southern New England Along With Blizzard Conditions to Eastern Southern New England

Days after a rather intense coastal storm dumped major snowfall accumulations across a large chunk of southern New England another system is set to impact the region Saturday night and bring another round of snowfall accumulations and unlike this past storm, we will be dealing with an all snow event.

Our main area is focus is this piece of energy which is digging into the southeast.  As this energy continues to dig into the southeast, the associated trough will continue to dig and this will lead to further intensification of the energy.  The energy will then work off the Carolina coast, and while doing so, it will draw in moisture from the Atlantic.  The system will then continue working northeast, off the US coast where the system is expected to undergo rapid cyclogenesis (strengthening).  The system is expected to track far enough off the coast to support a cold enough thermal profile for all snow but track close enough to allow for heavy precipitation to wrap around into southern New England:

Over the past day or so, computer forecast guidance has really gone...wild with this system just off the coast.  In fact, we are looking at yet another major (widespread 8-12'') to significant (12''+) snowstorm across the region with the highest snowfall totals perhaps confined to eastern MA, including the Cape.

The westward extent of the highest snowfall totals is all going to depend on the exact track.  Right now there is some disagreement as to the exact track of the storm and that does make for some shaky confidence with regards to how far west the heaviest snow will track.

Besides a several hour period of moderate to heavy snowfall winds will also be of concern, especially across eastern MA where blizzard conditions are likely as winds will be sustained 25-35 mph at times with gusts perhaps as high as 60 mph.  The exact track of the storm will also determine how far west we see these strong winds occur...something else to keep in mind.

Computer forecast models also briefly work in a pocket of instability in the mid-levels of the atmosphere.  This instability pocket riding overhead, coupled with very intense lift from the system could lead to some embedded pockets of convection leading to thundersnow.

Snow is expected to break out across western portions of southern New England as early as 3-4 PM and this could actually happen as early as 2 PM.  Once the snow begins, it will rapidly increase in intensity and the height of the heaviest snowfall is expected to occur between about 5 PM and 1 AM (lasting until 3-4 PM for eastern sections of southern New England).

Below is what to currently expect as far as snowfall totals are concerned:

Major to Significant Winter Storm To Impact southern New England Thursday into early Friday

Over the past few days confidence has vastly increased that a storm system developing across the southeastern United States will work up and track close enough to the coast to impact southern New England.  With several sets of computer forecast guidance suggesting a rather strong system, these impacts will be rather significant as portions of the region will see major (8-12'') to significant (12''+) snowfall accumulations.  Given the expected track of the system, we will also have to contend with plain rain, sleet, and perhaps some freezing rain.  This system will also be accompanied by some strong winds as well.

While confidence is extremely high of a storm track close enough to the coast to impact the entire region, there is still a great deal of uncertainty with regards to the exact track of this storm system.  This is ultimately going to be absolutely huge when it comes to determining who remains all snow, who mixes or changes to rain, and where the axis of the highest snowfall totals will occur.  Unlike some of the past storms we have had this winter, we don't have a high pressure system to our north to keep colder air locked in place across the region and with the models strengthening this system very rapidly off the Carolina coast, this will allow for warmer air to try and work into portions of southern New England.  A track difference west/east of as little as 30-50 miles will be the difference between something like 3-6'' of snow and potentially as much as 12''!

What we also have to watch is the track of the 700mb low.  This is extremely important for two different reasons.  1) The highest snowfall totals along with the heaviest banding of snowfall occurs just to the northwest of the low pressure track.  This is where lift is typically the strongest along with very strong forcing.  2) Areas virtually in the direct path of the low track and just south and east typically end up being in the dry slot.  This is where drier air from aloft descends downward or when drier air gets wrapped into the system.  The drier air eats at the precipitation and the moisture and areas within this zone see very little to no precipitation and something like this can vastly affect snowfall totals.

Given how close to the coast this system will track and with warmer air that will be working into portions of the region, this will create a tight baroclinic zone (greater temperature difference over a small area).  This also works to vastly increase lift and forcing, just to the west of this zone and where this zone sets up is where the heaviest snows and higher totals will occur.

With a potent storm system which will continue to strengthen as it works up the coast this will also increase the pressure gradient across the region and a period of strong winds will be introduced.  Winds could gust as high as 35-45 mph region wide, and especially across the coast.  This will lead to isolated to scattered power outages along with perhaps blizzard conditions in some spots.

The first impacts from the storm will begin very early Thursday morning, right around dawn or just a bit after with everyone starting off as snow.  The timing of the storm will likely lead to a great deal of school cancellations throughout the region.  For those heading to work the travel commute will deteriorate very quickly and expect numerous delays.

The height of the storm will occur between about 10 AM and 10 PM.  During this time snowfall rates may approach 3''/HR in the areas where the heaviest banding occurs.  Winds will also increase as well as the afternoon goes on with gusts approaching 35-45 mph, especially towards the coast.  For areas south and east snow the snow will eventually transition over to sleet and then rain thus the lesser snow totals.

Given everything mentioned above, this is what I'm currently thinking as far as snowfall totals go:

Storm System to Bring Major Snowfall Accumulations to Much of southern New England Wednesday, February 5th, 2014

The active start to February continues on Wednesday as yet another storm system is expected to impact southern New England.  This system is forecasted to be a bit stronger than what we just dealt with as the system will have a bit more organized structure with much stronger lift and a further northward track will mean increased moisture as well.  All these ingredients will come together to bring the potential for major (8-12'') snowfall accumulations across a chunk of the region.  We can't even rule out some areas perhaps picking up over a foot of snow as well.

With a storm track which is expected to be virtually just south of CT moving across the extreme outer portion of the Cape this is going to allow for some warmer air to work northward across southern CT, southern RI, and far southeast MA (including the Cape) and this will reduce snowfall totals across these parts thanks to more in the way of mixing.  We also may see some sleet mixing in as far north as the CT/MA border for a period of time.  A storm track like this as well means an increased thermal (temperature gradient) in the mid/lower levels of the atmosphere which also means stronger forcing and lift over the region:

  When looking at how much moisture is associated with this system, computer forecast models are all in agreement there will be loads and loads of moisture associated with it.  In fact, the consensus between the NAM/GFS/Euro models all have about 0.50'' to 1'' of liquid falling in a 12-hour period!  Forecast soundings also show the snowgrowth zone will be located between about 15,000-20,000ft up which is what you want to see and the soundings do show some good lift within this zone at times.

While all of these are certainly positives for a pretty sizable snow event, there are a few negatives as well which may reduce the potential for a much higher snowfall accumulations.  Computer forecast models really begin to dry out the mid-levels of the atmosphere rather quickly during the late morning into the early afternoon hours as dry air races in from the west and southwest.  This dry air is not only a precipitation shield eater but it vastly reduces snowgrowth and ratios.  There are also some signs of some subsidence zones ( caused by "negative lift") which will mean there may be some spots where virtually no snow is falling (or the intensity is extremely light) while only a few miles away it's snowing quite heavily:

Given what we are presented with right now this is what I am currently thinking for snowfall accumulations.  The highest snowfall totals will be confined across far northern sections where dry air will be less of an issue and snowfall ratios will be a bit higher.  Interior northeastern MA back to Worcester will also be another jackpot area as dry air will be less of a factor here as well and guidance suggests the best banding may pivot over this area.

Snowfall Forecast Update: Forecast Still on Track

Reviewing data as it has become available since yesterday's forecast, at this time there will be no changes from the previous forecast and everything still appears to be on track.  Computer forecast guidance has remained quite consistent with how this system will evolve with the exception of a few minor changes but I don't think these changes will have substantial impacts on snowfall totals.  If anything, perhaps far western areas of the forecast might receive a bit less in the way of snow but at this time I see no need to make adjustments.  What I'm also watching out for is the possibility of an inverted trough developing late tomorrow morning which could prolonged periods of heavier snowfall across far eastern MA/Cape areas...if this is the case the potential would exist for more snowfall than currently forecasted for these areas.

Forecast soundings indicate we will be dealing with very good snowgrowth which will lead to beautiful dendrites...nice big fluffy snowflakes which will accumulate very quickly.  We will also be dealing with some incredible snowfall ratios which could perhaps exceed 15:1 or even 20:1.  This will lead to the higher totals we will see.

What to expect as we move through today:

• Snow will begin falling across the region from SW to NE as the afternoon progresses.  While snow will remain predominately light across the region, areas across the CT/RI coast may experience bursts of moderate to heavy snow as models have some pretty strong lift across these areas.  As the afternoon progresses, these bands of heavier snowfall will work northward. 

• Given very cold ground and cold temperatures, snow will begin sticking immediately, this will make more very slick roadways and hazardous travel. 

• The heaviest of the snowfall will occur between the hours of 7 PM-7AM tomorrow for much of the region with the exception of eastern MA/Cape where heavier snow may linger past 8-10 AM.  

• During the height of the storm winds will begin to increase as well with gusts potentially up to 35 mph across the region.  Across far eastern MA/Cape area, winds could gust to near 50 mph with sustained winds 20-30 mph.  This will not only lead to blizzard conditions but dangerous wind chill values as well.

As of now, no changes have been made from yesterday's snowfall map.  Any adjustments will be made as necessary as the storm is occurring and we start to see where the strongest forcing/lift will reside over and where exactly the heaviest banding sits over.  Typically you have to wait until the storm is occurring to determine this as these are what we call "mesoscale" factors.  This means they occur on a very small scale and our computer models have a very difficult time handling these features well.

Coastal Storm to Bring Accumulating Snowfall to southern New England Tuesday into Wednesday

Confidence in a coastal storm bringing accumulating snowfall continues to increase as computer model guidance over the past few days has come into further agreement on a further northward track.  The time of impact will be from late tomorrow morning/early afternoon lasting into mid to late morning Wednesday with the height of the storm occurring late evening through the overnight period.

Shortwave energy will be entering the United States from the northern Plains during the overnight period, rotating around a piece of the polar vortex which is situated north of Maine in Canada.  As this energy digs into the United States it is expected to strengthen and the response will be for surface low pressure development.

Throughout the day tomorrow this piece of energy is expected to strengthen as it continues digging into and through the Ohio Valley into the mid-Atlantic states and as it does show the energy/trough will become negatively tilted.  This will not only allow for further intensification (cyclogenesis) of the low pressure system but this will help to keep the track close enough to the coast for impacts.  We will also see the system slow down a bit as it passes just off the New England coast.

Late evening tomorrow and through the overnight period, as the system works northward just off the coast, it will undergo rapid cyclogenesis and we will see a rather intense and mature storm system develop.  This is evident as computer forecast models are developing closed off circulations at the 925/850/700mb levels.  When this occurs we will begin to see a very intense band of extremely heavy snowfall develop and where this band traverses is where the highest snowfall totals will occur.  At this time this appears to be across portions of SE CT/RI and into far SE MA which includes the Cape.

What's going to happen is light snows may begin to break out as early as late morning/early afternoon with periods of moderate bursts of snow during this time.  While a few inches may drop during the afternoon hours the heaviest of the snow is expected late evening/overnight.

While we have a rather cold airmass in place which is a positive for higher snowfall ratios, forecast model soundings are also highly suggestive of very favorable snowfall growth and snowfall rates.  This will lead to extremely fluffy snowfall composed of rather big snowflakes.  This will also be a positive for higher snowfall totals.

With an intensifying storm system just off the coast this will lead to a tightened pressure gradient which means increasing winds, especially across eastern MA.  This will lead to blizzard conditions across eastern MA and this can't even be ruled out for areas further west.  Given such an intense system with such strong lift and forcing, some thundersnow is also possible.

At the height of the storm, snowfall rates may approach as much as 2-3'' per hour, especially considering the snow will be so fluffy.  The highest snowfall totals should be confined to eastern southern New England as this is where the consensus is for the higher precipitation totals.

Below is what I'm expecting right now:

What is the Polar Vortex?

The recent blasts of very cold Arctic air which has invaded a large chunk of the United States, giving areas some of the coldest weather they've experienced in years has driven the media to discover the term "Polar Vortex" and run absolutely wild with the term.  With that,  I thought this would be a great time to dedicate a post about the polar vortex and explain exactly what the polar vortex is as well as discuss it's role in meteorology and in weather forecasting.  While this post will get a bit technical, everything will be explained as clear as possible and as simple as possible.  

What is the Polar Vortex?

As defined by the American Meteorological Society (AMS), the Polar Vortex is, The Planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere.  In other words, the polar vortex is an area of low pressure which is centered over the polar regions of the Northern Hemisphere and the polar vortex ALWAYS exists.  In the Northern Hemisphere, the polar vortex typically has two main centers of circulation with one typically located near Baffin Island and the other center of circulation typically located over northeast Siberia.  

The polar vortex (which remember is an area of low pressure with two circulation centers) varies in intensity/strength throughout the course of the year and the location of the two main centers also changes throughout the course of the year.  The polar vortex is typically more "active", meaning it's typically a more dominant signal, during the Northern Hemisphere winter as this is when the temperature gradients between the Equator and the North/South Poles is the greatest.  This leads to stronger wind gradients along with stronger pressure gradients.  

How is the Polar Vortex Measured?

We measure the strength of the polar vortex by the phase and strength of the Arctic Oscillation (AO).  The National Climatic Data Center (NCDC) defines the AO as, a large scale mode of climate variability, also referred to as the North Hemisphere annular mode.  The AO is a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55N latitude.  The Arctic Oscillation is measured in three different phases; negative, neutral, and positive.  Each of these three phases define the state of the AO and each of these phases influence the weather pattern not only across North America but other areas of the world as well.  Below we will take a brief look into each phase and what they mean in terms of weather.  

• Negative AO phase -  When atmospheric pressures up at 500mb (about 18,000ft AGL) associated with the Polar Vortex circulation are weaker than normal (meaning HIGHER atmospheric pressure values compared to the average) and zonal wind circulation around the vortex are weaker than normal, this indicates a negative phase of the AO.  During this phase, it becomes much easier for colder Arctic air to invade the northern tier of the United States.  Below are the monthly averaged 500mb heights/zonal winds for the Top 10 most negative AO January's along with the temperature anomalies for the month of January for each of the top most negative AO January's:

 

• Neutral AO Phase - The phase of the AO is termed neutral when there is no clear phase signal.  Typically, you're dealing with height anomalies in the Arctic regions which are much more neutral (not much above or below normal).  The graphic below displays 500mb height anomalies for the 6 January's with the three highest negative values and the three lowest positive values.  Looking at the height anomalies over the Arctic regions we see more neutral heights: 

• Positive AO Phase - The positive phase of the AO is a mere opposite of the negative phase.  500mb heights associated with the positive phase are lower than normal and the zonal wind circulation around the vortex is stronger than normal, meaning stronger westerlies.  These stronger westerly winds work to keep colder air bottled and locked up across far northern latitudes and increases the likelihood of above-average temperatures across the northern tier of the United States.  Below are the monthly averaged 500mb heights/zonal winds for the Top 10 most positive AO January's along with the temperature anomalies for the month of January for each of the top most positive AO January's:

Now that we have taken a brief look into what the polar vortex is and how the strength of the polar vortex is measured let's do a quick review.  

• Polar Vortex is an area of low pressure centered over the polar regions of the Northern Hemisphere and ALWAYS exists.  
• The strength of the polar vortex is measured by the Arctic Oscillation (AO).
• The Arctic Oscillation had three distinct phases; negative, neutral, and positive.  
• Each phase influences the weather pattern across North American and across other parts of the world as well.  

We've discussed what the polar vortex is and how the strength is measured but we will now take a look into other aspects of the polar vortex, such as movement.  While the polar vortex is normally situated over the polar regions of the Northern Hemisphere, certain atmospheric phenomena can actually cause the polar vortex to shift locations or even split into two separate areas of low pressure.  None of this is that uncommon either.  

In the case of this past Arctic Outbreak which plagued the United States east of the Rocky Mountains the first week of January, the weather pattern across the Northern Hemisphere allowed and favored for the polar vortex to actually sag southward into southern Canada and northern-tier of the United States, bringing down the brutally cold air we had.  Below are the average 500mb heights across the Northern Hemisphere, along with the average 500mb zonal winds from January 2nd, 2014 to January 6th, 2014.  The main areas of interest are all numbered.  Numbers 1-3 are showing the area of higher heights, indicating the ridging, while number 4 shows the very low heights, indicating troughing, associated with the polar vortex.  Number 5 shows the negative, or easterly zonal winds which allows the polar vortex to sink southward:

 What we all can take from this is, the polar vortex is nothing new and it's not some brand new creation caused by "Global Warming" or "Climate Change".  The polar vortex has always been in existence and was first discovered as early as the year 1853.  Despite alot of the talk about it within the media as well as social media, this isn't scary and it does not signal anything about the future.  It is not uncommon at all to have the polar vortex sag southward towards or even into the United States and this has happened many times before.  With all of this, hopefully there is now a better understanding of what the polar vortex is, how we measure it, what it does, and how it affects the weather pattern across North America.   

Winter Storm Set to Bring Major/Significant Snowfall Accumulations, Blizzard Conditions to southern New England

A rather potent coastal storm will bring major (widespread 8-12'') and significant (12''+) snowfall accumulations to southern New England beginning tomorrow and lasting into the first half of Friday.
A piece of shortwave energy is diving down from Canada into the central Plains as a trough develops and continues to amplify (strengthen).  This will allow the piece of shortwave energy to rapidly strengthen as well.  At the same time, a shortwave, albeit weaker, is moving out of Mexico into southern Texas and into the Gulf Coast states.  This shortwave will spawn an area of low pressure in the Gulf of Mexico and will begin to strengthen as it's influenced by the stronger shortwave.

As the trough continues digging into the Plains and begins advancing eastward, the trough will continue to amplify and dig, becoming quite strong which will allow for rapid cyclogenesis (deepening of the low pressure) just off the East coast.  The amplifying trough will also allow for the low pressure to track close enough to the region to allow for heavy snowfall.

What We Are Looking At:

• Periods of light snow are expected to develop early AM Thursday morning with pockets of moderate/heavier snowfall as well.  As the day wares on periods of moderate to heavier snow will become much more common. 
• The heaviest of the snowfall will be occurring during the overnight hours of Thursday into Friday morning.  During this time snowfall rates could potentially exceed 3'' per hour (especially with any thunder snow)
• With a strong area of high pressure off to our north and west and a rapidly deepening area of low pressure, this will create a several hour window of very strong winds across the region.  At times winds could gust as high as 35-45 mph.  This will lead to periods of blizzard conditions.  
• The heaviest snowfall accumulations are expected to occur across RI, eastern MA, and the Cape
• With very strong lift associated with this system along with some elevated instability, the potential will exist for thundersnow as well which will lead to locally higher snowfall rates.
• Temperatures will be quite cold during the duration of the storm as well with temperatures ranging from the single digits to the lower teens.  This coupled with favorable conditions aloft will lead to very high snow ratios, potentially as high as 15:1.  
• The snow will be very light and fluffy, allowing snow to accumulate very rapidly.  Given how light/fluffy the snow is along with very strong winds this will work to 1) Make snowfall measuring very difficult, if not impossible 2) Allow for drifting snow 3) Ratios could be negatively affected as strong winds may rip apart dendrites which could also have an impact on snowfall totals.  
• Based on what the computer forecast guidance is showing, this is what I'm currently expecting: