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.