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.