Summer has once again returned to southern New England as a warm front lifted northward on Sunday bringing in much hotter and humid air to the region. While the duration of the heat/humidity will be rather brief it will be fairly oppressive at times. Temperatures on Monday reached 90F across much of the region and more of the same can be expected Tuesday and Wednesday. In fact, Tuesday looks to feature many location reaching the mid to even upper 90's! Wednesday will be the final day of 90's and heat/humidity, however, it will not go away quietly.
A fairly potent shortwave moving through eastern Canada will be swinging a cold front towards New England Tuesday and Wednesday. On Tuesday, the cold front will be situated to where northern New England will see a threat for widespread t'storms, some of which will become quite strong to severe with damaging winds and large hail the main threat. in fact, across extreme northern New England, especially near the US/Canadien border the possibility of an isolated tornado will exist as this area will be much closer to the strong piece of energy/associated surface low which will help to locally back the low-level winds and create some pockets of stronger helicity.
As Wednesday rolls around the front will continue sagging southeastward and this will place central/southern New England in the threat for storms/severe weather. With a very warm low-level airmass in place and overnight lows on Tuesday expected to be very mild temperatures will shoot through the 70's/80's very early Wednesday and reach the lower 90's by early afternoon. With a very moist low-level airmass in place dewpoints should be right around the 70F mark. The combination of very hot temps and high dewpoints situated under a fairly steep mid-level lapse rate environment with 850-500mb lapse rates up around 6.5 C/KM will yield to a moderate to extremely unstable airmass across the region. Several computer models are developing anywhere from 2500-3500 J/KG of SBcape along with 1500-2500 J/KG of MLcape, 2000-3000 J/KG of MUcape and LI values as low as -8C.
Along with the potential for moderate to extreme instability winds aloft will also be fairly strong as well with a solid amount of speed shear (winds increasing with height). While the strongest shear values will remain north of the front shear across central/southern New England is expected to be adequate enough. Computer models have about 30-40 knots of vertical shear values across the region along with 850mb winds of around 20 knots, 700mb winds around 30 knots, and 500mb winds around 40 knots. Winds throughout the atmosphere are expected to be unidirectional as there will be a westerly component to the wind at all levels.
While the combination of shear/instability looks rather impressive there are some factors that have to be considered. Timing of the front may be an issue. Over the past few days several computer models actually moved the front through the region too early which would certainly lessen the chance for severe weather across the region. It does appear though that the front should not come through too early as models have slowed the front down. In fact, some new data actually moves the front too slow which would actually mean it would be once again northern and central New England as the main threat area on Wednesday.
Another issue could be a wind shift occurring too early in the afternoon with surface winds shifting from southwest to a more westerly direction. If this were to occur we could actually see some drier air aloft mix down to the surface resulting in dewpoints lowering a bit. While the low-levels are rather moist dewpoints shouldn't drop more than 2-3F, however, that still is enough to really knock down instability numbers.
While not likely to be a major factor we do have to watch to see if there are any cloud debris from previous day convection.
Given the above the potential will exist on Wednesday for widespread t'storms across the region and the potential will certainly be there for severe weather. Given how the winds aloft will be unidirectional with a good presence of speed shear we could see a pretty potent bowing squall line develop and the main threat for this would be damaging winds. While freezing levels are a bit on the high side and helicity values expected to be on the lower side with the presence of the unidirectional flow large hail is not expected to be a major threat, however, it certainly can't be ruled out. With high dewpoints and available moisture torrential rains will be likely with any storm and some isolated flash flooding can't be ruled out either, although this should be confined more to the normal flood prone/poor drainage areas.
Once the front clears the region we will return to more comfortable weather with much lower humidity.
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