Tuesday, July 12, 2022 Thunderstorm Potential Update

Scattered showers and thunderstorms are expected to develop across New York and Pennsylvania Tuesday afternoon and progress east-southeast across the Northeast into New England ahead of an approaching cold front and developing pre-frontal trough. With the pre-frontal trough now in the picture, this may lead to earlier storm development. 

Severe Thunderstorm Potential 

The potential for widespread severe weather is exceedingly low, however, the potential exists for isolated-to-scattered pockets of severe weather across central/eastern New York down into Pennsylvania with severe weather potential more localized across New England. The greatest risk is for damaging wind gusts. 

In terms of the set-up there are actually some impressive ingredients. An unseasonably strong mid-level flow will overspread the Northeast with 50+ knots of flow at 500mb and 40+ knots of flow at 700mb. This will contribute to bulk shear values in excess of 40-50 knots over the region. This shear would be more than supportive of thunderstorm organization and with a predominately unidirectional flow this would support quick upscale growth into one or two line segments. Shortwave forcing is also pretty decent with much of the better forcing right along the international border:

Combination of surface temperatures climbing into the 80's with dewpoints surging through the 60's will contribute to about 500-1000 J/KG of MLCAPE into New England with upwards of 1000-1500 J/KG of MLCAPE farther west across New York and Pennsylvania. There are indications the pre-frontal trough may become established relatively early and well east. This would yield earlier cloud development and scale back temperatures and instability. Poor mid-level lapse rates and warm mid-level temperatures will yield a low ceiling on how much instability can develop.

How thunderstorms should evolve

With surface-based CAPE values approaching 2000 J/KG this will aid in the rapid acceleration of parcels to the LCL and subsequently the LFC as convective temperatures are reached. However, due to very poor mid-level lapse rates (5.5-6 C/KM), rather warm mid-level temperatures (500mb temps ~-8C), and mixed-layer CAPE only in the 500-1500 J/KG range, parcel acceleration through the mid-levels of the atmosphere will be reduced greatly. This will preclude more robust convection on a widespread basis. This is evident by tall skinny CAPE profiles which suggest a higher degree of water loading which will actually minimize the true CAPE.

Convection will begin to rapidly fire by early afternoon across central New York and Pennsylvania. The strong shear aloft will help aid in thunderstorm organization, however, the processes described above will hinder convection from becoming very deep and thunderstorms will struggle to intensify. With this said, combination of some steep low-level lapse rates (but this too could be impacted if full heating isn't materialized) and strong low-level winds will promote the risk for isolated-to-scattered damaging wind gusts within the strongest of thunderstorms. Given some signals for low-level winds to back a bit around the trough, a tornado can't be ruled out, especially if any thunderstorm can remain discrete. 

Tuesday, July 12, 2022 southern New England Thunderstorm Potential

A developing upper-level trough digs into the upper-Midwest region through the day Monday meanwhile at the surface an area of high pressure slides east off the Northeast coast. The result will be a developing low-level southwesterly flow across the Northeast and New England. The result will be strong theta-e advection associated with a hotter and much more humid airmass.

On Tuesday, the upper-level trough amplifies as it progresses east across the Great Lakes and southeastern Canada. At the surface a cold front slowly advances south and east across the Northeast. Out ahead of the cold front the airmass will be hot and humid with high temperatures away from the coast pushing into the mid-to-upper 80's. Dewpoint values will likely be pushing lower 60's by early Tuesday morning with mid-to-upper 60's by late afternoon to early evening. 

Instability

An area of steeper mid-level lapse rates (> 7 C/KM) will traverse portions of the Northeast Tuesday morning, however, move off the Northeast coast by early afternoon with mid-level lapse rates ranging between 5.5-6 C/KM. These poor mid-level lapse rates will hinder overall instability potential, however, combination of temperatures pushing well into the 80's with dewpoints climbing through the 60's should be enough to yield 1000-1500 J/KG of mixed-layer CAPE. While the highest dewpoint values may occur past peak-heating these higher dewpoint values will help offset the slight cooling that occurs towards evening. 

Wind Shear

An unseasonably strong wind max rounds the base of the upper-level trough characterized by 50+ knots of wind from the west-southwest at 500mb and 700mb which should yield bulk shear values of 40-50+ knots overspreading the region. This shear will be more than supportive for thunderstorm's to become organized. Forecast models are also suggesting the potential for some directional shear with winds in the lower-levels of the atmosphere and at the surface coming out of the southwest. 

Severe Potential

Thunderstorms are expected to rapidly fire within New York and Pennsylvania by early Tuesday afternoon thanks to an unstable and weakly capped airmass. Combination of mixed-layer CAPE values in the 1000-1500 J/KG range, strong bulk shear, and steep low-level lapse rates due to strong surface heating will result in the potential for the strongest thunderstorms to produce damaging winds. Given shear vectors storms should grow upscale into one or multiple line segments which will increase the potential for damaging winds in the strongest cores that can develop. While mid-level lapse rates will be weak along with warm mid-level temperatures (-8C to -9C) which will limit hail growth and potential, some hail can't be ruled out during the early evolution of storms, especially if any initial storms take on supercell characteristics. This could also come with the potential for a brief tornado. 

The greatest overall potential looks just west of southern New England across New York and Pennsylvania as the timing of the front closer to southern New England is more during the evening. Thunderstorms will likely begin to weaken as they move into southern New England due to the loss of daytime heating. However, given strong bulk shear and high dewpoints, there may be enough instability to keep the potential for an isolated severe thunderstorm into central Massachusetts and Connecticut through the evening. Given the weak lapse rates and warmer mid-level temperatures thunderstorms even across New York and Pennsylvania will really struggle to develop very deep cores even with the stronger shear. This will keep the wind damage potential more scattered. Cloud-to-ground lightning will also occur with the strongest thunderstorms but the degree of lightning will also be scaled back due to lower CAPE within the hail growth zone and warm mid-level temperatures. Noting the Storm Prediction Center (SPC) has the best chance for any severe thunderstorms just to the west of southern New England and given the above descriptions can't disagree. Thunderstorms will rapidly diminish as they move east of the Connecticut River Valley through Tuesday evening.