Mesoscale Update: Severe storms, heavy rain likely today

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A warm and humid airmass has settled into the region this morning, ahead of a large mid level atmospheric trough digging through Southeast Canada and into parts of the Northeast States. With a surface low well to the northwest of the area, a cold frontal boundary will shift eastwards through Pennsylvania and New York later today, providing lift and forcing for widespread thunderstorms to develop.

These thunderstorms are likely to be fueled by a very warm and humid airmass which is already in place this morning. A deep southwesterly flow has become established throughout the region, leading to dew points in the upper 60’s and lower 70’s throughout the Northeast states. Temperatures this afternoon will rise into the 80’s, further enhancing the instability in the atmosphere.

Forecast model soundings suggest precipitable water values (the amount of moisture content in the atmosphere) will approach or exceed 2″ during the early afternoon hours.  Ample theta-e advection in the lower 7500 feet further confirm the deep southwesterly flow in place. The instability axis is likely to be angled from southwest to northeast from Eastern Pennsylvania into New Jersey, Southeast New York, and Western New England, with less instability near the coast and on Long Island.

Height falls in the atmosphere, associated with an approaching front, are expected to gradually begin during the afternoon hours with re-generation of thunderstorms in Eastern Pennsylvania. With little to no convective inhibition in place (to prevent storms from developing), updraft development should be fairly robust. As updrafts develop, models suggest storms will become rapidly organized over Eastern Pennsylvania between 1 and 3pm this afternoon.

The initial storm mode remains in question, and will depend heavily on a multitude of factors from mid level heights to pre-development convective debris. Careful analysis of forecast models suggests subtle hints that storms may develop in a discrete fashion initially, possibly as supercells, over Northeast PA and the Lower Hudson Valley. This is supported by a mid level atmospheric speed max trailing the storm development by 200-300 miles — and the presence of favorable effective shear. In laymans terms, there is enough shear for storms to be organized but not too much so that they quickly form into a line. (Technical: Despite inflow and effective shear vectors running nearly parallel to the front, updrafts re-developing ahead of the frontal zone should have some discrete parameter space for 90-120 minutes).

During this time, the severe weather potential may become locally enhanced in Northeast Pennsylvania, far Northern New Jersey, the Lower Hudson Valley and Western Connecticut. Here, subtle low level turning is apparent on mesoscale model soundings. With southeast inflow shear of 40-50kts, effective 0-6km shear of 40-45 knots and apparent instability of 1750-2000 joules at the surface, supercell thunderstorms over this area could track from southwest to northeast. These would be capable of producing damaging wind gusts, small hail, and a few tornadoes. Models also suggest interaction with a diffuse boundary pushing westward from the coast, in assocation with more stable air. While these low level boundaries serve to keep severe weather potential limited near the coastal plain, they can also enhance low level turning among pre-established storms.

After 22-23z, thunderstorms should organize into multi-cell clusters as wind fields push the storms further east. This suggests clusters and lines of storms will progress eastwards towards New Jersey and into the NYC Metro Region after 4pm. These storms will be capable of producing torrential rains, dangerous lightning, and strong wind gusts that may damage property. Localized flooding will also be possible.

The Storm Prediction Center has placed the region in an Enhanced Risk for severe weather this afternoon, supporting the discussed threat for strong wind gusts. Thunderstorms will weaken as they approach Long Island, but shear vectors and persistent moisture will support the threat for rain continuing throughout the area even after the severe weather threat has ended. This will support the potential for localized flooding, as precipitable water values are highest where moisture pools near the front. The front gradually pushes eastward and the weather will slowly improve by Tuesday morning.

Stay tuned this afternoon and evening for the latest updates and information regarding the evolving threat for strong and severe thunderstorms. We expect the issuance of several watches and a multitude of short fused severe weather warnings. Our social media accounts are the best place to keep up with warnings as they are issued.

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