10/9 Isolated Severe Threat
NAM computer model forecast SCP values and 500 and 850mb wind barbs for 5 p.m. EDT Friday.
A low-end threat for severe thunderstorms exists on Friday across the Middle Atlantic region and Central Appalachians, perhaps extending as far north as southeastern New York and southern New England. As we move through October, the risk for severe weather tends to diminish with northward extent. While instability is generally low to non-existent in the late season, stronger wind fields can often support some damaging wind gusts, even if some of the other severe weather parameters look less than impressive.
The setup for Friday is somewhat messy and far from significant in terms of severe potential. Nonetheless, at least marginal instability is expected to develop ahead of an approaching cold front Friday afternoon. A robust low-level jet will swing through New York and New England Friday morning through midday, at the same time as a trough passes through. The timing is far from ideal, as instability is forecast to be very low at the time. While an rogue severe wind gust or two could occur early on, a more appreciable severe threat may develop further south and southwest in the afternoon.
Increasing instability and wind shear may support a few marginal supercell structures and bowing segments, particularly across the Middle Atlantic region to central Appalachians. The focus is mainly on the Virginia-Maryland-Pennsylvania-New Jersey area. Here, a few discrete to semi-discrete cells may fire ahead of a cold front. These storms can produce damaging winds and brief hail. While there may be some localized subtle backing of wind fields, low-level rotation is not expected to be a major concern with these storms. Mid-level lapse rates also appear to be lackluster. Further west, a broken line of thunderstorms should develop just ahead of the cold front and these storms may produce some strong wind gusts as far west as eastern Kentucky.
While a brief tornado could potentially occur in the Middle Atlantic region, the odds appear to be less than 50 percent for at least one tornado report. It is one of those cases where more often than not, a tornado will not occur, but given the wind fields and pockets of instability, the possibility cannot be entirely ruled out.
