The Sept 18th weather threat.

The weather tomorrow is going to look a lot like what we saw on the 8th, only a bit more to the south. We will have a strong cold front that will be intensifying as it moves out of the Great Lakes, there will be several short-waves moving along it. A surface low will develop in the Tennessee and Ohio valley and move north. The low will pulling a lot of southern moisture into the Northeast. The cold front and the low will interact with each other. We will have marginal instability, with ML CAPE values of 200 to 750 J/Kg. Overhead the upper and lower jets will be very potent in the afternoon and evening; the lower jet will be in the neighborhood of 60 to 70 knots, with a upper level Jet parked just to the west of around 140 knots. However, the mid level lapse rates look very unimpressive at this time, around 5.5 C/Km. The shear profiles are such that CAPE values of 500 J/Kg would be more than enough to create a squall line.

In the animation you can see a very dynamic squall line quite clearly on the simulated radar. The line will stretch from NYS to North Carolina. The line will be thin but it will be very intense with extremely heavy rains and strong gusty winds. When the line goes through the winds will whip-up to over 40mph.

The timeline will be something like this. We run the risk for a few scattered showers after midnight, this threat will last until the early morning hours of tomorrow. As the morning progresses we will see more in the way of steady rain from west to east. The heavy rain will be falling in western NYS and Finger lakes in the morning; moving into areas east of Lake Ontario in the afternoon.I think the squall line will be approaching the Tug Hill around 4:00 pm and should be impacting the western Mohawk Valley around 5:00 pm. It should start to move into the Capital District after 8:00 pm Rainfall totals for the event will be 1-3 inches. But in spite of the heavy rain, I don’t think flooding will be a huge issue, do to it being so dry. However, there could be some small stream flooding in poorly drained areas.

As for winds, boundary-layer instability looks to be just enough to allow some of the wind energy overhead to mix down to ground level, especially for those at or above 1800 feet.
The thunderstorm threat is low, but those south of Albany have the greatest chance of seeing any storms that do develop, it looks like the greatest instability will be over Ulster, Dutchess, and Litchfield Counties. Any storms that do develop will carry an additional wind threat.
The lack of instability will be caused by pre frontal clouds and rain and weak lapse rates. But as we’ve seen in the past, that the models tend to under-forecast instability in this type of setup. We will be dealing with a chance for low topped convection, this can lead to significant convective weather, if things come together just right, so the instability will have to be closely monitored tomorrow in order to assess the tornado potential. The main threat will be the southern Mid Atlantic during the afternoon, with a much lesser threat in southern New England during the overnight as the squall line moves eastward. The greatest tornado threat will be from southeastern Pennsylvania into North Carolina during the afternoon and evening. CAPE values look to be 750 to 1200 J/Kg in Pennsylvania, and 1500 - 2000+  in the Southeast.