Evening Thoughts
Steady rainfall, heavy at times, is concentrating along a stationary frontal boundary over the Carolinas with a strong enhancement from the remnant moisture of Fay. The rainfall extends from the North Carolina coast back to eastern Kentucky with the low level circulation continuing to move through east-central Tennessee.
The model guidance for the next 66 hours is not exactly what I would call in strong agreement. If one would follow the NAM, then nobody will see any rainfall at all. Meanwhile, the Canadian produces well over an inch of rainfall through Saturday afternoon with the exit of the cold front from the Plains.
These are the cases where too much model data is a bad thing. You get bogged down with all the data and end up with tunnel vision. So, let’s step out of the tunnel and think about this forecast for Thursday and Friday.
In order to make this forecast, I first nailed down the main players here. At the upper levels, the two main players are the trough over the Great Lakes and the upper low off the New England coast. The upper trough that is currently digging into the northern Plains is directing the remnants of Fay towards the Mid Atlantic Coast. Meanwhile, the upper low off the New England coast is supporting strong convergence and confluence at 250 MB over much of the Northeast, which is producing sinking air. The sinking air is producing an atmosphere that is relatively cool and very dry throughout the atmosphere.
The set up we are observing here is basically like a train (the warm front and tropical moisture) running right into a wall (the high pressure system and convergence over the Northeast). So the question is which player in this forecast will be the dominant force? That answer depends on where you are located.
If you are in the Hudson Valley, Connecticut, northeastern Pennsylvania, northern New Jersey, and New York City then the high pressure and confluence will be the winning fighter in this contest. High pressure will hold off much of the precipitation in these areas. Although I won’t rule out scattered showers for New York City, Long Island, northern New Jersey, and northeastern Pennsylvania; I think the heaviest rainfall will remain to the south. Meanwhile, locations in the Hudson Valley and Connecticut may not even see showers and possibly even some breaks in the clouds through Thursday and Friday. The warm front never moves north of these locations and the winds will remain from the east and northeast, producing cool temperatures and low humidity.
The winner of the fight for locations over central and southern New Jersey, the Philadelphia metro, and much of southeastern Pennsylvania is the warm front and the steady rainfall. In these locations, the isentropic lifting will be strong enough to overwhelm the initial dry air, which in fact will enhance the isentropic lifting through Thursday. Now, this does not mean that every location in the area above will see heavy rainfall. Just that this area can see a higher influence of showers and thunderstorms with steady rain at times. The further south and west you go, the better chance for a prolonged period of steady, heavy rain.
The interaction with the upper low off New England and the remnants of Fay will be difficult for the models to handle for the next 24 hours, then we should see better agreement by tomorrow evening.
