NY NJ PA Weather Forecasts

This Evening:  Tonight, I discuss what is producing the showers and thunderstorms over the forecast area this evening and what to expect going forward into tomorrow.  Also, a look at the weekend and the threat of tropical weather impacts for the East coast for next week!

Thursday August 14th, 2008

A strong upper level disturbance, seen to the left on the map, is currently located over eastern Pennsylvania and will move through the region tonight.  This disturbance is currently destabilizing the warm, moist atmosphere at the surface via strong lifting and the interaction of colder air aloft.  As a result, these thunderstorms have been capable of producing small hail, frequent cloud to ground lightning, wind gusts exceeding 55 mph, and heavy downpours.  

While the upper level dynamics are certainly enhancing the potential for severe thunderstorms, the features at the small scale or mesoscale, are what is focusing the thunderstorms.  A developing low level jet is pushing thunderstorms through central New Jersey and interacting with an out flow boundary sinking through northern New Jersey and the Hudson Valley.  This is producing an area of low level convergence from the Long Island Sound, through western Long Island, New York City, Mercer County in New Jersey, Trenton, and then west over Allentown and Harrisburg.  There is a high potential for flash flooding from these developing thunderstorms along with damaging wind gusts and large hail.  The main threat of course will be the heavy rainfall with rainfall amounts easily exceeding an inch to possibly 4 inches in some isolated locations based on recent Doppler Radar precipitation estimate trends.  

Through the rest of the evening, the disturbance will move through the region with widespread showers and thunderstorms for the entire forecast area.  Not everyone will see the heavy rain or flooding conditions like the areas described above, but everyone should see rainfall.  

Another trough will move through the forecast area tomorrow with the potential for additional showers and strong to severe thunderstorms in the afternoon and evening.

The main threat for severe weather will remain over the southern Plains as a strong upper low and jet maximum produces strong vertical wind shear over the region.  Although the low level clouds may inhibit instability initially over the region, by the afternoon the cap should be broken with a significant threat for large hail, tornadoes, wind gusts exceeding 60 mph, heavy downpours, and cloud to ground lightning  can be expected in the strongest thunderstorms.  

Severe weather is not expected over the rest of the country, however thunderstorms will still be possible over other locations.

Over the northern Plains, the digging trough will support weak wind shear, divergence aloft, and low level convergence which will lead to showers and thunderstorms along an advancing cold front.  

Meanwhile, an upper low rotating through the eastern Great Lakes and towards the Northeast will produce unstable conditions over the region.  Showers and thunderstorms will develop in the afternoon as cold air aloft supports the development of hail, strong wind gusts over 50 mph, and heavy downpours.  The showers and thunderstorms will be scattered in nature.

There are three areas of interest that I’m keeping an eye on.  One area is of immediate concern for much of the East coast, the other is of general concern, and the third is of long range possibilities.  

First, a tropical disturbance roughly 175 miles east of the Leeward Islands.  This disturbance is becoming much better organized over the past few hours and will likely become a tropical depression by sometime today.  This disturbance is forecasted by practically every model guidance to strengthen into a tropical storm or hurricane over the next several days and to be positioned somewhere over or to the north of the Bahamas.  A trough moving through the Northeast on Tuesday will pull the tropical system to the north for a short time before the ridge rebuilds into the Northeast, thus I would say that there is a growing threat for the East coast of the United States from this tropical disturbance.  The upper level winds and sea surface temperatures strongly support the potential for strengthening over the next 48 hours.  This disturbance will have to be monitored very carefully.

The next disturbance is located roughly 1350 miles east of the Leeward Islands.  This tropical disturbance is not as organized as the first with showers and thunderstorms remaining scattered and the low level circulation rather weak.  However, the upper level winds over the central Tropical Atlantic will remain weak with no significant wind shear expected.  Also, sea surface temperatures remain rather warm and should support further development.  However, given the recent track record of this tropical disturbance, I’m still expecting development to be slow.  

Finally, a tropical wave that has just exited the African coast bares watching over the next several days.  This tropical disturbance does exhibit a low level circulation with plenty of showers and thunderstorms.  As with the other disturbances, wind shear will be weak if not non-existant, with warm sea surface temperatures.  Although I don’t expect any development over the next 48 hours, this disturbance will have potential to develop further through the weekend.

The local forecast discussion is updated!

The Nor’easter remained a near miss, however showers have developed over the forecast area.  The upper low responsible for spawning the surface low off the coast will rotate through the region over the next 24 to 36 hours, and then the change begins!  Here are the thoughts I’ll cover in the technical forecast discussion:

1. Heat no, humidity yes.

2. Troughs and other nuisances.

3.  Tropical trouble?