NY NJ PA Weather Forecasts

Evening Thoughts- The rain begins

8:40 PM

Surface Map at 8 PM- Penn State E-Wall

This evening, a warm front is starting to move northward as moisture from the Gulf of Mexico surges towards the Mid Atlantic and Northeast.  Overall, my thoughts about the development of the warm front and cold front over the next 36 hours has not changed.  It is clear that the high pressure over the Canadian Maritimes is playing a major role in enhancing the pressure gradient over much of the Mid Atlantic, Great Lakes, and Ohio Valley which in turn is producing increasing easterly winds over the northern Mid Atlantic around 10 to 20 mph with higher gusts.  The majority of the rainfall this evening is focused over eastern Pennsylvania, central/northern New Jersey, and the New York City metropolitan area.  The isentropic lifting over southern New Jersey is relatively weak due to the warmer conditions in place over this area in the first place and the best upper level dynamics are remaining well to the northeast, thus southern New Jersey has been and will continue to remain relatively dry this evening.

The water vapor satellite image this evening tells us a lot about this upper level trough and the overall structure of this cold front.  Note the dry air surging into

Water Vapor This Evening- NWS/NOAA

the Ohio Valley this evening, which is ahead of the surface cold front.  It is pretty clear that without any additional infussion of moisture to this upper level trough, the rainfall with this cold front would not be nearly as impressive as the bulk of the original moisture with this cold front is moving into New England this evening.  This is why the disturbance over the Bahamas this evening is so important to the development of heavy rainfall for tomorrow afternoon.

As the upper level trough tilts to a negative axis, the tropical moisture from this disturbance will be drawn northward and interact with the cold front tomorrow afternoon through the evening hours.  Due to the nature of the moisture moving into the Mid Atlantic and the strong lifting at all levels of the atmosphere along this cold front, a wall of heavy rain is expected to develop initially over western New York and central Pennsylvania.  This wall of rain will march eastward through the afternoon and evening hours.  In fact, the development of this wall of rain is already forming over central and southern Ohio in the form of a line of strong thunderstorms.  As the cold front progresses to the east, embedded thunderstorms will lead to heavy downpours, rapidly reduced visibility, and the threat of flash flooding.

What could lead to a drier forecast?

The timing of the moisture from this disturbance over the Bahamas is key.  If the moisture interacts with the trough just 3 to 6 hours later than forecasted, then the heavy rain will develop later in the period with a much faster progression of the cold front overall.  So the key overnight is to keep an eye on that disturbance.  If upper level interaction does not start by tomorrow morning, than a drier forecast is about to unfold.

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