NY NJ PA Weather Forecasts

High pressure gives way as rain is on the way

7:27 AM

The pleasant weather conditions from yesterday are about to come to an end.  So is my confidence in model guidance, by the way.  However, before I get to that, let’s take a look at the precipitation that is charging towards the forecast area. 

High pressure currently centered over eastern Connecticut and Rhode Island, will continue to slow slide to the east, which is supporting a well developed easterly fetch off the Atlantic.  Meanwhile, a warm front is approaching from the Ohio Valley.  Note how the precipitation drives through southwestern Pennsylvania and Virginia, but comes to an abrupt stop over central Pennsylvania through coastal Virginia, almost like an invisible wall is up to protect the forecast area.  In a way, one could describe the high pressure system like a wall.  Dry, sinking air from the surface high pressure and the exiting trough over the Canadian Maritimes is suppressing the advance of cloud cover and precipitation this morning.  However, as the day progresses, high pressure will begin to lose influence and clouds will increase at first with very high clouds (seen this morning over much of eastern Pennsylvania) and then with mid level and low clouds by this afternoon and evening.  Rainfall will hold off until early on Thursday morning as the best isentropic lifting with the warm front will remain to the west of the forecast area.  

The forecast for Thursday is still a tricky forecast for me, and I think the model guidance is missing some key details.  I am suspicious of the exact placement of the warm front by Thursday afternoon.  The air mass behind this warm front is humid and very unstable, and I think will have a big influence on the development of precipitation over the forecast area.  Meanwhile, the marine air mass along the coast will continue to have some support from the exiting high pressure system through at least the early afternoon on Thursday.  The low level jet stream at 850 MB will be increasing to 50 KT from the south and southeast through Thursday afternoon, while winds at 500 MB will be from the west.  Note the potential for strong directional and speed shear over the forecast area.  So let’s put everything together.  The entire forecast area will be under the influence of strong wind shear vertically via direction and speed shear.  The low level jet stream will bring plenty of moisture into the forecast area from the Gulf of Mexico and the tropical Atlantic (note the moisture building over the coastal waters of Florida).  The marine air mass will be slow to give way along the coast, meanwhile locations like the Delaware River Valley and much of eastern Pennsylvania will be under the influence of a very warm, humid, and unstable air mass.  Just these influences alone would suggest periods of heavy rain throughout the forecast area.  However, a wild card here is if any breaks in the clouds develop behind this warm front is significantly destabilize the atmosphere, thus producing strong to severe thunderstorms.  These thunderstorms would then progress east and northeast towards the coast, become elevated over the weakening marine air mass, and produce very heavy downpours.  Oh, and did I forget to mention the strong lifting at 500 MB from the upper level trough.  So clearly, Thursday afternoon will exhibit two distinct precipitation types over the forecast area, convective and isentropic precipitation.  

The fun does not end on Thursday though.  I have some doubt on the accuracy of the model guidance for Friday through the beginning of next week.  While I agree that a trough will be present over the Northeast, I have doubts on the strength of this trough that is seen on the ECMWF and GFS model guidance.  I took a blend of these guidance with the GEFS guidance, however my confidence is low on the position and strength of low pressure system through this period.  The main problem is that the model guidance is over-amplifying the trough in the East due to a disturbance exiting out of the Southwest which interacts with the trough.  Model guidance for what ever reason always has a difficult time accurately forecasting the development of upper level patterns with energy coming out of the Southwest.  At any rate, I still expect a threat of showers and thunderstorms each day from Friday through Tuesday.  There is some thought that the trough axis will move over the forecast area, which would bring a cool, cloudy, and damp marine air mass into the region by Monday afternoon and Tuesday.  For now, I am keeping the same forecast that I felt comfortable with before the latest runs of model guidance, and will go with a cooler forecast for Tuesday to illustrate the back door front potential.

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