NY NJ PA Weather Forecasts

The Roller Coaster Begins

The step down to some really cold conditions will begin after Thanksgiving.  I’m using a blend once again of the GFS and ECMWF for the period of Friday through Monday.  

A deep trough will dominate the pattern through this period and for some time afterward over much of the East.  For the most part, the pattern will exhibit disturbances from the southern and north branch interacting off the coast and to the north of the forecast area.  As a result, I don’t expect any significant precipitation events for the forecast area through Monday.  However, the development and intensification of these low pressure systems over New England is very important.  

You see, we are in a period of the negative NAO blocking pattern developing.  As a result, the storm track of clippers from the northern branch are still a bit too far north and the storm track of the southern branch disturbances are a bit too far east, thus the development off the New England coast rather than the Mid Atlantic coast.  However, once again we find important clues showing up for the coming month of December.  First, there is a pattern showing up where Ohio Valley lows and Gulf low are tending to redevelop off the East coast.  The second is that the warm ups ahead of the fronts are much weaker than forecasted and the cool downs behind these fronts have been much more intense than forecasted.  Note last week when the forecast area never got out of the 30’s while the forecast 48 hours ahead of the period had temperatures in the lower 40’s.  

As for this time period, the first clipper and redevelopment will come on Friday.  For the forecast area, the majority of the precipitation will be focused to the north and east of the region.  However, scattered showers can be expected over the region on Friday as the cold front passes through the forecast area.  I expect winds to increase as a new low develops to the east of New England, which will enhance the CAA over the forecast area on Friday night.  

The next low pressure will be the result of phasing of the northern and southern branch.  Once again this is more of a New England threat, especially away from the immediate coastline.  A gulf low that develops along the old stalling cold front will move up the coast and towards New England.  This wave of low pressure is generally weak and does not produce a great deal of precipitation until the phasing of the northern and southern branch disturbances begin.  The main impact on the forecast area will be much colder temperatures for Sunday night and Monday.  I decided to go below MOS guidance for this period and side withe ECMWF 850 MB temperatures which range from -4 to -8 degrees C, similar to temperatures of early last week.  

The development of these storms are very important.  A negative NAO blocking pattern does not develop quickly, at least not a sustained true west based one.  For the past several weeks, the Atlantic pattern has produced a strong Atlantic ridge, which has allowed cold air to enter the forecast area, but not remained sustained.  In order to develop a true 50/50 upper low, the first step was to develop a strong PV over southeastern Canada.  These storms will do just that and the developing 500 MB lows with these storms eventually lead to a 50/50 low developing and building heights over Greenland.  The pattern is coming together, and I still like the idea of the first real threat for snow and frozen precipitation around the December 4 to 8th time frame via a Miller A storm track.

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