NY NJ PA Weather Forecasts

Pattern changes and turkey dinners

7:45 AM

A weak yet pesky low pressure system continues to move away from the New Jersey coast this morning, however the persistent northeast to easterly winds associated with the low is keeping overcast skies and damp conditions over much of the northern Mid Atlantic.

Scattered showers continue to move through much of eastern Pennsylvania and New Jersey this morning, but will weaken as the upper disturbance supporting the low pressure system continues to minor out.

A much more impressive upper level disturbance is diving south towards the Gulf Coast this morning.  This upper disturbance is eventually going to lead to a storm that will start to trigger a major pattern change that will take place through much of December.

Tonight through tomorrow afternoon, weak high pressure from the St. Lawrence River Valley will produce dry yet overcast conditions for one of the busiest travel days of the year.  A weak disturbance and cold front will slide through on Wednesday afternoon with an isolated showers, but will not be a significant impact on travel conditions.  Temperatures however, will be rather pleasant in the 50’s and lower 60’s throughout the region.

The forecast becomes rather tricky on Thanksgiving.  The strong disturbance will carve out an impressive trough over the eastern third of the United States.  The disturbance is expected to move from the Ohio Valley towards the northern Mid Atlantic coast from Thursday afternoon to Friday morning.  The old primary low associated with this disturbance will weaken over the Great Lakes, however a new coastal low will develop off the North Carolina coast.  There is general agreement on this from all model guidance.

Up to this point, Thanksgiving will be dry and relatively pleasant.  In fact, temperatures on Thanksgiving will run well above normal making for perfect conditions for the Macy’s Thanksgiving Day Parade.

On Thanksgiving night, the low pressure system will intensify significantly from a 1006 MB low to a low dropping to 998 MB by Friday morning and down to 988 MB by Friday evening.  The question is where does this occur?  The operational models take the storm further east, keeping a large majority of the precipitation (but not the wind) further off the coast.  However, ensemble guidance is not as conclusive with several members bringing the storm closer to the coast.  One model of interest to me is the SREF guidance, which I have had gaining trust in with these types of forecasting problems.  The SREF also brings this storm closer to the coast as well.

Besides the model guidance, physically I find a better argument for a closer coastal storm track.  The reason being is that this upper low over the Ohio Valley will be significantly intensifying on Thursday night through Friday, which will pull the surface low to the west as the best dynamics will be focused towards the coast.  The operational guidance goes as far as redeveloping surface low pressure system as a result of this effect.  The best divergence at the upper levels is over the northern Mid Atlantic in this time period along with the best mid level forcing as the 700 and 850 MB lows develop.

At this time, the forecast idea I had presented yesterday stands.  The coastal low will move through the New Jersey coastal waters on Thanksgiving night with rain entering southern New Jersey first and working north and west through the night.  Winds will increase through the night as well, ranging from 15 to 30 mph with higher gusts along the coast.  The rain will be heavy at times, especially along the coast, and the threat for coastal flooding will have to be monitored.

On Friday morning, the low will continue to intensify as the low moves into New England.  Cold air will rush into the coastal plain, forcing temperatures to hold steady or fall through the day.  A much colder day can be expected on Friday compared to the pleasant conditions on Thanksgiving.  With colder temperatures at the surface and aloft, there is a threat for snow and sleet to mix in with passing rain showers through the day on Friday.  The higher elevations will be at risk for some light accumulations due to this threat, especially above 1500 feet.

High pressure will take hold by Friday night, making for a cool and dry period of weather for the weekend.  Temperatures will range from the lower 40’s over the interior to lower 50’s along the coast, which will average near to slightly below normal.

Another storm will be on the way for early next week.  There is some uncertainty on the exact track of this second storm as models have wavered between a coastal track and a storm track through the St. Lawrence River Valley.  This storm is key for one reason, this is the storm that drives the cold air south for the start of December.  I would not be surprised if this storm takes a track further east if the storm on Friday intensifies to the point of setting up a 50N/50W 500 MB low.  If that does occur, look for redevelopment towards the coast as the primary drives towards Ohio.  The new experimental GFS (to replace the operational guidance on December 15) Regardless, this storm is a rainstorm due to the lack of cold air ahead of this storm for the northern Mid Atlantic.  By Tuesday, the low pressure with this storm is expected to move through central Pennsylvania and then towards the St. Lawrence River Valley.  The cold front will move through with periods of heavy rain and another drop in temperatures through the day.  High temperatures, depending on the timing of the cold front, may occur in the morning followed by falling temperatures through the afternoon.  Significantly colder air will follow this storm.  However, the question is for how long.  This pattern is not the winter pattern that will lock in for late December through February, but a step towards that pattern.  As a result, after a few days I expect moderation and with moderation, a threat for another storm in the time period between December 5th and December 8th.

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