High pressure dominates, but not for long

9:40 AM

High pressure is in full control over much of the eastern third of the nation this morning and remain in control through tomorrow evening.  As the high pressure moves northeast of the northern Mid Atlantic on Sunday and a developing low pressure over the Mississippi Valley approaches, winds will veer to the southeast and support moderating temperatures throughout much of the region.  Temperatures today and tomorrow will push into the lower 40′s along the coast and upper 30′s over the interior, leading to a continued melting of the once deep snowpack over the region.

The low pressure system that drives into the Ohio Valley on Sunday night will continue to move north and east towards Lake Erie on Monday evening.  As the low pressure system runs into the negative NAO block over eastern Canada, a coastal low will develop east of Delaware on Monday night.  This low pressure system will track to the east of New Jersey on Tuesday morning and continue to rapidly intensify and becoming the primary low pressure system on by mid Tuesday morning.

The strong southeasterly winds from the surface to 850 MB ahead of the coastal low becoming the primary low, will significantly warm much of the coastal plain into a significant portion of the interior, leading to this storm being a primarily rain event for these locations.  Rainfall of a half inch to as much as 2 inches of rain over the Philadelphia and New York City metropolitan area will support the threat of flash flooding of rivers, streams, and urban locations.  Remember, a season’s worth of snowfall is going to be melting into the ground, saturating the soil and leading to over saturation of the ground throughout the coastal plain.  As a result, the additional one to two inches of rain will have to run off into the rivers and streams, thus creating the flooding issues.

Further north and west over northeastern Pennsylvania, the northern portions of Sussex County, the Hudson Valley, and interior Connecticut, cold air at the surface will hold on through Tuesday morning.  As the coastal low becomes established, the warming influence of the Atlantic will come to an end, thus keeping much of these locations below freezing throughout the precipitation event and supporting an all frozen precipitation event.  Many of these locations will have a mix of snow, sleet, and freezing rain leading to a significant icing problem in these locations.  Snowfall and ice accumulations will be dependent on the predominance of precipitation type, with a preference for more ice south and more snow north.  Further north towards the Mohawk Valley of New York through the central Hudson Valley and northern Connecticut, temperatures at all levels will be below freezing supporting an all snow event.

As the low pressure exits on Tuesday evening, strong cold air advection will drive towards the coast, potentially changing the rain over to snow before exiting, however no accumulation is expected along the coast.  High pressure will briefly take hold on Wednesday ahead of the next potential storm.

A strong upper low is expected to form over the Tennessee Valley via a phase of the Sub Tropical and Polar disturbances moving through the Mississippi Valley on Wednesday night.  As this upper low deepens and the trough axis tilts to a negative position, a coastal low will form off the North Carolina coast.  The details of where this coastal low tracks is still very much in debate as there is a wide spread in solutions on the model guidance.  However, considering that a ridge axis over the central Rockies has strong agreement on the model guidance along with a fresh new cold air mass over the northern Mid Atlantic, there is a high chance that a very slow moving coastal low will be impacting much of the Mid Atlantic from Virginia to Connecticut on Thursday and Friday with widespread snow if not frozen precipitation, strong northeasterly winds, coastal flooding, and generally unpleasant driving conditions.

So enjoy this weekend as much as possible as next week will not be pretty for many throughout the Philadelphia and New York City metropolitan areas.

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