7:32 AM

This morning, an impressive cold front is moving through central Pennsylvania and producing heavy rainfall throughout the Philadelphia and New York City metropolitan areas.  The cold front will continue to progress east this morning and exit the New Jersey coast by 12 PM and eastern Long Island by 2 PM.  The rainfall with this cold front will be heavy at times, leading to flash flooding of streams, small rivers, and urban locations at times.  Temperatures ahead of this cold front are very warm with most locations in the lower to mid 50′s and Philadelphia in the upper 50′s pushing close to 60 degrees this morning!  However, as the cold front exits, strong cold air advection will force temperatures to fall through the 50′s, 40′s, and eventually the 30′s by late this evening with a gusty northwesterly wind.

A much colder regime will take hold by tomorrow with temperatures in the 30′s over the interior and lower 40′s along the coast for afternoon highs.  A few isolated snow showers will move through the northern Mid Atlantic as a passing disturbance reenforces the new fresh Polar air mass, however most locations will remain dry with variable cloud cover.

Another cold front will move through on Wednesday with another shot of Polar air and a few additional snow showers.  Temperatures on Wednesday afternoon will struggle in the 30′s, however high pressure will quickly exit the Mid Atlantic coast by the evening, allowing for a limited fall in temperatures on Wednesday night under the influence of a developing southwesterly wind.  High pressure will continue to move east on Thursday with scattered clouds and temperatures break 40 degrees along the coast under the influence of the moderating southwesterly wind.  However, the brief period of highs above 40 degrees will end quickly.

The upper level pattern is rapidly changing with a new Polar Vortex diving south through central Canada and towards Ontario.  The Polar Vortex will drive a strong Arctic cold front through the northern Mid Atlantic on Thursday night, which will leading to temperatures crashing through the 30′s and into the 20′s for Friday morning lows even at the coast.  A very strong thermal gradient will set up over the Mid Atlantic, which the Philadelphia and New York City metropolitan area on the cold side of this gradient.  Temperatures will struggle to break into the lower 30′s along the coast on Friday afternoon as a result.

Meanwhile, a strong Sub Tropical disturbance will be organizing over the Gulf Coast and will interact with the stalling cold front  on Friday morning.  The position of this cold front will be key in the overall track of the surface low along the Gulf Coast, however current indications strongly point to the low tracking towards the southern Mid Atlantic coast by Friday night.  This Sub Tropical low pressure system will be loaded with moisture from the Gulf of Mexico with an increasing southwesterly flow at 700 and 500 MB over the northern Mid Atlantic.  With cold temperatures in place and well supported, the threat for a prolonged periods of light to moderate snowfall will be possible Friday afternoon through Friday night ahead of the developing low pressure system.  The process that will produce the snowfall is called isentropic lifting and can lead to periods of heavy snowfall in locations where mesoscale banding develops.

The low pressure system will transfer to the coast by early Saturday morning and move towards the bench mark of 40N/70W by Sunday morning, leading to the potential of a moderate to heavy snowfall throughout much of the Philadelphia and New York City metropolitan area.  With temperatures forecasted to range from the upper 10′s to lower 20′s over the interior and lower to upper 20′s along the coast Friday night through Sunday morning, liquid to snow ratios are likely to range from 12:1 to 15:1, which will enhance snowfall accumulation potential.  However, the key for this event will be the eventual location of the Arctic cold front  and the speed at which the cold air will build at the surface and mid levels ahead of the moisture advection.  Clearly though, this storm will have the potential to produce a significant snowfall.