Cold front brings cold air back

7:08 AM

A cold front is approaching the Philadelphia and New York City metropolitan area this morning with snow showers for most locations as the front progresses to the east.  This cold front will move through the northern Mid Atlantic today with scattered rain and snow showers.  Temperatures this afternoon will peak in the lower 40’s along the coast and upper 30’s over the interior ahead of the cold front, causing the precipitation with this cold front to turn to rain showers as the line reaches the coast.

The Polar air mass behind this cold front will set up a potential moderate winter storm for Friday.  High pressure behind the front will move from western Ontario to the St. Lawrence River Valley this evening through Thursday evening, producing scattered clouds, dry conditions, and supporting the building Polar air mass over the region.  As a result, high temperatures on both Wednesday and Thursday will range from the lower to mid 30’s throughout the region.  This air mass is not an Arctic air mass and temperatures will be marginal for frozen precipitation, but a clear thermal gradient from the southern Mid Atlantic to the northern Mid Atlantic will be established which will lead to a tricky forecast period Thursday night through Friday night.

While the Polar jet stream reintroduces cold air back into the northern Mid Atlantic, a Sub Tropical disturbance will enter the southern Plains and interact with a stationary frontal boundary.  This frontal boundary is the same front that is moving through the northern Mid Atlantic today.  An area of low pressure will develop along this front and travel eastward towards the Tennessee River Valley on Thursday night and then jumping to the southern Mid Atlantic coast on Friday morning.  Now, here is where this storm gets interesting!

As I discussed last night, the timing of several features will be key in determining precipitation type throughout the region and the intensity of that precipitation.  This is clearly a storm that will be tracking west to east through the Plain.

ECMWF-Thursday PM

The first key to keep an eye on is that high pressure system over Ontario.  The stronger the push of the low level and mid level cold air from this surface high pressure, the more likely the precipitation will be snow or sleet for many locations from Philadelphia to New York City.  The 500 MB pattern for the Polar jet stream does provide for convergence and confluence aloft, albeit in a rather unusual way compared to a normal pattern for snow production.  The cold air mass is introduced via a trough over Ontario that progresses towards the Canadian Maritimes.  The position of this trough and the ridge over central Canada leads to a pattern where surface high pressure is supports over Ontario, Canada.  The developing Sub Tropical disturbance over the Mississippi Valley and entering the Tennessee Valley enhances this convergence aloft, thus allowing the surface high pressure system to remain in place.  The position of this high pressure system gives support to keeping a low to mid level cold air mass in place.  This set up is support on the majority of model guidance.

So next key is the Sub Tropical disturbance.  The 500 MB Sub Tropical disturbance is rather strong, which has me concerned about precipitation types along the coastal plain.  A strong low pressure system over the Tennessee Valley may force an easterly wind along the coast depending on the location of the high and low.  This would lead to warming of the boundary layer and as a result the precipitation would start as rain on early Friday morning.  However, if the surface low over the Tennessee Valley remains relatively weak as a new coastal low forms over the Virginia/North Carolina coastal waters, then the combined influence of the high pressure system to the north and the developing coastal low to the south would enhance a northeasterly wind along the coastal plain and thus support a colder solution.  As the low pressure system intensifies off the Virginia/Delaware coast on Friday morning, precipitation will become heavy as the cold air mass drives towards the deepening area of low pressure.  This solution is seen on the Canadian guidance very well along with some support from the UKMET and ECMWF guidance.  If this solution is verified than a moderate to even a heavy snowfall is possible on Friday throughout the Philadelphia and New York City metropolitan area with the exception of extreme southern New Jersey where boundary layer temperatures would likely remain above freezing at all levels.

The primary lifting mechanism for precipitation will be isentropic lifting until the coastal low can become established.  As such, the thermal gradient ahead of this developing low pressure system will be key and likely will not be handled properly not only at 850 MB but at the surface as well until 24 to 48 hours ahead of the event.  Given the factors above, there is just as much chance as the majority of this precipitation to be all rain as it is to be all snow or a mix of snow, sleet, and rain.  Much of this forecast depends on the interaction and development of the high pressure over Ontario and the Sub Tropical disturbance exiting the Plains.  Changes in the forecast are likely and confidence is on the low side.

After this storm exits, the Canadian high pressure is expected to continue to build south, providing clearing skies and dry conditions on Saturday along with highs in the 30’s throughout the region.  The high pressure system will slide to the east of the region on Sunday, allowing for some modification of the air mass.

A very strong cold front will approach the region on Monday.  Strong warm air advection ahead of the cold front on Sunday night and Monday will support very warm conditions in the morning with some locations breaking 50 degrees ahead of the cold front passage.  However, as the cold front exits, temperatures will fall rapidly through the 40’s and into the 20’s and 30’s.  Heavy rainfall can be expected with this front with the potential for even some convective (thunderstorms) embedded in the heavy rainfall along with the potential for flash flooding.