Thursday, February 16, 2012

Sunday Storm Update

Last night's computer modeling shifted around a bit, showing the increased potential for at least some snow for the Delaware Valley on Sunday as low pressure intensifies while passing to our south.  It's a good track for snow but it's not an ideal setup in that there is a lack of cold air present at the surface and a relative lack of cold air in the lower atmosphere (that "850" zone that is often talked about seems to feature temperatures just below freezing).  This potential storm would need to rely on dynamics and rely on explosive development to generate heavier precipitation and also draw down colder air from the mid layers of the atmosphere.   While both tracks are better for snow, it is not a dead-on certainty that these tracks will produce snow.  Low pressure development would need to be rapid in order to get heavy enough precipitation in place to cause that changeover to snow.

The GFS shows the pieces for this storm in a bit more detail this morning.  This is a 500 millibar map -- what we call midlevel, showing the various shortwaves of energy (red and orange features) that work through the atmosphere.  The feature that will set up the short-lived 50/50 low is currently positioned over Minnesota and will work through Southern Canada over the next couple of days.  Feature #1, the main ball of energy for this storm, is currently over Southern California, with northern stream energy in features 2 and 3 just off of the Western Canada coast.

The GFS takes piece of energy #2, dives it southeast, and feeds it into #1 on Friday afternoon and evening, resulting in development of heavier precipitation across Texas and Oklahoma as the low intensifies in the Southern Plains.  Wave #3 tracks through Canada and then dives down into the Midwest on Saturday.  We talked about how the 'dig' on this wave as it moved down in the northern jet was critical to help enhance the southern feature and intensify it.  If wave #3 phases and digs too soon, the storm gets too strong and would be the left "goalpost" track of rain in the city, snow farther northwest.  If wave #3 doesn't dig at all or is too progressive, it will keep the southern system just far enough south to miss altogether (the "suppression goalpost").   Assuming a middle, or goldilocks type solution, takes hold, the question then becomes how quickly can the storm intensify to draw colder air in.

Given the lack of stronger high pressure over Canada and given the lack of cold in place at the onset of the storm, we may see a rain to snow scenario in the region, with light rain changing to snow as the storm intensifies.  How much snow we ultimately get, I think, depends on how quick the intensification of this low is and how expansive the precipitation field ultimately gets.  The potential exists for a couple of inches of snow accumulating in some spots to Philly's south if a more southern track takes hold to perhaps several inches if the storm develops rather rapidly and closer to the region.  As the upper features in Western Canada move into better data sampling I think we will see more fine tuning in the models today and especially tonight regarding what ultimately happens.