Tuesday, February 12, 2013

Timing Out Tomorrow's Snow...or Rain...or Both!

We're still holding to our forecast accumulation thoughts into this evening but I fully reserve the right to revise these tomorrow morning based on how modeling comes through tonight.  That said, the dynamics at work (or not at work) could lead to a few things going wrong tomorrow night.

One, dynamics not coming together perfectly.  The most recent run of the GFS from late this afternoon shows that scenario at work -- a low pressure center that tracks a bit farther north, not intensifying quick enough, bring rain that is slow to convert over to snow.  Notice that on the GFS run the freezing line aloft is across South Jersey and brushing against Philadelphia, with the surface freezing line farther to the northwest.  Such a  scenario does paint a couple of inches of snow, mainly to the north and northwest of the city, but leaves South Jersey and Delaware dealing mostly with rain that ends with snow.

Other scenarios, such as the NAM model are more favorable for snow.  Notice the freezing line aloft is much farther southeast as precip begins -- as opposed to being along I-95 at the height of the event, the freezing line is down near the coast and will drive farther away.  The NAM depiction is colder, a bit farther southeast with the low, and thus more favorable for snow.  The NAM is the happy medium between the GFS' northwest track (and more consistent look) and the Euro's far southeast suppressed and not a lot of anything look.  The NAM and GFS have both been relatively consistent in their look, with the Euro really struggling to maintain any sense of consistency with track and detail on this system.

The details of the system are such that we'll likely start as rain since temperatures during the afternoon will be around or above 40 at the onset of precipitation between 4 and 7 PM.  It will fall mainly as rain on the front end for most of us, with some snow or rain mixed across hills to the west over Lancaster County.  That changeover will take place during the evening hours, likely between 7 and 9 PM...HOWEVER, if the GFS is right that changeover is slower to take place because the system isn't as dynamic and surface cold slower to take over.  We're leaning towards the changeover taking place sometime in the 7 to 9 timeframe, for now, but that could change.

The key thing is that your PM commute, while damp in spots, should be mostly snow free...and even if snow starts to fall towards the tail end of the commute, roads should remain wet.  We're still watching for the potential of some banding where higher accumulations could set up overnight...and that will determine who gets the higher accumulations.

We'll have another update tomorrow morning.