Saturday, March 23, 2013

Updating Sunday Night & Monday's Storm System

Sunday night into Monday's storm system is still there...and if you don't believe the NAM computer model, by and large your expectations for a cold rain and some snow are still in check.

The bad news for those hoping snow would be the dominant precipitation type is that the storm system is moving in during the morning hours and not completely at night, which means the bulk of precipitation falls during the day. Given our sun angle is post-equinox, solar strength is pretty strong and given the bottom portions of the atmosphere will not be terribly cold to begin with, you need heavy precipitation to get snow through the atmosphere and for it to stick.  There will be precipitation but it may not be heavy enough north and west of the city to do much in terms of stick outside of on the front end and tail end of the event.

The Euro and GFS (farther below) both show what's going on -- low pressure will push into Ohio before running into an atmosphere block as the jet pushes the low then transfers its energy to a second low pressure system off of the Virginia coastline, which pushes on into the Atlantic.

The Euro has waffled around in intensity and has been relatively inconsistent in how quickly the low organizes itself in the Atlantic -- some runs it jacks up the precipitation pretty quickly and in runs like last night's it does not.  The GFS has generally been consistent on the precipitation aspect overall but between the Euro and GFS, you get a pretty good sense of timing and track.

1) Bulk of precipitation is along and south of the Pennsylvania Turnpike and I-195.
2) Some elevated areas will get some accumulating snow -- probably west through just north of the city but that could extend into the hillier parts of Philadelphia.  This looks like a 1-3 type event for them, with coating type accumulations in lower elevated valleys and in the city.
3) It's mostly rain for those of you south and east of I-95.  It may end as snow on Monday evening.
4) Precipitation moves in early Monday morning (pre-dawn) and continues into Monday evening before ending.  During the day, we may see a chunk of this fall as a mix of rain and snow...or in snow areas, not really stick all that effectively.  That's why the accumulation totals that we're talking about here aren't that high overall, with the highest totals likely on the hilltops to our city's west (Chester County on into Lancaster County).

We'll provide another update on this tomorrow, with an updated snowfall map.