Update Tues 07:41 PM— Today’s GFS, GEFS, CMC and ECMWF have become available and continue with the trends discussed this morning:

Snow starts mid morning Saturday instead of before daybreak.  A changeover from snow to rain occurs between 3 and 5 PM Saturday.

The GFS is maintaining 4 inches just west of the city before being tamped down with the rain late in the afternoon. The GEFS (Statistical “Ensemble”) version of the GFS has 2.3  inches.  The Canadian CMC GDPS has 1.8 inches and the European ECMWF has less than an inch. All have a  mix and change to rain 3-5 PM.

It’s important to keep in mind that the models have over-predicted snow accumulation for every storm so far this season.  The storm is still 100+  hours in the future.  We probably can’t make definitive snow forecasts until Friday.  This will be a minor snow at best.

GEFS Snow Depth Forecast for 7 PM Saturday


Update Tue 08:53 AM — The EKDMOS is great for temperature forecasts, but it also gives a good indication of the degree of uncertainty in the coming days’ forecasts.  Notice the increase in “spread” between possible maximum temperatures on Saturday- Sunday (1/18, 1/19) compared to the two days prior—

EKDMOS max Temperature forecast

…From earlier Tues:

Last night’s GFS, GEFS, CMC and ECMWF have become available. Here are the current trends with the storm forecast for Saturday—

All models are showing the snow to start somewhat later now.  Instead of starting before daybreak, the models have it starting about 9-10 AM Saturday. With the later start and daytime warming, warmer air moves in somewhat earlier.

A changeover to a mix may occur sooner around the city and immediate suburbs.   That said, the GFS is still showing 4.2 inches (Ft Washington and KLOM- Blue Bell) Canadian 2.6 and ECMWF 1.2 inches snow before mixing with sleet and rain.  The Canadian has a burst of heavy precip over us late afternoon, resulting in snow falling through dynamic cooling, despite the fact that surface temps will have moved above 32º.

A changeover to rain is expected between 4-6 PM Saturday and precip should end by midnight.  It doesn’t look like cold air rushes in immediately behind this storm.

These numbers are provided simply as a guide.  It’s important to know that this doesn’t appear to be a major storm for the PHL area. Accurate Snow Forecasts more than 24 hours in advance are almost impossible to do in this area.

Areas north and west of Allentown/Reading will have several inches.