GFS Update 10:40pm
Tonight’s GFS paints a very different picture than the NAM. It has a QPF 0.35 inches water and a colder profile, maintaining snow through 10am. That would amount to 3-4 inches of snow before a changeover to sleet later in the morning.

I’m staying with the NAM forecast described below, but this GFS difference makes this a low confidence forecast.

Things continue to evolve with the forecast. Tonight’s NAM and NBM data just became available.  QPF values of less than 0.08 from now until the changeover to sleet about 7 AM is expected to limit snow accumulations to a coating or less than 1 inch.

Temperatures at the surface will be marginally cold enough to prevent much if any accumulation in the city proper, except on some surfaces.

North and west of the city, temperatures are forecast to be 31-32, but the increase in precipitation rate about 7-8 AM will correspond with the changeover to sleet and some freezing rain.  Some accumulation of sleet is possible.  Larger accumulations of sleet and snow will be an issue primarily in the far northwest suburbs. 

This will be a messy storm, but most of the precipitation will eventually fall as rain in most areas.. A mix of precipitation is possible until mid to late morning, then rain.

The GFS will be available in an hour or so. I’ll update if the GFS is significantly different.


Late Afternoon Update
A quick look at the afternoon NAM and HRRR suggests the snow will be very light tonight.  The NBM has snow starting about 8-9 PM, but the latest NAM holds it back even later.

The HRRR and RAP have about 1-2 inches of snow by daybreak here, before the changeover to sleet and some freezing rain about 7 AM. The sleet may be heavy for about 2 hours, then heavy rain.  As for wintery precipitation, this storm will have the biggest impact in the (far) northwest suburbs.  I’ll update this evening. 

from earlier today:

The latest NAM and GFS data has become available.  Both models have returned to their earlier predictions of having the precipitation suppressed to our south for the balance of the day by cold air that is building in from the northeast.  So cancel that snow for the daytime hours.

Both models maintain a very cold low level flow from the northeast, while upper winds bring warmer temperatures aloft.  A complicated forecast.

For Philadelphia and immediate suburbs:   Both models have the precipitation moving back to the north Monday evening.   The NAM and GFS has snow returning northward to our area between 8 and 10 PM tonight.   Depending upon your model of choice, about 2-4 additional inches of snow will fall tonight before 5-7 AM Tuesday.

Temperatures at critical levels of the atmosphere will rise and cause the precipitation to mix with and fall as sleet.  This should happen between 5 and 7 AM Tuesday morning, but somewhat later in the far northwest suburbs. 

Sleet will fall in the early morning and mix with and change to freezing rain, and then all rain by about noon.   There is the possibility that this transition to rain may be delayed, especially in the far northwest suburbs.

Heavy rain for the balance of Tuesday.  Total QPF for this storm will be about 2.0 inches of water, falling as snow, sleet and rain as outlined above.

Updates later this evening.


Things have changed…..So, we had the approximate predicted snowfall last night, with snow coverage further north than predicted. However, it doesn’t  appear to be ending early morning, as has been predicted last night.

While we slept, the 1 AM run of the models changed their tune about a persistent band of snow that originally was predicted to be pushed to our south.   It has now set up shop over us.

Radar as of 7 AM Monday morning, courtesy of

Some of the statistical models suggested this yesterday, but most missed it.

An additional QPF (based on the 1 AM runs) of 0.12 inches water may fall as additional snow today.  So an additional 1.5 inches of snow is possible during the daytime hours on top of what we have.

The radar image above shows the general area where the snow will fall today.

Things will change tonight with the approach of the main low pressure system.  Already, the 1AM model runs suggest additional warming in the upper atmosphere that was not predicted earlier.

I’ll update later this morning, after the latest model runs have become available.