I’ve had a chance to review the 2 PM run of the NAM model as well as the latest HRRR (High Resolution Rapid Refresh) which is derived from the NAM.  Here’s what I’m seeing:

The QPF values continue to increase, now almost 1.80 inches water.  However, the upper atmosphere is expected to warm up to levels that typically don’t support snow, not only in Philadelphia, but also in the immediate suburbs to the north and west of the city.

(Areas near Reading, Allentown, and  Lancaster will have all snow.)

For a large period of time with the heaviest precipitation, from 5 AM to 9AM, I think the precipitation may fall as a mix of sleet and freezing rain instead of snow.   I believe that this will significantly reduce the snow totals in Philadelphia and the immediate suburbs to the north and west.  Even areas as far north as Trenton, NJ and Doylestown will have the same issue with freezing rain/sleet instead of snow.

It will turn back to snow before ending around 1 PM, but the heaviest precipitation will have passed by.

With snow going to sleet and freezing rain, and with high winds,  it won’t any less of a serious storm, but I’m beginning to think that snowfall totals in Philadelphia and the immediate surrounding suburbs to the north and west may only be 7-10 inches by the time the precipitation tapers down in the early afternoon. 

Areas south and east of the city will get significantly less.

The afternoon runs of the NAM can be fickle, so we really have to wait for the next model run. The next NAM model data becomes available after 10PM and I’ll update this site by 10:30 PM.

Mon 11:15AM update: Latest NAM model output has increased the QPF further to over 1.70 inches water!

Other complication is the influx of warm air in the upper atmosphere, which suggests a changeover to sleet and freezing rain for several hours around daybreak and into the morning in Philadelphia, which will reduce accumulations .   If this is all snow, the expected snowfall is now around 15-20 inches, but if there is the changeover to sleet, 10-12  is a good guess in Philadelphia.

[stbpro id=”alert” caption=”Current Snowstorm Predictions 11:15 AM”]In Philadelphia 10-12 inches, areas outside the city (northwest suburbs) 15-20 inches[/stbpro]

Last night’s 2AM runs of the NAM and GFS models were remarkably similar and it’s rare to see such similarity about an upcoming storm.   This means there’s an extremely high confidence in the severity of this storm.

QPF values have significantly increased with the NAM having about 1.55 inches water and the the GFS having 1.64 inches water.

The key features: Both models have a period of warmer air intruding at the upper levels of the atmosphere between 6 AM and 10 AM, while the lower levels are just at or below freezing.

This might allow a period of freezing rain and sleet to develop for that time period, reducing snow totals due to compaction.  Total snowfall totals, extremely difficult to estimate, but best guess is 10-15 inches, depending upon how long the period of sleet/freezing rain lasts.  

Areas outside of PHL and its immediate counties to the north and west will have less sleet and rain and potentially 17-18 inches.

Both models show a return to all snow, heavy at times, before tapering and ending in the early to mid afternoon.  This will likely be a heavy, wet snow with high winds, possibly causing power failures.

I’ll update early afternoon if things change considerably.  Count on an update about 10:30 PM tonight.