Forecast Update Sun 8AM —  Quick update.   Snow starts between 2PM and 4PM this afternoon from south to north.  Accumulations 9-12 inches from Philadelphia, including much of the city and northward.   Higher (12+) accumulations northern Montgomery and Bucks and Lehigh  counties.  I’ll have more info based on the morning’s models later .   Stay tuned. 

Next Update about 1PM Sunday

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Forecast Update Sat 6 PM— This remains a high uncertainty, low confidence forecast. 

The NAM group of models has a significant mix with sleet and rain, but maintains surface temperatures below freezing in most of Philadelphia. More freezing rain.  The NAM was wrong in the same direction this past week and its snow totals are running 2 inches or more less than the GFS model (shown just below).

 

The GFS group of models is warmer at the surface, but also has a rain sleet snow mix for almost 8 hours from midnight Sunday through part of Monday morning.  Despite a QPF of 1.5 inches of water, it cranks out only about 6 inches of snow (Blue Bell).

 GFS snow totals based on latest model run this afternoon—

 

The Canadian models have 2-3 inches more snow than the above.  The NAM 2-3 inches less!

 

Look for an new update Sunday morning!

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from Sat morning…

This remains a complicated forecast.  There are uncertainties about the low pressure system track, timing and degree of intensification.    All of these uncertainties affect the amount of moisture falling as snow, the temperature profile (meaning 3-D profile) and the duration of the storm.

The general sequence has not changed.  A “warm air advection type snow (warm  front)  will occur Sunday afternoon where warm air rides above a cold layer at the surface.  About 1 ” of snow is possible by early Sunday evening in most areas. 

Meanwhile, a secondary low pressure system will develop along the coastline and will move slowly northeastward, being blocked by a high pressure ridge in the mid Atlantic.  This will result in additional snow, lasting into late Monday night!

The main complication is the amount of warm air at 3000-6000 feet that moves in with the initial warm front and the subsequent development of the coastal low bringing those same 3000-6000 ft temperatures below freezing.

Further complicating the picture is the development of the secondary low circulation that is multilobular with at least two circulation centers—

Switching to the model blend (NBM), which is designed to take most available models and statistically weight/blend them based on history and hourly changes, it shows mixed precipitation Monday morning with surface temperatures above freezing—

This mixed precip will further complicate accuracy of snowfall prediction.

All models have the mix changing back to all snow during the afternoon and evening Monday. 

The current European (ECMWF) model has the following snow totals—

 

My own snow totals, based on variation of the NBM 50 percentile snow accumulation—

I expect changes as the storm time horizon falls into the range of more short range models.  Stay tuned.

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Just added Sat 10 AM: NAM PTYPE with 32º isotherms at critical 3 D levels