The latest NAM data is coming in as I type this. Looking at the trends-

QPF values have stabilized at about 1.00 inches water.

Temperatures at critical levels levels of the atmosphere decrease enough to support a changeover to snow about 9-11am in Philadelphia, earlier north and west of the city.

Prior to the changeover to snow, expect continued sleet and freezing rain.  So snow changeover may be a bit later than previously thought .

Based on the actual QPF expected to fall as snow and factoring in March sun angle through clouds, I’m thinking that about 6-9 inches fall in Philadelphia and immediate suburbs, 10-12 in far northwest suburbs.

Winds will be high at 30 -35 mph with higher gusts.  

So, the snow transition starts a bit later here and QPF values have decreased somewhat and winds remain a big problem.

important update 11:30pm. Correct winds to 20-25 mph with higher gusts. Reduce snowfall to 4-7 inches in Philadelphia  based on GFS NAM blend 


I’ve had a chance to look at the morning’s model data. As expected, the mid levels of the atmosphere were too warm to support snow in and around PHL today.  We did have some sleet as the rain fell through colder lower levels and that could continue into the evening.

According to this morning’s profiles, the thermal profiles chill down to support snow about 6-9 am Wednesday morning, earlier north and west. Current NAM QPF is about 1.05 inches water, which in winter,  would be over a foot of snow!   In March, with all sorts of magical guessing, I can see as much as 9 inches of snow on grassy surfaces, with less accumulating on roadways by Wednesday evening.  So snow starts In the morning and continues through the day into evening on Wednesday, 

That’s my current call.  The deciding model run data becomes available between 10pm and 10:40 tonight. I’ll update then.

5 pm update- this afternoon’s NAM (“off-hour” model runs not always reliable) has increased QPF to 1.65 inches water falling as snow.  The trend is increased snow totals, possibly over 12 inches.  Have to wait for tonight’s NAM run.