Update

Latest NAM model data in.  Here is the trend.   QPF values remain about the same at 0.67 inches water, but the heaviest preciptation has shifted to the afternoon and lingers later into the evening.

With the heaviest precip occuring during the warmer afternoon hours, somewhat lower snow totals in the city and the the immediate suburbs.  Two, possibly four inches of snow on grassy surfaces in the immediate suburbs, less in the city.  Less on roadways.   Up to 6 inches in far northwestern suburbs.  Some mixing with rain possible late afternoon.  Precip ends about 9 pm

This snow will look worse from your window than it will really be.   A messy day, weatherwise.

Friday Winter Weather Update

Latest afternoon model runs maintain the likelihood of snow starting around 8 AM and ending between 6 and 8 PM.

Here are the model trends: Both the NAM and GFS have a QPF of  0.65 inches water equivalent.  The NAM (which did so well with the last storm) has trended colder, with the heavier precip in the afternoon. The GFS is still a bit warmer, but still supports snow.

4 inches  (maybe 5) of snow just outside of the city is likely on grassy and shaded surfaces.  Significantly less on dark asphalt roadways.

With the change to Daylight Savings Time, the next NAM data will become available after 10 PM with final data about 10:40 PM.  The GFS early data doesn’t start rolling out until 11:25 PM DST.  (That’s an hour later for us  than winter forecasts on Eastern standard time.)

I’ll update about 10:45 PM

Friday Winter Weather Update

Last night’s models continued with the following trends:

QPF values continue to increase with each model run.  The current NAM (the NAM was excellent with our last snowfall) continues with almost 0.80 inches water equivalent.   All thermal parameters from the NAM FOUS data support precipitation as snow in PHL, and certainly north and west;   Areas to the north and west should be thermally colder, but will likely have somewhat less QPF values.

The only ‘piece of the puzzle’ less than perfect for snow is the 500 mb thickness level which suggests that the upper levels of the atmosphere are just a hair too warm.  But I’ve seen higher thickness levels support snow in March.

So a heavy, wet snow it is, unless the models change.

If this were a January snowstorm, or a storm at night,  we’d be talking about 8 inches of snow.  BUT, things play out differently during the daytime in March with the sun angle so high behind the clouds.  Accumulation on dark paved roadways is challenged and short-lived.  Temperatures at the surface will hover around 32.  So 4-6  inches is possible just outside of the city, mostly on grassy surfaces and shaded areas.  That amount is just a guess.  Frankly there are too many parameters here to accurately predict accumulations.  Any accumulations will rapidly melt in our next day of sunshine.

Update

Latest NAM model data just became available.  (Available an hour later due to daylight savings time. )

Thermal profile for PHL still supporting snow, but marginally.  Latest QPF has increased to 0.75 inches water equivalent.  Starts about 8am,  ends about 6 pm

Storm Forecasts & Other Things "up in the air".