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

Friday Weather Update

This morning’s models continue showing precipitation starting Friday morning about 8AM  and ending Friday evening.

Temperatures at the surface will be around freezing (the MOS lately has not been very accurate with temperatures).  Temperatures in the lower levels of the atmosphere will support snow through early afternoon.  Temperatures in the upper atmosphere just marginally support snow.  QPF values have increased to over 0.65+ inches for both the NAM and GFS.   With heavier precipitation rates, dynamic cooling might make temps a bit colder.

Areas to the north and west of Philadelphia are more likely to have snow accumulate.

So a messy day for Friday.  Right now it looks like much of the day it will fall as wet snow, but accumulations will be limited, especially on dark asphalt roadways.  A change to rain is likely after 3 PM.

Everything is right on the edge of being a rain event, so it would not take much for the expected snow to fall as a wet mix or just rain.  Right now, I’m betting on it coming down as snow, likely changing to rain during the mid afternoon on Friday.

Winter Weather Update

Both the NAM and GFS have low pressure moving along a frontal boundary to our south and intensifying somewhat as it moves over the coast Friday.

Thermal profiles in both models support snow, however temperatures at the surface will be close to 32.  With QPF values of about 0.45 inches water, this would be a significant storm IF it were winter.

However, it’s March, the sun angle is high and insolation through the clouds challenges accumulation on roadways.  The snow will be occuring,  not a night, but during the morning and afternoon.   So  accumulation on grassy surfaces and sidewalks is possible,  but accumulations on roadways will be limited and short-lived.

Too many factors to really be able to predict accumulations accurately.  Best guess is an inch or two on grassy surfaces,  and a slushy coating on roadways.   The precipitation may be heavy during the afternoon, so visibilities while driving may be poor.

This time of the year, weather forecasting has its lowest accuracy, so changes are likely.

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