This morning’s models continue to change regarding placement of maximum precipitation bands, with the general trend for lower amounts of wrap around snow Friday afternoon in Philadelphia. Most of the action will be north of our area. I expect things to continue to change. Stay tuned to tonight’s forecast, about 9:30 pm
SNOW, NOT NECESSARILY ACCUMULATING, ON FRIDAY
The 1 AM runs of the models (06 Z) show some differences with the earlier evening runs regarding the snow for Friday. The GFS has much less wrap-around moisture than the NAM. This run of the NAM delays the changeover to later in the morning or earlier afternoon.
That said, wind-driven rain will change over to wet snow by afternoon, even in Philadelphia. However, falling snow does NOT necessarily translate into accumulating snow. Ground temperatures will be above freezing, as will surface air temperatures. Solar insolation through clouds is high in March, further reducing the chance of accumulations on roadways and dark surfaces.
So a slushy accumulation at most on roads expected. The NAM has light snow into the evening, where some roadways and sidewalks may get some accumulation, if this really occurs.
Won’t attempt actual accumulation totals with this storm- meaningless in this scenario at the current time.
SIGNIFICANT WET SNOW POSSIBLE FRIDAY
Tonight’s NAM data has just become available. Details of the coastal storm continue to change, but some aspects have clarified somewhat.
Here’s the current outlook: Rain starts on Thursday late afternoon, becomes very heavy Thursday evening and continues through the night. Rain continues into Friday daybreak, where things then get interesting—
Temperatures in the critical levels of the atmosphere drop about 8-10 AM Friday morning, with a changeover to wet snow in Philadelphia which may become heavy during the afternoon and last into the evening.
March snowfall totals are exceedingly difficult to pin down.
Precipitation may fall as heavy wet snow, but falling during daylight hours in March with ground temperatures above freezing and air temperatures near the surface at or just above freezing will make accumulations difficult to occur. So accumulations on roadways will likely only occur if it extends into the evening.
That said, the storm may look impressive from the window with wind driven wet snow, but roadways will likely remain wet and slushy for much of the daytime hours.
Still too soon to pin down specifics, but current QPF during the snow chill down is an impressive 0.75 inches water. Stay tuned.
Below is a meteogram showing temperatures at critical levels falling below freezing about 10 AM Friday— (Philadelphia)
