A quick update on the snow for Friday morning.
There are large differences between the NAM and GFS models which is making the forecast to be one of lower than average confidence.
Basically, the GFS now has increased its QPF value to over 0.55 inches water, starting well before daybreak. The NAM has less than 0.20 inches and doesn’t start until after 7 am.
Will need to wait for tonight’s models to clarify.
Thursday AM update: Last night’s 1 AM run of the GFS model shows some interesting trends- a significant increase in the QPF to over 0.40 inches water and a somewhat earlier start. The GFS is running colder too. The NAM also showed a change, staying at a lower QPF of 0.15 inches water, but now starting later, after 7AM.
This looks like it might be a tougher forecast than previously thought. Will have to wait until tonight’s model runs to pin this down.
Today’s model runs continued with the likelihood of snow starting during the hours before daybreak Friday and continuing into the morning.
Some of the precipitation before daybreak will fall as rain mixed with snow, but a change to all snow is likely by or before daybreak Friday. The NAM has rejoined the GFS in having snow. QPF values have been in the 0.10- 0.25 inch water amount.
(Tonight’s NAM model just became available and has reduced the QPF value to 0.15).
As discussed in my post from Tuesday, predicting accumulations in March with such low QPF values is really an academic exercise.
Surface air temperatures may not fall to 32 and will rise during the morning.
That said, it appears that a coating is possible at daybreak Friday, mostly on grassy surfaces. Not much more than that will occur.
Driving conditions will be minimally impacted in the immediate PHL area and surrounding suburbs, as little is expected to stick on roadways, especially after daylight returns.
Temperatures are expected to plummet later Friday afternoon and nighttime.
Another post from over a week ago suggested a possible major snowstorm for the period around Monday March 13th. That scenario seemed to fade, but the latest models are again suggesting the possibility of a significant coastal snowstorm on Tuesday March 14th.
Continuity of the model forecasts have been poor this season, but the situation next Tuesday needs to be watched.
I’ll update Friday’s forecast Thursday night, about 9:45 PM.
Wednesday AM Update: Last night’s NAM has backed off on the snow quite a bit, with much of the precipitation moving past us to our north. The GFS continues with light precip early Friday morning. Will have to wait on this forecast .
Sunday’s potential forecast snow has also changed, with a more definitive movement to our south, missing us.
The newest possible storm is Tuesday to Wednesday timeframe. As mentioned many times this season, the lack of model continuity lately has made forecasting beyond a two day timeframe very difficult.
Some snow is looking likely for late Thursday night into Friday morning as a disturbance rides along a stalled frontal boundary.
Despite much recent model run inconsistency, the forecast for this early Friday period has become more consistent
The models are cranking out about 0.30 inches of water falling as snow. It’s extremely difficult to predict accumulations in March, as solar insolation through clouds tends to melt all but the heaviest snows on dark asphalt roadways.
If this were January, we would be talking about 3-4 inches but in March, snow accumulation discussions become more academic with “grassy” surface vs road accumulations. Ground temperatures are starting very warm, further reducing accumulation potential. That said, expect some accumulation at daybreak (due to the onset occurring at night) and messy driving during the morning commute. A coating to an inch, maybe two, is possible.
Looking ahead, a storm on Sunday is expected to keep much of its snow in Maryland and Virginia, but that could change.
Another potential snowfall is possible next Tuesday night.
Stay tuned.