The arctic front has moved through the area and very cold air will filter in for the weekend and much of next week.

Saturday will be sunny but will also  have some instability cloudiness, especially during the late morning and mid day hours.  It will be windy and cold.  High temperatures near 31.

Sunday will be similar to Saturday with windy conditions and some periods of instability stratocumulus, especially mid-day. High temperatures near 35.

Following a cold weekend, attention will be turning to a strong coastal low pressure system that is expected to develop late Monday night from an upper air disturbance dropping down from the northwest and interacting with an inverted coastal front off the southeastern coastline.   This potential system was remarkably predicted by the GFS model as much as two weeks ago.

If things continue to play out, this system will be the most intense to affect our area this entire winter.   The current question is how close to the coast will it develop; too close and we will have mostly a mix of snow and  heavy rain, with high winds.   Slightly off-shore and we might have a major snowstorm.  The latest GFS model favors a large snowstorm, with up to 15 inches of snow around PHL.

That said, the devil is in the details and the long range experts at the NWS feel this one is going to be tough one to predict accurately, even in the short range.

This storm will differ from today’s snow maker.   The ground temperatures will be cold and the QPF values will be much higher.  The March sun angle is an issue, but a large portion of the snow may occur before daybreak on Tuesday.     Stay tuned.

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.