The model depiction of the small low pressure system expected to affect us on Thursday night into Friday continues to evolve.  The NAM and GFS differ on timing and coastal development.  The GFS has a more substantial coastal storm developing on Friday and coming very close to PHL, but the heasiest precipitation shield still misses us.

It should be noted that the degree of coastal development and the degree of upper air troughing continues to change, with the trends leaning to the possibility of more snow than the currently expected 1 to 2 inches.

Put another way, it wouldn’t take much change for the snow in our area to be more significant.  It also wouldn’t take much change for it to be just a period of snow showers.

To give perspective on the model’s continuity, tonight’s rainfall, originally expected to be quite heavy, doesn’t appear that it will be be very significant.  So confidence in the forecast for Friday continues to be lower than usual.

11 pm –tonight’s GFS has increased the QPF to 0.33 inches water. The NAM has reduced it to 0.07.  Large model differences!

The models have been showing a fast moving low pressure system moving to our south Thursday evening and redeveloping off the coast as it moves eastward Friday morning.  Snow would start Thursday evening  and end early Friday morning

Accumulations appear to be light at the current time from this system, but the trend has been for increasing amounts.  There is still uncertainty regarding this potential snowfall.  Stay tuned.

Tues AM: Last night’s models has a QPF of 0.14 inches water for Thursday evening into daybreak Friday morning.   That’s about 1-2 inches of snow accumulation by Friday morning.

Had this amount of QPF fallen during daylight hours, there would be even less accumulation due to solar insolation from the March sun, but because it is falling at night at the lowest diurnal temperature range, the 1, maybe 2 inches is a possibility,

The GFS continues with snow showers during the daytime Friday, but little additional accumulation from these is expected.

There is still uncertainty about the specifics with this low pressure system, which should become clarified over the next several days.  (To give a sense of the uncertainty, tonight’s expected rainfall was supposed to be heavy, but the latest models have little QPF.)

Better agreement on the models for Monday through Tuesday.

Light snow starts about noon on Monday.  About 2, maybe 3, inches of snow by late afternoon, then a shift to sleet and freezing rain around 5 PM.   (Total QPF on the NAM is 0.26 inches water during this period)

Here’s where it gets tricky, especially north and west of Philadelphia- there may be a period of sleet and freezing rain early evening Monday, with possibly several hours of sleet and freezing rain north and west of the city.  The models often rush the warmup of surface temperatures, so driving conditions could get very difficult for several hours early Monday evening and during the night, north and west of PHL.

A definite changeover to all rain late evening or night on Monday.   Tuesday- heavy rain mid day, ending towards evening.  High 51!

Update Sunday 6pm:  This afternoon’s model runs continue the general trend of snow starting in the morning Monday.  The NAM QPF is somewhat higher again, over 0.30 inches water during the “snow phase”. The GFS and NAM suggest an earlier start, perhaps early morning.

The issue won’t be snow accumulation, which still remains about 2-3 inches.  The concern still remains that surface temperatures remain at freezing at PHL until 6-7 pm while the upper atmosphere warms, suggesting a possibly extended period of sleet and freezing rain during the evening rush hour. Areas north and west could have this mix last longer through late evening.

I’ll update with the next model run.   The NAM model does best with this and the NAM FOUS data becomes available about 9:35pm or so.