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.

The models have been consistently predicting the development of a low pressure system, moving in from the south, during the Tuesday time-frame.

Over the past few models runs, the timing of the onset of precipitation has moved up to Monday afternoon, with the eventual track of the storm just west of our area, allowing a southerly wind to bring temperatures up for rain.

 Sat 6 PM: The latest NAM has the snow starting Monday morning.  Several inches of snow possible by Monday evening.  While the NAM often overestimates QPF in this timeframe, based on the NAM QPF it looks  like  3-4 inches possible in PHL and more north and west.  The biggest concern is the possibility of an extended period of freezing rain Monday evening.

Sat 11pm: the NAM has backed off of its high QPF and it now is similar to the GFS.  The NAM still has snow starting earlier, in the late morning.  The model’s QPF before a change to sleet  around 7 pm is 0.28 (NAM) and 0.19 (GFS).   So 2-3 inches of snow by evening.  Both models are predicting a rapid warmup to occur in the evening, but sometimes this is  predicted to occur too quickly.

Various elements of this storm have changed recently.

• The onset is earlier on Monday, allowing the precip to start as snow.  The models (as always) vary with QPF values, but the NAM has ~0.40 inches and the GFS ~0.25 inches water falling as snow before a changeover to sleet and freezing rain.  So snowfall amounts by Monday evening (before the changeover) may be as much as 3-4 inches, and more north and west of the city.
• The changeover occurs sometime Monday evening or night, to sleet and possibly a period of freezing rain in some northern and western areas.  Changeover in south Jersey will be sooner.
• The total precip, originally expected to be almost two inches of water, has diminished in the current model runs to half of that, suggesting the modeling of this storm is not final.

By daybreak Tuesday, the precip should be all rain, assuming no further change in the track of the storm.  Previous model runs suggested a changeover back to snow at the end, but that is less clear.