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.

I’ve been thinking that the Tuesday snow from the ‘secondary low’ wouldn’t amount to much (1-3inches) but the latest NAM and GFS data is available and both models crank out 0.50 and 0.54 inches respectively, as the QPF.

That ordinarily translates to about 5-7 inches of snow, but the models have it coming is dribs and drabs over a 30 hour period from late Monday night into Tuesday night and I’ve seen those sort of things over-done with other storms.

4:45pm: Based on this afternoon’s NAM, I’m still thinking 1-3 inches total by Tuesday eve, despite higher model QPF values.

That said, the words to keep in mind are “low confidence” and “notoriously unpredictable” when it comes to a secondary low precipitation QPF.   Not only is the QPF unreliable, but it is especially so for any particular location.

So, according to the latest,  the snow starts after midnight early Tuesday morning, we wake up with about 2-3 inches of snow Tuesday and light snow continues throughout the day into the evening.   The temperatures support snow instead of rain or a mix.    I’ll update again this evening.

An update on the potential snow for Monday through Wednesday.

The models have come together in having the southeast coastal storm pass east of us, missing us early Monday.

The secondary low expected to develop late Monday into Tuesday looks weak and diffuse.  Exact location of development and the amount of precipitation is uncertain, but looks to be light, at most, 1-3 inches, according to the NAM. The GFS cranks out much more, with a QPF of 0.48 inches water, but I think it’s overdone.