Based on this afternoon’s model runs, (yes, the “off-hour” runs that tend to be a bit off), a mix of precip should start between 6 and 8 PM.
The NAM graphic below still is reasonably close to where rain vs a rain-sleet mix will fall, just north and west of the city.
Most models have the changeover to all rain in the immediate area by about 9 PM. I’m interested to see if that occurs, because there’s a layer of cold air at 3000-4000 feet that is predicted to remain below freezing until midnight. Anyhow, it will change to all rain eventually. Latest QPF is back to over 0.40 inches water.
from earlier today…
The latest NAM and GFS model data has become available. QPF values have reduced on the NAM to about 0.36 inches water and 0.43 inches water on the GFS for gridpoint Philadelphia.
NAM thermal profiles (which I prefer) are borderline cold for sleet or a mix of sleet and rain when the precipitation starts. It then changes to all rain by midnight. No accumulation expected in or around Philadelphia.
The GFS is warmer and somewhat wetter. The GFS has no sleet for our area.
So this is a lower confidence forecast for sleet.
As for the start time, here is a NAM-NEST graphic for precipitation moving in from the west about 8:30 PM. Notice that the precip may start far north and far south of Philadelphia, before it fills in near Philadelphia.
(Sometimes these ‘holes’ in the precipitation field are artifacts, but several model runs have showed this.) I guess we’ll see what happens.
With no accumulation expected, this is really a non-event from a winter weather perspective
I’ll soon be turning my attention to the well-advertised storm expected for Wednesday.
As we can see, even with tonight’s rain storm, predicted QPF values can change over 12 hours, as can thermal profiles, especially with these transitional storms. So it’s really just speculation regarding the specifics until we’re in the 24 hour time frame.