Those of you who follow this blog know I’ve been wrestling with this forecast for Saturday evening. There have been too many borderline conditions to accurately predict the precipitation type and accumulation amounts.
The latest NAM data has become available. QPF values have been incredibly consistent at about 0.60 inches water. Surface temperatures appear to be above freezing for much of the day and near to above freezing during the storm. Mid-level atmosphere temperatures appear slightly too warm to support snow, while lower temperatures are at or below freezing.
After an initial start as light snow between 4-6 pm, it appears to be a mostly sleet and rain storm in Philadelphia and its immediate suburbs before changing back briefly to snow before ending after midnight.
A cause for error in this forecast might be dynamic cooling due high precipitation rate, which might increase the snow possibility.
Warm surfaces will further reduce accumulations, except on grassy surfaces. A wet, slushy 1 to 3 inches is a best guess on grassy surfaces, less on pavement.
There’s still 24 hours for things to clarify.
11:30 pm – Tonight’s GFS even less impressive QPF and warmer surface temperatures. Mostly a sleet and rain event for PHL and surrounding areas.
Yet another forecast snow bites the dust. The 1 AM runs of the GFS and NAM show the low pressure development Saturday night to be very unimpressive. The track is further south, the intensification is less and the precipitation rate is lower. Surface temperatures appear warmer.
With the current trends, I’m significantly scaling down the snow forecast.
In the immediate PHL area and immediate suburbs,
little accumulation is expected on roads and paved surfaces and perhaps a wet coating on grassy surfaces. It looks to be principally rain mixed with some wet snow here in PHL and the immediate suburbs.
Things may change again. I’ll update this evening.
10 AM Friday Update- Latest NAM again has 0.60 inches QPF , starting about 6 PM. Temperatures in Philadelphia appear just on the borderline too warm for snow at the start, but my concern about dynamic cooling might come into play here again. So I may have to backtrack to last night’s forecast with a few inches of snow, mostly on grassy surfaces in the PHL immediate area!
Sorry for the flips back and forth on the forecast, but this forecast is plagued with boundary conditions that could go either way. That said, this morning’s NAM was looking colder than its previous run. We won’t know until tonight at the earliest, and it may need to wait until Saturday morning for the final handle on things.
The latest NAM model data shows a consistent scenario of low pressure developing along a frontal boundary to our south and east Saturday evening. QPF values still in the 0.60 inch range.
The usual rain-snow line appears to run through Philadelphia. Here’s the difficulties with the snow forecast — temperatures at the surface may initially be above freezing, limiting accumulations, especially on paved surfaces.
With the usual “critical thicknesses” for snow where they are, I would ordinarily be forecasting a mix of rain, sleet and some snow. BUT, with the precipitation rate as high as forecast (0.60 inches/6 hours), I am thinking that dynamic cooling may become a big factor with this event. As a result, I am leaning towards more wet, large flake snow than I would ordinarily forecast. From Philadelphia north and west, 4-5 inches appears possible on grassy surfaces, less on paved surfaces. Areas far north and west may have 6 -7 inches.
Again, I’m counting on the NAM high precipitation rate for dynamic cooling for this to be snow in Phladelphia; otherwise it will fall mostly as a wet mix with significantly lower accumulations.
Precipitation starts about 5-6 pm Saturday and ends about 3 am in the morning Sunday. Whatever falls starts melting Sunday.
Still 48 hours before this starts, so expect changes in the forecast.
The latest NAM data is becoming available. Precipitation, very light, expected to begin about 7 am Wednesday. As I’ve been saying since last night, temperatures in the upper atmosphere will be too warm to support snow in Philadelphia and the immediate adjacent PA counties and south Jersey.
In the lower levels of the atmosphere and at the surface, temperatures remain below freezing until 9-10am in and around PHL, so precipitation will fall as a brief period of light sleet and then light freezing rain, changing to all rain by late morning.
The issue for tomorrow morning will be icy conditions on untreated surfaces, not accumulations. Even a thin coating can be slippery.
One more thing…QPF values are very low in the morning, less than 0.09 inches water. The heavy rain will be during the afternoon.