The latest NAM data has become available. Here are the trends:
The precipitation moves in earlier, now beginning between 3 and 5 PM. The NAM is showing a warming trend in all levels of the atmosphere that will extend further north than previously predicted.
Precipitation possibly starts as snow but quickly mixes with sleet and rain and changes to all rain by 7 PM in Philadelphia and by 9 PM in the northern and western suburbs. A coating to less than an inch of accumulation in and around Philadelphia followed by heavy rain Saturday night. Total QPF values about 1.90 inches water.
The arctic front moves through about 8 AM Sunday morning. Some changeover to snow showers possible, but little or no accumulation in and around Philadelphia. High winds and gusts. Temperatures rapidly plunge to below freezing by 1 PM and into the teens by evening.
With the high winds, open roadways may dry but areas of pooled water or drainage will quickly freeze by afternoon.
So this is really looking like just a heavy rain storm and a sharp cold front for Philadelphia and the immediate areas.
Too warm for snow tonight and the NAM has reduced the QPF in Phladelphia to zero.
First, will the Eagles win again with Foles? Yes.
That said, let’s turn to tonight’s forecast which may be a more difficult call. A front will pass through tonight between midnight and daybreak. Weak low pressure is predicted to move along the front. Each model run has increased the intensity of this wave of low pressure.
The models are consistent with low temperatures being above freezing (36 degrees in Blue Bell). The latest NAM has several critical temperatures of the atmosphere supporting snow, except for the lowest level. QPF values are now 0.07 inches water. Currently the models have a general consensus of light snow with a slight grassy coating in the areas in blue –
If there is further strengthening of the low, dynamic cooling may move this area of snow closer to the city. Right now, the areas shown are the best bet, but it wouldn’t surprise me if there’s some very light snow falling closer to the city. Since it will occur before daybreak, we’ll have to see if anything is left on grassy surfaces. This is the best we can do this year for a white Christmas.
Of more interest is the period around New Years Day. For almost a week, the GFS and FV3-GFS have been showing some sort of major storm (rain, ice or snow) in the Jan 1-2 timeframe. Something I’m watching.
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.