The last night’s 06Z data has become available- both the NAM and GFS models have increased the QPF values to an average of about 0.50 inches water.
This morning’s NAM data also becoming available- the trend is for heavier QPF, and warmer atmosphere!
The NAM has increased the average temperature of the mid-level of the atmosphere to a level that is too warm for snow formation for much of the storm in Philadelphia and immediate northwest areas, as depicted by this map. (Lower levels of the atmosphere (except at ground level) are at or below freezing, allowing sleet at times.)
Here’s the current best forecast-
Precipitation starts between 4-6 PM today, possibly as light snow or a mix of rain, sleet and snow.
A changeover to rain and rain-mixed with sleet will occur early. Much of what falls will be rain and possibly some sleet; at times, some snow may mix in at peak intensity if dynamic cooling occurs. This event will be mostly wet, not white, in the area depicted south of the red line in the inset map.
Precipitation may changeover to a brief period of light snow before ending about 2 PM.
Accumulations- zero to a coating, mostly on grassy surfaces.
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.
This afternoon’s models continue with the fact that this is a difficult forecast—too many boundary conditions which affect the precipitation type and amount. The GFS has a QPF of only 0.30 inches water, the NAM has a consistent QPF of 0.60 inches water.
Here are the issues making this forecast difficult-
Temperatures during the daytime Saturday are expected to be well above freezing, with the GFS much warmer than the NAM.
Ground temperatures, especially on paved surfaces are slow to drop, reducing accumulations.
Critical temperatures in the lower atmosphere support snow, BUT the models agree that critical thickness levels (a measure of the upper atmosphere temperature and density) are a bit warm to support snow at several points during the 7 hour precipitation window.
As a result, it looks like a mix of snow, sleet or rain is likely in PHL and the immediate suburbs. So what falls from the ground may be snow, but it may switch over to sleet and rain and then end as snow. Add the warm ground temperatures to the mix and it’s truly not possible to predict accumulations.
More specifically, it looks like it starts as snow about 4 PM, but I think it will mix with sleet and freezing rain for a portion of the duration, then switching back to snow before ending about 2 AM Sunday. Most accumulations will be on non-paved surfaces.
Areas in northern Montgomery and Bucks county will likely have more snow.
Temperatures are key- the next model run with the data I need will be available about 9:45 PM.
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.