The latest GFS data has come in. The GFS has raised the QPF to over 2.50 inches water. It’s a bit colder than the NAM at the surface, but like the NAM, thickness levels rise to be too warm to support snow by daybreak. Slightly more snow possible before a changeover to sleet/rain.
Difficult to nail down the snow totals, but expect it to be in the 4-5 inch range, maybe 6, before a changeover to sleet/rain before daybreak, significantly reducing further snow totals in the immediate PHL area and the immediate surrounding suburbs.
Areas near Reading, Allentown and Lancaster will have the heavy snow that’s been predicted.
A change back to snow will occur before ending about 2 pm, but the heaviest precipitation will have passed us by.
The latest NAM data has become available. There appears to be significant changes in the forecast. (Although consistent with several aspects of past NAM runs.)
The latest QPF values have increased to over 2.0 inches water, BUT temperatures in both the upper and lower atmosphere now appear to be too warm to support snow for much of the height of the storm.
It now appears that snow will begin shortly, but a change to sleet and freezing rain and just plain rain in Philadelphia and the immediate NW suburbs will occur. The change will occur before daybreak and it’s likely that it will be all mixed precipitation, mostly sleet and rain for much of the morning before changing back to snow before ending.
With atmospheric thickness levels as high as predicted by the NAM, and warmer temperatures at the surface, total snowfall may be as little as 4-5 inches in the immediate PHL area and surrounding NW suburbs within 10 miles of the city
I know I’m bucking the hype of the tv forecasts, but this is my call. I think this will be a big rain/sleet storm, not a blizzard in PHL
Major Change in the Forecast
Much of the expected snow will likely fall as sleet and rain in Philadelphia and immediate surrounding suburbs up to 10 miles from the city. Snowfall totals may be only 4-5 inches.
So the storm for tomorrow has fallen into the time frame of the HRRR short range model. I usually find the HRRR very reliable in predicting snow totals. Here is the current snow totals for the HRRR expected by noon on Tuesday:
Here’s my forecast dilemma: I’ve checked the GFS and NAM model data and both show a parameter called “1000-500 mb thickness” to be above the level that supports snow formation from about 5 AM to about 10AM in Philadelphia and the immediate surrounding suburbs.
This “thickness” parameter relates to density of the upper atmosphere and basically captures temperatures in the upper levels of the atmosphere above 18,000 feet. Higher thickness levels generally mean that it’s too warm for snow formation. So I’m having real trouble with the “official” snow forecasts. I expect a significant changeover to sleet and freezing rain during the height of the storm, keeping snow totals below that shown in the above graphic.
I might have egg on my face with this forecast, but that’s why you read this blog. I have to go with my experience on this one. Hey, I may be wrong. It won’t take long to find out.