Daily Archives: Tuesday, December 15, 2020 9:56 pm

Winter Weather

SNOWSTORM UPDATE

The most important weather model runs, based on global weather balloon (“radiosonde”) upper air measurements, are done twice a day— at 7 PM EST (00 UTC, also called 00z)  and 7 AM EST (12 UTC, also called 12z).

These new upper air measurements often account for any significant change in the model forecasts. The latest NAM-NEST has a decrease in snow totals due to significantly more sleet.

Here’s the latest 00z model data, based on these new upper air measurements.

The 00z HRRR (High Resolution Rapid Refresh) shows snow developing between 12 noon and 2 PM Wednesday and ending 7-10 AM Thursday. It will become very WINDY Wednesday night.

The HRRR continues the trend for sleet to mix in with the snow in a large portion of the area during Wednesday evening, and then change back to snow before ending.

HRRR sleet, rain snow precip type.  9 PM Wednesday  (Click on image for a larger view.)

 

Despite the changeover to sleet at times, the latest HRRR has increased snow totals by Thursday morning—

HRRR snow total forecast  (Click on image for a larger view.)

 

The 00z NBM (Model Blend) shows a similar snow total forecast—

NBM Snow Totals  (Click on image for a larger view.)

 

Several models are showing wind gusts in excess of 50 mph Wednesday evening.

Speaking of model forecast changes, the NAM NEST just available has more sleet and much less snow—

NAM NEST Snow  (Click on image for a larger view.)

I think the NAM-NEST may be too warm with too much sleet.   We’ll have to see the trends into tomorrow morning.

Right now I’m sticking with the HRRR and NBM snow forecasts above.

Updates

A QUICK SNOW UPDATE

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Tue 04:31 PM Update — This afternoon’s NAM captures the expected decrease in total final accumulation (3-5 inches in the PHL area immediate suburbs) due to a changeover to sleet and even rain for several hours during Wednesday evening.  (It will change back to snow)—

NAM Precipitation Type at 11 PM Wednesday  (Click on image for a larger view.)

 

NAM Snow totals by Thursday morning (Click on image for a larger view.)

I’ll be doing an update around 10 PM this evening with the latest model info.

Another update: here’s the latest ECMWF (European) snow total forecast:

European (ECMWF) snow total forecast  (Click on image for a larger view.)

from earlier today…

This morning’s later models (Canadian HRDPS and Regional GEM, GFS) continue with the trend of the NAM and ICON models from last night and this morning— of having more warm air aloft in the PHL area and immediate western suburbs reducing forecast snow totals

No time right now to provide a graphic, but snow totals have reduced to 3-5 inches in the immediate PHL area and immediate suburbs with a rapid tail-off lower going east.

I’ll be updating later…