Mon 08:32 PM Forecast Review —We had more snow than forecast.  The models (with the exception of the Canadian GEM model) underestimated the QPF and didn’t fully take into account the cold upper level atmospheric temperatures, which resulted in more snow and less rain. The NAM erroneously hastened the transition to rain, despite having the QPF somewhat in-range.   

The Canadian “Regional GEM” model is one model I always look at, but it it isn’t necessarily a model that is more often accurate.  I’ve seen it over-estimate snow totals many times.   Last night’s GEM had these higher snow totals, but as I said, “Not sure what to make of it” 

Here are the NWS official snow totals:Here’s the graphic posted yesterday afternoon from the GEM model—

CMC GEM Model Snow totals 


Here’s the GEM model that I looked at 11 PM last night but I didn’t know what to do with—

Last night’s Canadian GEM snow totals.  (The GEM shows snowfall as snowfall rate in Kg2/m2)  These numbers are correct.  The 4-5″ mentioned in last night’s  “update” was a quick, mental approximate conversion to inches of snow.)





Update— Tonight’s Canadian model continues to forecast 4-5 inches of snow in central parts of Montgomery county.  Not sure what to make of it. 

Tonight’s latest models (HRRR, RAP, NAM)  shows the following changes— snow changeover a bit delayed northwest of the city, although snow totals are pretty much the same.  (It might be snowing an hour longer, but it will be increasingly difficult for accumulating snow.)

Dark, paved roadway surfaces will likely see less accumulation than grassy surfaces.

Snow starts about 9-11 AM from west to east and changes to rain from southeast to northwest by 2 PM.    Precip ends 3-4 PM.

The differences with this storm and recent previous storms  were outlined in an earlier post:

  •  The models have been consistent with total water falling (QPF) will only be 0.3-0.4″ water equivalent or less in the immediate PHL area, in  contrast to much greater amounts in previous storms.
  • The thermal structure of this storm is vastly different.   This storm will have southerly winds at the surface and surface temperatures will be at or above freezing for most of the area once things get going.   (Previous storms had northeasterly or easterly winds at the surface.)
  • In contrast, upper air temperatures will be colder northwest of the city, supporting snow formation but the surface temperatures will go against accumulation.
  • We’re into the final part of February.  The solar angle is higher and “insolation” effects through clouds are more likely to interfere with and cause melting on dark asphalt road surfaces when the precip occurs during the daytime hours.

Things can be summarized with this graphic slide show:

  • RAP model 10 AM Precipitation moves in
  • NAM model 1 PM Changeover northwest of the city. (note 32º surface temp white line)
  • RAP model Snow totals 4 PM


RAP model snowfall on grassy surfaces.  (Click on image for a larger view.)




2 thoughts on “MONDAY’S SNOW TO RAIN”

  1. Looks like the Canadian model run from last night might be the winner this time. I have about 4” now at my home in Collegeville and it is still coming down pretty hard.

    1. I am in northern Montgomery county for work today and we have at least 3-4 inches. Yesterday when I posted the Canadian model, it was just a curiosity and a suspected outlier. Although last night’s Canadian had similar high snowfall and I didn’t know what to with it. So I just shared it as potentially useful information.

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