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: