This afternoon’s NAM model just became available. This is the “off hour” model run that does not include new weather balloon upper air (radiosonde)  data.

Here are the changes—Temperatures are warmer and precipitation ends earlier, about 4 AM Monday. Precipitation starts about 4 PM Sunday, probably as snow or a mix.Temperatures at the surface are above freezing, so the first amounts won’t accumulate very much.

Temperatures at critical levels of the atmosphere become too warm for snow about 8:00 PM in Philadelphia and move north.

8:00 PM Critical Temperatures

The warm air aloft moves north to a position shown below by midnight. This is the northern extent of the sleet/rain line.

Areas  north and west of the white line below are all-snow for the storm: 

Midnight Sunday critical temp- Maximum northward movement- white line is transition line

By 2 AM, cold air line moves back through Philadelphia:

Sunday 2 AM critical temp
So this means that between 8:30 PM and 2 AM, areas below the white line in the middle graphic are a mix of sleet and rain instead of snow.

This will seriously limit the snow accumulations in the immediate Philadelphia area. Combine this with marginally freezing surface temperatures in the 32-33 degree range means limited snow.

This is an off-hour run.  We really will need to wait until Sunday morning’s model runs for the final call.




  1. Hi Glen, first just want to say I love this blog and your analysis!! But I was just curious and maybe it’s just me but is there something specific that could be attributed to this year that puts us on right on that rain/snow line so often? Not that I’m complaining hah just wondering why we seem to be on the warmer side of these storms compared to other years.

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