Daily Archives: Sunday, February 14, 2016 9:51 pm

Weather Updates

Philly Winter Weather Update

The latest NAM data just became available.   It shows a QPF of 0.06 before 1 pm and a QPF of 0.20 water between 1 pm and 7 pm.   Temperatures at mid levels of the atmosphere rise above freezing in the late afternoon, changing the snow to sleet.  So all of this QPF won’t fall as snow.

Based on this data and the latest HRRR and GFS LAMPS  forecast, it looks light snow flurries in the mid morning, then light to moderate snow early afternoon.  Approximately 2 maybe 3 inches of snow before mixing with sleet and then rain early evening. The NAM data still suggests a period of sleet and freezing rain in PHL late afternoon and evening with an extended period of the same north and west of the city.  A full transition to rain by midnight as temperatures rise.

Again, the major impact will not be snow accumulation, but rather the potential for icy conditions during the late afternoon and early evening, especially areas north and west of the city.

Winter Storm Outlook

Philly Winter Weather Update- Sunday

Better agreement on the models for Monday through Tuesday.

Light snow starts about noon on Monday.  About 2, maybe 3, inches of snow by late afternoon, then a shift to sleet and freezing rain around 5 PM.   (Total QPF on the NAM is 0.26 inches water during this period)

Here’s where it gets tricky, especially north and west of Philadelphia- there may be a period of sleet and freezing rain early evening Monday, with possibly several hours of sleet and freezing rain north and west of the city.  The models often rush the warmup of surface temperatures, so driving conditions could get very difficult for several hours early Monday evening and during the night, north and west of PHL.

A definite changeover to all rain late evening or night on Monday.   Tuesday- heavy rain mid day, ending towards evening.  High 51!

Update Sunday 6pm:  This afternoon’s model runs continue the general trend of snow starting in the morning Monday.  The NAM QPF is somewhat higher again, over 0.30 inches water during the “snow phase”. The GFS and NAM suggest an earlier start, perhaps early morning.

The issue won’t be snow accumulation, which still remains about 2-3 inches.  The concern still remains that surface temperatures remain at freezing at PHL until 6-7 pm while the upper atmosphere warms, suggesting a possibly extended period of sleet and freezing rain during the evening rush hour. Areas north and west could have this mix last longer through late evening.

I’ll update with the next model run.   The NAM model does best with this and the NAM FOUS data becomes available about 9:35pm or so.