Category Archives: Winter Storm Outlook

Winter Storm Outlook

Winter Weather Update 9:40 PM

Latest NAM FOUS data just became available.

Precipitation starts at midnight as rain, gradually mixes with and changes to snow towards daybreak.   Snow ends between 7-9 AM.  Total QPF is now 0.46 inches water.  Most of this will fall as rain or rain mixed with snow, becoming all snow by 5-7 AM.   Still impossible to predict accumulations, because of warm ground temperatures, melting due to mixed precipitation and varying amounts of cold air at the surface.  (Temps expected to fall to 32 by 7 AM).   More QPF at the coast, but warmer temps.  Precipitation drops off far north and west.

While impossible to predict accumulations with any confidence,  if you’re looking to hang your hat on a number, I’d say between 0.5 to 2 inches, especially on grassy surfaces outside of Center City Philadelphia and near suburbs.  This is a low confidence forecast.

Tomorrow’s precipitation is a perfect example of how poorly the models have been handling the extended range forecasts (more than 4-5 days out) this winter.  Just 3 days ago, this precipitation had been expected to stay to our east, just brushing the coastline.  Things clearly have changed. 

With short range forecasts so changeable, the “long range” period- Monday through next Wednesday- presents a large forecasting challenge, as there are several potential coastal low pressure systems forecast to develop,  with low confidence in each scenario.

Philadelphia Weather Outlook, Winter Storm Outlook

Upcoming Active Weather Pattern

The next week or two looks to have a very active weather pattern, probably the most active this winter.

Following the possibly heavy rain this Wednesday, the jet stream undergoes several deep amplifications with cold air diving south as moisture emerges from the Gulf of Mexico. The first such amplification on Friday allows the moisture to miss us, but subsequent cold outbreaks next week will result in some impressive storms.  It will be a roller coaster regarding temperatures.

Too early to tell if it will be snow, rain or a mix for each one but it will certainly get interesting.  Nothing looks like the “blizzard” we just had, but the storminess and temperature swings looks to be impressive.  Stay tuned.

Tuesday 8 pm update- The low pressure system this Friday still expected to just miss us, moving to our east.  One model has it grazing the jersey shore. A large coastal storm still possible Monday-Tuesday timeframe but timing and track has high variability with different models.  Much uncertainty here.

Deep upper air trough with very cold weather next week; we may hear the term “polar vortex”by the end of next week.