Monthly Archives: February 2018

Winter Storm Outlook

SNOW FORECAST UPDATE 7 PM EDITION

This afternoon’s models continue with the fact that this is a difficult forecast—too many boundary conditions which affect the precipitation type and amount.   The GFS has a QPF of only 0.30 inches water, the NAM has a consistent QPF of 0.60 inches water.

Here are the issues making this forecast difficult-

  • Temperatures during the daytime Saturday are expected to be well above freezing, with the GFS much warmer than the NAM.
  • Ground temperatures, especially on paved surfaces are slow to drop, reducing accumulations.
  • Critical temperatures in the lower atmosphere support snow, BUT the models agree that critical thickness levels (a measure of the upper atmosphere temperature and density) are a bit warm to support snow at several points during the 7 hour precipitation window.

As a result, it looks like a mix of snow, sleet or rain is likely in PHL and the immediate suburbs.    So what falls from the ground may be snow, but it may switch over to  sleet and rain and then end as snow.   Add the warm ground temperatures to the mix and it’s truly not possible to predict accumulations.

More specifically, it looks like it starts as snow about 4 PM, but I think it will mix with sleet and freezing rain for a portion of the duration, then switching back to snow before ending about 2 AM Sunday.   Most accumulations will be on non-paved surfaces.

Areas in northern Montgomery and Bucks county will likely have more snow.

Temperatures are key- the next model run with the data I need will be available about 9:45 PM.

 

Winter Weather

GOTTA SCALE DOWN THIS STORM FORECAST

Yet another forecast snow bites the dust.   The 1 AM runs of the GFS and NAM show the low pressure development Saturday night to be very unimpressive.   The track is further south, the intensification is less and the precipitation rate is lower.  Surface temperatures appear warmer.

With the current trends, I’m significantly scaling down the snow forecast.

In the immediate PHL area and immediate suburbs, little accumulation is expected on roads and paved surfaces and perhaps a wet coating on grassy surfaces.   It looks to be principally rain mixed with some wet snow here in PHL and the immediate suburbs.

Things may change again.  I’ll update this evening.

10 AM Friday Update- Latest NAM again has 0.60 inches QPF , starting about 6 PM.   Temperatures in Philadelphia appear just on the borderline too warm for snow at the start, but my concern about dynamic cooling might come into play here again. So I may have to backtrack to last night’s forecast  with a few inches of snow, mostly on grassy surfaces in the PHL immediate area!

Sorry for the flips back and forth on the forecast, but this forecast is plagued with boundary conditions that could go either way.   That said, this morning’s NAM was looking colder than its previous run.  We won’t know until tonight at the earliest, and it may need to wait until Saturday morning for the final handle on things.