Daily Archives: Friday, February 16, 2018 10:28 pm

Winter Weather

WINTER WEATHER FORECAST

Those of you who follow this blog know I’ve been wrestling with this forecast for Saturday evening.  There have been too many borderline conditions to accurately predict the precipitation type and accumulation amounts.

The latest NAM data has become available. QPF values have been incredibly consistent at about 0.60 inches water.  Surface temperatures appear to be above freezing for much of the day and near to above freezing during the storm.  Mid-level atmosphere temperatures appear  slightly too warm to support snow, while lower temperatures are at or below freezing.

After an initial start as light snow between 4-6 pm, it  appears to be a mostly sleet and rain storm in Philadelphia and its immediate suburbs before changing back briefly to snow before ending after midnight.

A cause for error in this forecast might be dynamic cooling due high precipitation rate, which might increase the snow possibility.

Warm  surfaces will further reduce accumulations, except on grassy surfaces.   A wet, slushy 1 to 3 inches is a best guess on grassy surfaces, less on pavement.

There’s still 24 hours for things to clarify.

11:30 pm – Tonight’s GFS even less impressive QPF and warmer surface temperatures. Mostly a sleet and rain event for PHL and surrounding areas.

 

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.