Monthly Archives: February 2018

Winter Storm Outlook

WINTER WEATHER UPDATE- MOSTLY RAIN FOR PHL

The last night’s 06Z  data has become available- both the NAM and GFS models have increased the QPF values to an average of about 0.50 inches water.

This morning’s NAM data also becoming available- the trend is for heavier QPF, and warmer atmosphere!

The NAM has increased the average temperature of the mid-level of the atmosphere to a level that is too warm for snow formation for much of the storm in Philadelphia and immediate northwest areas, as depicted by this map.  (Lower levels of the atmosphere (except at ground level) are at or below freezing, allowing sleet at times.)

Areas south of the red line- mostly rain, maybe some sleet.

Here’s the current best forecast-

Precipitation starts between 4-6 PM today, possibly as light snow or a mix of rain, sleet and snow.

A changeover to rain and rain-mixed with sleet will occur early.  Much of what falls will be rain and possibly some sleet;  at times, some snow may mix in at peak intensity if dynamic cooling occurs.   This event will be mostly wet, not white, in the area depicted south of the red line  in the inset map.

Precipitation may changeover to a brief period of light snow before ending about 2 PM.

Accumulations- zero to a coating, mostly on grassy surfaces.

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.