Monthly Archives: March 2016

Weather Updates

Winter Weather Update

Changes in the models……Last night’s models show a less-sharply amplified upper air configuration., allowing faster movement of the expected coastal low and less of a track hugging the coastline.  We still get some light snow Saturday evening and night possibly accumulating mostly on grassy surfaces, but right now, less precipitation lingers into Sunday.

Interesting winter to spring weather, but NOT a snowstorm in any sense of the word.  I’ll update later tonight.

Fri Noon Update- Precip starts late afternoon or evening Saturday, may start as rain or rain mixed with snow.  Temperatures in the upper atmosphere support snow after midnight Saturday, but near surface temperatures (and ground temperatures) will be at or just above freezing.  The thermal profile supports snow Sunday morning, and some grassy accumulation possible, but insolation through the clouds will limit accumulation on roadways considerably.

So, when the atmosphere is cold enough for snow by daybreak Sunday, conditions will be marginal to allow accumulation.  The best type of snow- looks pretty, little accumulation.

Philadelphia Area Weather Forecasts

Winter Weather Outlook

As I’ve mentioned,  over the past two weeks, the GFS model has been showing the return of cold air and the possibility of snow for this coming weekend.

The models continue to show this possibility.  The GFS model QPF for late Saturday night into Sunday is well over 0.40 inches water equivalent, but the exact track and timing of the coastal low’s intensification is still uncertain and continues to be difficult to pin down.  What does seem likely is that the upper air thermal profile supports snow, vs rain.

Even if the expected QPF value gets consistent, predicting snowfall amounts this time of year is very difficult.  So much will depend upon the timing (daytime vs nighttime) ground temperatures (which are high for this time of year),  near surface air temperatures and rate of precipitation (which affects dynamic cooling.)

So this is NOT a snowfall where you multiply the QPF by 10 and you get the snowfall totals.  Instead, all of those factors above turn into a general crapshoot predicting accumulation.   To simplify, snowfall during the daytime hours reduces accumulation significantly, especially on dark paved surfaces.   Grassy surface accumulation almost always exceeds accumulations on sidewalks, unless precipitation rates and QPF amounts are very high.  Right now, it doesn’t look like a ‘big event’.

Both the NAM and GFS models have light snow starting Saturday evening and continuing into at least Sunday morning, maybe more prolonged. Temperatures will be cold and below average late Saturday through Tuesday.

Thurs 11PM – NAM QPF looking impressive at 0.56 inches water equivalent.  Accumulating snow looking more likely during the day Sunday.

Philadelphia Weather Outlook

More colder weather

The GFS model has been showing the development of an eastern upper air trough, allowing cold air to filter into our area again this coming weekend.

This trough will develop at the same time that moisture is streaming up from the Gulf of Mexico.  As a result, low pressure is expected to develop near us, in the Saturday -Sunday time frame.

With some precipitation occurring at night and with atmospheric thickness levels falling below 540 dm, it is possible that we will see some light snow, unlikely to accumulate, if the precipitation occurs during the night time hours.

Wednesday Update: The latest GFS continues with the possibility of snow, with the timing occurring during Sunday afternoon and evening. It’s not a sure thing, but the models have been somewhat consistent.

It should be noted that the GFS model has been depicting this possibility for almost two weeks.  And it should be noted that a week ago, the Channel 6  weather people emphatically declared no more snow for our area this season.