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Saturday AM Update: This morning’s models have fallen into line with the Sunday forecast from yesterday’s GFS — most of the rain remains off the coast on Sunday. Philadelphia will be cloudy, windy and chilly. There’s a chance of very light sprinkles, but mostly dry for the Eagles game late afternoon.

High pressure builds into our area on Saturday, bringing cold and mostly sunny skies. High temperatures on Saturday will be unseasonably cold with a high of 41.

For Sunday, the main models are still not in full agreement about the westward extent of precipitation Sunday afternoon. That said, the trend is for the precipitation to stay east of Philadelphia until very late afternoon or evening. So the Eagles game should be dry if the global models are correct.  It will be, cloudy, windy and chilly, with highs near 44.

GFS hourly model Simulated Radar Forecast for 2 PM Sunday

The NAM, CMC and SREF have significant rain moving in Sunday evening and night, while the GFS and ECMWF keep much of the rain east and along the coast.  I’ll be updating over the weekend to fine tune this forecast.

NAM Nest high resolution Simulated Radar Sunday 3 pm forecast


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Fri 7 AM Update: Last night’s NAM has joined the statical models in their forecast for showers /rain on Sunday afternoon and evening.

Saturday’s weather looks good, Sunday’s is uncertain.

High pressure builds into our area on Saturday, bringing cold and mostly sunny skies. High temperatures on Saturday will be unseasonably cold with a high around 41.

For Sunday, the same high pressure system is thought by the deterministic models to block the northeast movement of a coastal low from bringing rain to our area on Sunday. So the GFS and NAM keep us dry on Sunday.

Unfortunately, the statistical ensemble models, especially the short range model (SREF)  and more recently the GFS based GEFS, have been showing a westward track and a chance of light showers/rain moving in during Sunday afternoon and especially Sunday night.

SREF Model precipitation forecast for Sunday afternoon 4:30 PM

So there’s significant uncertainty with Sunday’s weather forecast.

I’ve been watching the trends and I think the ensemble forecasts with a chance of showers during Sunday afternoon is a strong possibility.   I’ll update Friday evening, when things should come into better focus.


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Anthony Wood, writer for the Philadelphia Inquirer, wrote a good article on the weather models and snow forecasts in today’s Philadelphia Inquirer. In the article, he discusses the new GFS model.  I do agree that the new GFS model (known as the FV3-GFS in development) seems a little off, especially with temperatures lately.  That said, the graphics I posted in previous days from the European and Canadian models show they didn’t do much better with today’s forecast.
Monday Update 10 PM:  Tonight’s high resolution WRF models have reduced the QPF even further to 0.10 water.   The WRF has light snow showers possible until 11-12 noon.  Still no accumulation expected. 
Monday Update 7 PM:  This current “storm” doesn’t merit the time and attention I’ve given it, but I see it as a dry run for future winter storms.

The latest models have reduced the QPF to a small 0.12 inches of water, falling essentially as a short period of rain. Rain starts before daybreak and now may end as early as 9 AM.

A changeover to snow flurries appears possible, but an accumulation is unlikely in Philadelphia and the immediate suburbs.  Cold air moves in quickly behind the front with windy conditions.

Monday Update 10 AM: The latest models continue to show a changeover to snow late morning (~ 10 -11 AM) with temperatures dropping during the day.  The snow turnover will occur, but whether any snow accumulates is uncertain.

The total QPF for this “storm” is just about 0.30 inches water; only the last 0.08 inches of this water will fall as wet snow.

Some of the thermal parameters that I look at are NOT really supportive of accumulating snow — temperatures at the surface start very warm and the low QPF values reducing dynamic cooling. The “thickness level” is also a bit high, indicating a warm atmosphere.

Additionally, the models have the temperatures falling to freezing at critical levels just as the precip is ending.

Last night’s ECMWF (European) has been insistent on some light accumulation, a coating to 0.4 inches.

ECMWF snow depth at 3 PM Tuesday. Blue =0.4 inches

This morning’s NAM and NAMNEST  also show a coating of snow. Stay tuned.


Sunday Update 11 AM:  Latest models have moved the precipitation earlier starting well before daybreak Tuesday.

Total QPF has reduced to about 0.4-0.5 inches water. The change-over to snow still expected during the last hour of precipitation, but accumulating amounts look to be zero or a coating at best.

Latest NBM shows a 78% chance of a coating of snow by 11 AM Tuesday

A cold front is expected to move through on Tuesday, with coastal low pressure developing northeast of us.

Some things have changed over the past few days— the front will move through much earlier on Tuesday morning than previously forecast.

Some aspects of the forecast have moved back to a brief change to snow.

The NBM (National Blend of Models) continues to predict a coating to 1/2 inch of snow.  The statistical SREF  (Short Range Ensemble Forecast) has a similar forecast.

The latest GFS model suggests that rain will turn briefly to snow before ending mid morning.  It’s unclear whether anything will accumulate.  Warm surface temperatures will limit any accumulation perhaps to a coating, up to 1/2 inch.

Stay tuned.


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Sunday 2 PM Update: The upper air disturbance mentioned below to cause Sunday’s mid day cloudiness has moved through faster than predicted. This morning’s NAM and GFS has the cloudiness lingering even longer into the late afternoon.
Sunday Update: The upper air disturbance mentioned below to cause Sunday’s mid day cloudiness has moved through faster than predicted.  It’s cloudy at 7 AM and cloudiness should break by the afternoon.
Sat Update: There wasn’t as much cloudiness as predicted.  A brief period around noontime.

…from Friday—

High pressure that moved in behind the front on Friday will slowly move off the coast over the weekend.

Saturday: Increasing cloudiness and cold in the morning, and considerable cloudiness and cold in the afternoon. Calm winds Saturday morning will become light and southerly during Saturday afternoon.  A return of moisture aloft and an upper air cyclonic flow will create mid to upper level cloudiness for much of the afternoon on Saturday.  The cloud deck will keep the temps from rising above 43.

Sunday: Mostly sunny and milder.  A mild flow of air  from the southwest will allow mostly sunny skies and high temps near 55. (There may be a period of cloudiness mid-day as an upper air disturbance moves through.)

It now looks like there will be no snow for us from the coastal storm predicted to form on Tuesday — by the time the cold air moves in, the low pressure system will be too far to our north and east.

This was more accurately predicted by the new GFS model than the Canadian Global.  The European model was doing it’s own thing with this storm regarding timing.  The major models are showing better agreement on this today than they had several days ago.

This will be the first winter that we’ll relying on the new GFS model.  The new GFS, referred to as the FV3-GFS when it was in development, became operational this past June.

The GFS model was further updated and enhanced just this past week when it was upgraded to version 15.2

Storm Forecasts & Other Things "up in the air".