[su_note note_color=”#defcdc”]Fri AM Update: Last night’s GFS and NAM show the possibility that rain lingers into early Sunday afternoon, rather than being over by Sunday morning. I’ll update this evening.[/su_note]

Fast moving systems will affect our weather over the coming weekend.

High pressure builds in for early Saturday, after a cold front moves through with showers mid day Friday.

Low pressure approaches from the southwest on Saturday. Some sun through high clouds Saturday morning will be followed by increasing cloudiness late morning and during the afternoon.  High 49. Most of the daytime hours will be dry.   The models are in reasonably good agreement that rain moves in between 4 and 7 PM.

Moderate rain expected Saturday night into Sunday morning. Light showers may linger after daybreak Sunday, with the low pressure system moving rapidly away, but expanding and intensifying as it departs.  It’s expected that skies clear during Sunday afternoon. High 51.

[su_note note_color=”#bceaed”]Outlook for Thanksgiving: The models have been inconsistent in the long range. A large storm expected for Wednesday before Thanksgiving seems to have disappeared from the model forecast. The models currently show a storm for Sunday after Thanksgiving, but I would not be surprised if that changes. One thing that has been consistent is the appearance of very cold weather for the first week of December. [/su_note]

[su_note note_color=”#defcdc”]Fri 7 AM Update: Last night’s NAM has joined the statical models in their forecast for showers /rain on Sunday afternoon and evening.[/su_note]

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.

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.

[su_note note_color=”#bceaed”]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. [/su_note]

[su_note note_color=”#defcdc”]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.  [/su_note]

[su_note note_color=”#defcdc”]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. [/su_note]

[su_note note_color=”#defcdc”] 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.

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

[su_note note_color=”#defcdc”]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.

[/su_note]

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.