Category Archives: Weather Outlook


Last Updated

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.


Last Updated

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.


Last Updated

Updated Fri 7AM:  The GFS seems to be the most consistent model with the others trending towards its solution— A front moves through Tuesday, with some light rain, as the low pressure system develops to our north and east. The cold air arrives too late for much snow in our area.

Specifically, the cold air doesn’t come in until the precipitation is ending late afternoon or early evening on Tuesday. At best, a coating to under an inch of snow is possible Tuesday by 8 PM  with an emphasis on the lower range.

Capturing the trend is the National Blend of Models (NBM) which has been slowly reducing its snow forecast for our area.  Here’s the current NBM snow forecast for Tuesday 8 PM:

NBM (new pre-release NBM version 3.2) Snow total forecast Tuesday 8 PM

It’s likely the NBM forecast is over-done, even with its low snow totals of a coating of snow.

…from yesterday (Thurs)—

Updated Thurs 8 AM:  There’s still much uncertainty with a potential snowfall early Tuesday morning.  The Canadian CMC GDPS (Global Deterministic Prediction System) has sleet changing to snow here, while the GFS jumps the energy off the coast, mostly missing us.  Here’s the current Canadian model, showing the rain/snow line in red, moving over our area, changing sleet/rain to snow. Precip starts before daybreak and continues through Tuesday.  The current GFS forecast is very different with light showers, flurries.

Here’s the current GFS forecast for 8AM Tuesday, showing the precipitation has departed by the time the cold air moves in (lines depicted by arrows.)

GFS forecast for 8 AM Tuesday (Lines with arrows show critical freezing temps in the upper atmosphere.)

…from earlier this week:

As I mentioned over the weekend,  the period around Tuesday, November 12th looks to be interesting weather-wise.

Following the cold outbreak this coming Friday through Saturday, another cold outbreak is expected next Tuesday.  While this weekend’s dip in the jet stream will be broad and positively tilted, next week’s outbreak is likely to feature a highly amplified jet stream allowing development of a coastal storm.

The current Canadian model has a deep coastal storm with snow and sleet for us next Tuesday. The GFS had predicted something similar, but has recently backed away from this forecast scenario.

So there’s much uncertainty, but next Monday through Wednesday looks to be interesting.  Stay tuned.


Last Updated

Friday Update 7:30AM:  Earlier Forecast Outlook is on track.

Saturday will have considerable high and high middle level cloudiness.  Some sunny breaks possible, especially during the early morning and  later at times during the early afternoon.   Clouds thicken late afternoon.  Showers possible north and west of Philadelphia during the evening.   High 68.

Sunday: Rain, starting early morning, ending mid to late afternoon.  Rain may be heavy.  Mild. High 71

from earlier in the week:

I was tempted to post the weekend outlook earlier this week and I’m glad I didn’t.  The GFS weekend forecast has changed  since yesterday.

Low pressure moving up from the south was expected to approach on Saturday.  The latest GFS has slowed the approach of this system down to late Saturday— while the Canadian CMC and the European ECMWF hold the rain off until Sunday.

(When the GFS model was updated this past June to what was previously called the FV3-GFS in development, there were known biases in the new model which allowed some systems move up from the south too fast. 

The GFS is being updated on Nov 7th; it’s not clear whether this bias has been fixed.)

Currently, Saturday looks to be a decent day, with clouds moving in during the afternoon.  High 64.   Some showers possible Saturday late afternoon and evening.

Sunday looks to be rainy, especially by early afternoon.  High 67.

There is still some uncertainty with this outlook; the large spread in forecast high temps for Sunday from the EKDMOS captures the uncertainty:

Ensemble Kernel Density Model Output Stats (EKDMOS) high temperatures showing high spread for Sunday.
A week ago, I spoke of very cold weather arriving around Halloween.   The models still show this. Expect a winter-like cold to affect us around the end of this month.