Monthly Archives: December 2025

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

Accumulating Snow Likely Saturday night

Updated Friday 12/12/25 @ 10:07 AM — The latest Model Blend shows an increasingly likely snowfall for Saturday evening and night. Here’s the current blend mean snow accumulation—

Today’s NBM mean snow accumulation (Click on image for larger view.)


Light snow moves in about 9 PM from the northwest and tapers off early Sunday afternoon. It would surprise me if the totals are slightly higher

Possible Snow Saturday Night

Updated Thursday 12/11/25 @ 11:25 AM — Recent runs of several models have converged on a forecast for accumulating snow Saturday night ending Sunday morning.

ECMWF-AI total snowfall Sunday morning. (Click on image for larger view)

A cold front approaches Saturday evening. Low pressure is predicted to form along the front and temperatures which start out above freezing will dip below 32° as the low intensifies. About 0.25” water equivalent will fall either as wet snow, sleet, then changing to all snow before ending Sunday morning. Due to the temperature transition and the intensification, exact snow totals will be difficult to nail down at this time but 1-3” is looking very possible.

New US AI weather model becomes Operational

Updated Wednesday 12/10/25 @ 7:22 AM — Yesterday, NOAA announced the operational release of three AI weather models, to become available December 17th. Named the AIGFS, the AIGEFS and the HGEFS, these are models are based on the Google DeepMind Graphcast AI engine and draw on the same forecast historical database as the ECMWF-AI model. (These models have been recently available on AWS, but with a 16 hour delay.). The release version should become quite useful for forecasting and the HGEFS is a hybrid ensemble version should be especially useful when available realtime and compared to same runs of other models.

These models join the European AI model and the Canadian AI “spectrally-nudged “models for a new direction in forecasting.

Unfortunately, the the AIGFS and the other new models have limited parameters compared to the Canadian and European models.

Wednesday Light Rain and Outlook

Posted Tuesday 12/09/25 @ 4:24 PM — An approaching cold front will bring light rain on Wednesday. Rain begins late morning, perhaps as late as noon which will allow temperatures to rise above freezing. No frozen precipitation in our area is expected.

This is a fast moving system and any light rain will end by 7 PM. Less than a quarter of an inch of rain is expected, with rainfall totals decreasing from west to east.

Thursday looks dry and cold with some cloudiness, especially western sections, early afternoon.

Some light precipitation Friday into Saturday morning looks possible, but amounts are looking minimal.

The next chance of any light snow may be late afternoon Sunday into Monday morning. The ECMW-AI is showing a range 1.5″ and the Model Blend (NBM) about 3.5″. Of course, it’s way too early to hang our hat on this forecast.

18z ECMWF-AI forecast snow accumulation by 1 AM Monday morning. (Click on image for a larger view.)

Some family responsibilities may keep my regular posts and updates less frequent over the next couple of weeks.

Tuesday

Updated Monday 12/08/25 @ 9:18 PM — Watching the Eagles and “doing the weather”. Cold tonight!

NBM low temps

Tuesday will have more sun and much less wind than today. Mid level clouds similar to today around noon followed by sun again.

Wednesday’s light precipitation will likely occur after later than first forecast, meaning light rain

Originally Posted Sun 4:42 PM —This week’s weather highlights will be occurring about every two days.

First, a cold front, now in northwestern PA will move through our area slightly after midnight tonight (Sunday). Very cold air will move in behind this front with windy conditions, especially Monday morning. High temperatures on Monday will remain below 32º with wind chills near 20º

For Wednesday, a storm system will move to our north. While there will be cold air aloft, the northern track of this system will bring in milder air, likely changing any frozen precipitation to a mix or changeover to rain.

12z ECMWF-AI forecast for Wednesday at 2 PM. Cold air aloft (red, lavender contours) and warmer air near the surface (yellow contour and white 32º line) will allow a mix of sleet and perhaps some snow, possibly changing to all rain. (Click on image for a larger view.)

Another thrust of cold air moves in behind this system, in time for the next system to approach Friday night into Saturday—

12z ECMWF-AI forecast for Saturday at 7 AM. Again, colder air loft with above freezing temperatures near the ground will allow for sleet possibly mixed with wet snow. Some accumulation may be possible at the onset. (Click on image for a larger view.)

Right now, the colder than seasonal average pattern looks to continue into following week. No major snow or rain storms are forecast at this time.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Sunday Forecast

Posted Sunday 12/07/25 @ 10:31 AM — The forecast posted earlier for Sunday remains on-track. Mid level clouds move in later this morning.

Visible Satellite at 10:30 with superimposed MSL pressure and HRRR model relative humidity at 850 mb. (Click on image for a larger view.)
Saturday Forecast Update

Posted Saturday 12/06/25 @ 10:09 AM — As described yesterday, the light frozen precipitation forecast was “unusually low confidence”. The model forecasts for freezing rain turned out to be light snow in the immediate area and it has continued a few hours later than expected.

Basically, a band of slightly warmer moist air was brought into an area of weak upward motion, causing precipitation. My usual graphics use low level vertical motion (Omega) between two different pressure levels (typically 850mb -700 mb) which didn’t really show what was going on.

There’s another less common way of viewing atmosphere dynamics— instead of vertical movement between pressure levels (heights), it views atmospheric dynamics as movement along equal energy levels (as in Temperature) and equal Potential Temperature. Viewing the current situation along these lines, the area of vertical motion causing the light snow is clearly visible as the drawn yellow Omega contours—

9:30 AM Water Vapor with RAP model 700 mb wind streams (green) and Omega (vertical motion) across equal potential temperature (energy) levels at 330Ke and 320Ke (Yellow Contours) (Click on image for a larger view.)

The current bright spots and breaks of sun are forecast to fade behind clouds again for the rest of early and afternoon. Some clearing late afternoon.

Originally Posted Fri 7:49 PM —The forecast for tonight is an unusually low confidence forecast. A weak flow of warmer moist air near the surface may become trapped below cold air aloft. Some drizzle, freezing drizzle or even light snow is possible during the night time hours into early morning. All the models show a low probability of one of the above , but it’s unclear to me what’s driving this. The models are on to something because current radar and water vapor images show some slow moving light precipitation developing upstream from us—

Fri 7 PM Water Vapor with superimposed NexRAD radar/MRMS radar and RAP model Omega and 700mb wind streams. (Click on image for a larger view.)

Temperature rise during the night time hours. Any precipitation will be a function of the vertical thermal profile based on location.

Since the NBM was so good at forecasting our lack of any precipitation here, here’s the NBM with tonight’s forecast.

18z NBM with light freezing drizzle (red) and rain (pink) at 8 AM Saturday White contour is 32º Blue Contour is 36.5º (Click on image for a larger view.)

Here’s the REFS, which got today’s forecast really wrong here. See the caption for important info ——

18z REFS Conditional Probabilities of Freezing Drizzle, Freezing Rain, Rain and Snow. This parameter is deceptive. The entire area will not be getting this precipitation and not at the same time. This is just a conditional probability, meaning IF it’s precipitating, this is what will be falling. The actual amounts will be much more scattered and in this case, much lighter. (Click on image for a larger view.)

Saturday Forecast

Cloudy with lingering light freezing rain or rain in the early morning. The RRFS shows mostly cloudy with some breaks. Clearing late afternoon.

NBM high temperatures: Blue Bell, PA 41º Philadelphia, PA 43º
Average Uncertainty (based on standard deviation): ± 1.4º

Sunday Forecast

Sunshine through high clouds in the morning. Increasingly cloudy by noon. Cloudy by mid afternoon as another cold front moves in from the west.

NBM high temperatures: Blue Bell, PA 41º Philadelphia, PA 43º
Average Uncertainty (based on standard deviation): ± 1.4º