Daily Archives: Friday, December 5, 2025 7:49 pm

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