Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Sunday Forecast Update & Tropical Depression 9

Posted Sunday 09/28/25 @ 9:24 AM —The northward and westward extent of yesterday’s rain was greater than forecast, with the heaviest band of rain through our area rather than just east of the I-95 corridor. Here’s the MRMS rainfall totals—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

As for Tropical Depression #9, the European AI model (“ECMWF-AI-Single”) has it likely becoming a Tropical Storm or even a Category 1 Hurricane, but the model has joined the other models with it moving east and away from us.

ECMWF-AI 06z forecast for Tuesday. Likely at least a Tropical Storm, but its track is now eastward. (Click on image for a larger view.)
“Poor Predictability”

Posted Saturday 09/27/25 @ 5:20 PM — I began yesterday’s Weekend Weather Forecast with a prominent comment about poor predictability as a result of tropical systems in the mix. We’re seeing this first hand— the rain has advanced faster than any of the model predictions from Friday, or for that matter, this morning.

Radar and RAP model low level Omega about 5 PM. Rain is moving in faster than forecast, although the RAP model shows downward vertical motion (Omega – yellow dashed contours), which inhibits rainfall. What had been forecast for 10 PM and later is likely earlier. (Click on image for a larger view.)

The models are consistently showing rain to end around daybreak and skies to clear for sunshine Sunday morning. I think we can still trust this.

Saturday Rain Update and Tropical Disturbance #9 (Imelda)

Posted Saturday 09/27/25 @ 8:21 AM — Last night’s model guidance has shifted the areas of rain to east of the I-95 corridor, i.e, South Jersey. Only light showers are forecast for Philadelphia and northwestward with totals < 0.10 inches and many models showing < 0.05″.

It looks like the Phillies game won’t be affected with such light rain arriving likely after 10 PM.

Latest REFS (06z) 1 hour rainfall at 10 PM (Click on image for a larger view.)
Latest HREF (06z) 1 hour rainfall at 11 PM (Click on image for a larger view.)


As for the tropical disturbance expected to become tropical storm or hurricane Imelda, most models have the storm approaching the coast of South Carolina and then moving out to sea. In marked contrast is the ECMWF-AI model (which did so well with hurricanes last season) which has the storm in a track that would affect us by next weekend—

Latest ECMWF-AI (06z) shows the storm blocked initially, then moving up into the Mid Atlantic. This is an outlier forecast, but that said, the ECMWF-AI did very well last hurricane season. (Click on image for a larger view.)


Previously Posted Fri @ 5:11 PM — —Following a beautiful day on Friday, the weather will become unsettled (and perhaps somewhat poorly predictable) as low pressure in the Tennessee Valley moves off to the coast south of our area bring clouds and rain.

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Water Vapor Imagery reveals emitted water vapor infrared Temperature, NOT reflected light from clouds (Click on image for a larger view.)

The concern over predictability is based on the existence of two tropical systems on course to approach South Carolina.

NAEFS forecast for Sundat at 5 PM (Click on image for a larger view.)

The moisture outflow and the warm center highly localized energy of these tropical systems often seem to play havoc with model guidance. So I expect to see shifts in the forecast over the next good number of days.

Saturday Forecast

High clouds in the early morning are quickly replaced by lowering, thickening clouds by late morning and through the afternoon. While a few scattered sprinkles are possible as early as 3 PM, heavier rain appears to wait for the end of the Phillies game Saturday evening. Rainfall will be highest from South Philadelphia and southeastward.

Light winds increasing in the afternoon.

NBM high temperatures: Blue Bell, PA 79º Philadelphia, PA 79º
Above Average Uncertainty (based on standard deviation): ± 2.4º

Sunday Forecast

We’ll go with an optimistic Sunday forecast, but it wouldn’t surprise me if there is more cloudiness. Rain ends before daybreak, clouds break for sunshine. (The uncertainty in the NBM temperature forecast below captures the uncertainty in the cloud cover.)

Light winds.

NBM high temperatures: Blue Bell, PA 78º Philadelphia, PA 79º
Above Average Uncertainty (based on standard deviation): ± 3.4º