Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Storm Forecast Update

Posted Sunday 10/12/25 @ 8:24 AM — The center of low pressure hasn’t moved much since last night. The 3 hour pressure falls I looked at last night were associated with a trough moving in from the east—

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 as is the case with visible satellite imagery. Ocean trough (drawn as warm front) is moving towards us and will bring rain by afternoon. (Click on image for a larger view.)

There remains large differences in the model forecast of this system.

Some models show an elongated system rotating into our area with the main low hanging back.

The ECMWF and ECMWF-AI are more in the double low solution with a second low on the front rotating into our area later today—

The latest ECMWF-AI (06z) .

Base on last night’s analysis, I’m going to lean on the ECMWF-AI as the large scale synoptic forecast and the RRFS REFS for high resolution.

So. continuing with last night’s impressions, I’m forecasting less rain in the immediate Philadelphia area during the daytime hours. The large area of rain at the shore will shear up to the northeast of the Philadelphia area today; our rainfall today will be minimal, especially west of I-95 through 8 PM.

RRFS total rainfall through 8 PM Sunday. Much lighter than previously forecast . The REFS shows even lower rainfall totals. (Click on image for a larger view.)

The heavier rain previously forecast will move in before daybreak Monday.

Forecast Update

Posted Saturday 10/11/25 @ 9:43 PM — There haven’t been enough 00z models available at this time to know if there’s any change in the forecast. One 00z model out, the HRRR, show a break in rain early Sunday morning, with the rain returning by noontime. The rain near the city doesn’t look as impressive as first thought.

The other trend I’m seeing: in comparing the afternoon models with the current actual current conditions using the RTMA model, it appears that the model guidance is too fast moving the system towards us and too far westward with the center of the low. The 3 hour pressure changes on the RTMA also suggest the possibility of greater eastward movement away from the coast than previously forecast.

RTMA 9 PM position of low along with calculated 3 hour pressure tendencies (Click on image for a larger view.)

We’ll have to see. Updates tomorrow morning.

Weekend Storm- Latest Trends

Posted Saturday 10/11/25 @ 9:10 AM — The high uncertainty with the track of the complex coastal storm expected late Saturday into Sunday has been emphasized over the past several days. This uncertainty is due to the interaction with at least 2 upper air low centers, blocking high pressure, and even interaction with Tropical Storm Jerry’s circulation.

The trend from last night continues into the 06z (2 AM EST) model runs: most models now have the storm further off-shore as it makes it into this latitude and the previous jog to the west to rotate around an upper low is either missing or less prominent.

All models show high winds and wind gusts at the shore.

Let me get to the extremes in forecast guidance: The German ICON model as well as the Canadian models have the heavier precipitation with this storm just affecting the coast. The ECMWF-AI and ECMWF also have a greater eastward track with stalling near NJ, but little of the previous retrograde track to the west. Perhaps the most western track is the GFS, but it also keeps the flooding rains just along the shore.

06z ICON forecast model: Little in the way of wrap around rain to the west and an eastern track (Click on image for a larger view.)

Here’s the latest GFS model forecast with a slower surface low and a westward track—

06z GFS model forecast for 5 AM Monday. A much slower track. (Click on image for a larger view.)

Here’s the latest NBM total accumulation. The NBM continues on a slower storm with rain into late Monday.

12z NBM total rainfall through early Tuesday. (Click on image for a larger view.)

So, the uncertainty isn’t with the high winds. It’s going to be around the rainfall. With Philadelphia in a relative drought again, it wouldn’t surprise me if the rainfall here is considerably less than forecast by the NBM

Update Fri 10/10 10:10 PM —Several models are trending towards a more offshore track and a split of the system into two low pressure systems; one hangs back near the Carolinas. The rain may not be as intense as forecast earlier. This is a complex system with lower than usual forecast confidence.

Increasingly Stormy

Originally Posted Fri 5:41 PM —Our weekend weather will be influenced by a coastal low pressure system as well as an upper air low. The forecast uncertainty and evolution of this system has been well-described in my previous posts.

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 as is the case with visible satellite imagery. (Click on image for a larger view.)

It’s still unclear how close to the coast the storm will remain, whether it will retrograde westward and whether the (much-needed) heavier rain will reach west of the I-95 corridor.

Friday night through Saturday: The general trends described in those earlier posts remain in play— there will be an increase in clouds tonight and light widely scattered/isolated showers are possible Saturday with increasing probability. Most areas will remain cloudy but dry, but there will be exceptions as shown below. The latest REFS captures the highly scattered nature of the showers during Saturday‘s daytime hours . Increasingly windy towards evening and overnight.

REFS 12z Forecast total rainfall (very light, widely scattered) for the entire daytime hours through 5 PM Saturday. (Click on image for a larger view.)

(It should be noted that the latest NBM shows much more shower activity for Saturday.)

NBM high temperatures: Blue Bell, PA 69º Philadelphia, PA 70º
Slightly above average uncertainty (based on standard deviation): ±1.8º

Sunday: Moisture from the coastal low pressure system continues to move up the coast and into our area. Cloudy with rain throughout the day. A large pressure gradient will cause WINDY and GUSTY conditions with wind gusts > 40 mph

NBM high temperatures: Blue Bell, PA 63 º Philadelphia, PA 64º
High uncertainty, low confidence (based on standard deviation): ±3.7º

Total rainfall through Sunday night:
REFS Model

NBM Model