Philadelphia Area Weather Forecasts

THIS WEEK’S WEATHER

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Thursday Evening Storms

Posted Thursday 09/17/26 @ 5:41 PM — High pressure from the northwest will try to push the heat dome south of us this evening. Showers and thunderstorms have already broken out in north central PA.

Water Vapor image at 4:51PM with superimposed RAP model Mean Seal Level pressure (black contours), 500-1000mb Thickness (yellow contours), RAP potential vorticity contours (violet fine contours) with MRMS Radar with surface wind barbs, Note the position of the 576 heat dome edge which makes a sharp anticyclonic curvature near NJ. (Click on image for a larger view.)

On a larger scale, I’ll use the Canadian GDPS-AI to capture the big picture—

Today’s 12 z Canadian GDPS with AI spectral nudging. Heat dome edge near our area will be partly be suppressed to our south. Notice the curvature of the orange 576 thickness; it makes a sharp curve near NJ and the curve is ‘anticyclonic’ meaning it suppresses upward motion that produces rain. NJ is not expected to receive much rain with this frontal passage.

The humidity is high and the total column of water (“PWAT“- Precipitable WATer) is high—in the range of 2.2″-2.4″. Heavy rain is possible in any thunderstorms this evening.

Thunderstorms are forecast to move in from the northwest between 8 PM and 11 PM this evening with highest chance around 9-10 PM.

The best guess, based on this afternoon’s RRFS and HRRR. suggests the heaviest rains will be west of the city and later, both north of the city and south of the city.

18z RRFS forecast at 8 PM shows the strongest dynamics for rain will be near the white box areas I’ve drawn in. The storms are forecast to weaken as they move into NJ (Click on image for a larger view.)

FYI, there were heavy showers today, northwest of Allentown and through western sections near the city, that were not well-predicted by today’s models. So confidence is a bit lower than usual with this evening’s forecast.

Posted Thursday 09/17/26 @ 5:09 PM — High pressure is attempting to push southeastward from the north tonight, pushing a weak cold front through. Storms have broken out in north central PA this afternoon—

Posted Tuesday 09/15/26 @ 6:00 PM — Beautiful weather continues through Wednesday with the change being an increase in humidity and temperatures. A cold front approaches on Thursday. Over the past two days, the models have been inconsistent with their Thursday forecasts. The AI models were forecasting light showers while some of the deterministic models had little shower activity for our area.

It now appears that Thursday will have a mix of clouds and sunshine with increasing cloudiness towards late afternoon. According the latest RRFS,scattered showers are expected to move in from the west about 6-7 PM ahead of a cold front.

However the latest GFS has the showers dissipating as they move into our area. So there’s higher than average forecast uncertainty.

Here’s the GFS at 10 PM—

GFS eastern-most extent of precipitation at 10 PM Note the position of the 576 thickness line, the approximate edge of the heat dome. (Click on image for a larger view.)

In contrast, the latest RRFS shows some light widely scattered showers at 8 PM—

18z RRFS shows widely scattered showers at 8 PM Thursday evening. (Click on image for a larger view.)

The actual front moves through during the morning hours on Friday.

Originally Posted Mon 10:03 AM — High pressure has built in to our area and will remain in control through much of Wednesday. Comfortable temperatures and humidity for the next two to three days. An increase in temperatures and humidity expected later Wednesday into Thursday.

On Thursday, the front that went through will return as a warm front. Low pressure will move along the front bringing showers and rain for Thursday into early Friday morning.

06z ECMWF-AI forecast for Thursday 8 AM. Warm front returns with edge of heat dome (approximated by the 576 thickness line) over us. (Click on image for a larger view.)