Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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How Much Rain Did We Get?

Posted Friday 09/26/25 @ 9:09 AM — As always, the model forecast placement of rain and the actual rainfall coverage only match in a general way. Additionally, while some areas received large amounts of rain, some areas, over the past three days, received relatively modest amounts.

Here’s the MRMS final totals over the past 72 hours—

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.)

This weekend’s forecast will be tricky. A low pressure system to our south will move up towards us, blocked by high pressure to the north. Most models have come together with a forecast of light rain beginning as early as 3-4 PM Saturday, with much of the rainfall coming towards and after midnight.

How long the rain and clouds linger into Sunday is in question. Clouds and showers are expected to exit about 1-3 PM in northern sections, but clouds may linger from the city southward.

The model guidance is clouded by two tropical systems in the eastern Atlantic, whose moisture outflow and energy will undoubtedly affect the accuracy of the model forecasts over the next several days into next week.

ECMWF-AI forecast for Sunday at 2 PM. (Click on image for a larger view.)

Check back later this afternoon for my regular Weekend Weather Forecast.

Thursday Storm Update

Posted Thursday 09/25/25 @ 7:12 PM —So far, radar is not showing the weakening of the storms as forecast by the HRRR and RRFS. 7:43 PM — Signs that the line of storms is now weakening.

Posted Thursday 09/25/25 @ 5:38 PM — The last slug of rain is to our west. Despite the line of storms showing on radar right now, the model forecast is for significant weakening as it passes through Philadelphia.

Current Radar with RAP model vertical velocity pressure (yellow contours) and 700 mb wind streams (green contours) (Click on image for a larger view.)

Another image from a highly rated iOS app, “WeatherFront” —

High resolution radar image. (Click on image for a larger view.)

Here’s the latest available RRFS (19z). Its forecast has changed considerably since early this morning: Storms weaken considerably as they pass through the city—

19z RRFS forecast simulated radar at 9 PM. The line of storms weakens even more over the next hour.

The Phillies game may be minimally impacted, if we’re lucky.

Posted Thursday 09/25/25 @ 11:39 AM — As forecast several days ago, the heaviest precipitation with this stalled system has, so far, moved to our west. Another band of heavier precipitation developed in NJ. Here’s the current MRMS total precipitation received estimate up to this morning —

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

The models are still showing areas of instability to develop over our area with the potential for areas of heavy rain during the afternoon. It appears that the final major band of rain will move through our area between 6 PM and 9 PM, according to the latest RRFS and HRRR. This is ahead of the actual front that moves through before daybreak on Friday.

RRFS 1 hour precipitation at 7 PM (Click on image for a larger view.)

It should be noted that the HRRR is less robust with this final push of rain than the RRFS.

Friday looks to be sunny and quite nice.

Thursday Forecast Update

Posted Wednesday 09/24/25 @ 9:17 PM — The model forecast guidance for the next several days has changed. It appears that the heaviest rain will now occur between now and Thursday evening.

There’s a chance of severe thunderstorms (localized high winds, heavy rain) between noon and 4 PM Thursday. The heaviest activity will be north and west of the city.

Here’s the RRFS showing areas of strong upward motion—

18z RRFS at 3 PM with 700 mb RH and DZDT upward vertical velocity centers. (Click on image for a larger view.)

RRFS Calculated Severe Weather Index (SWEAT INDEX)

RRFS SWEAT index at 3 PM. Orange SX > 300 shows severe possible/likely (Click on image for a larger view.)

It looks like the front moves through late Thursday night and there will be clearing on Friday.

Wednesday Forecast Update

Posted Wednesday 09/24/25 @ 6:58 AM — Another wave of low pressure developing along the stalled frontal boundary will pull another warm front up into our area later Wednesday, along with showers and possible thunderstorms.

Most models show little rain activity before 4-5 PM, but the RRFS (experimental) underwent some recent revisions shows showers and thunderstorms popping up as early as 1-4 PM around the city. (I’m not sure if the recent RRFS revisions are moving the model in the right direction. This will be a good test.)

The rainfall tonight into Thursday may be quite heavy, with as much as 2+” of much-needed precipitation.

06z Canadian RGEM forecast total precipitation by Thursday. The experimental RRFS (not shown here) is similar, but with the axis of heavy rain nearer the city. (Click on image for a larger view.)

Another round of heavy rain again later Thursday afternoon into the evening. As mentioned on Monday, this final round of heavy rain may bring strong to severe thunderstorms Thursday evening.

Tuesday

Posted Tuesday 09/23/25 @ 8:20 AM — An indistinct warm front will slowly approach today, Tuesday, with the full effects not felt until Wednesday into Thursday.

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., I’ve drawn the approximate position of the warm front although this feature is not as distinct as drawn. (Click on image for a larger view.)

Current cloudiness should dissipate this morning and we should see increasing sunshine. High clouds move back in from the west and lower clouds move in in the far northwest suburbs by about 4 PM. Rain moves in this evening. As posted yesterday, most of the rain will shear off to our northwest, but the latest model guidance suggests a bit more rain may make it into Philadelphia.

Dew points (humidity) will increase today into the mid 60s and closer to 70º by Wednesday. Uncomfortably high dew points in the low 70s by Thursday.

Wednesday looks to be considerably cloudy with some light showers possible in the mid to late afternoon.

Thursday, especially Thursday evening will have somewhat heavy rain and possible severe weather.

Many models are showing total rainfall through Friday to be in the 1.5- 2.5″ range with locally higher amounts.

Mon 09:09 PM Update —While this is well into the future, there’s a strong signal for possible severe weather late Thursday afternoon into Thursday evening. I’m keeping an eye on this.

Posted Monday 09/22/25 @ 5:43 PM — A cold front will approach the area Tuesday, but it will stall somewhere near our area, likely just northwest of the immediate Philadelphia area.

Tuesday will have sunshine through high cirrus clouds. It will be warm, with highs in the low to mid 80s near the city.

Showers and thundershowers will develop in far western sections of Montco and Bucks counties about 6 PM Tuesday. Much of the rain and thunderstorm activity will slide just west of our area; much of the rain will miss us.

12z RRFS total rainfall through midnight Tuesday. Most of the rain stays to our northwest. (Click on image for a larger view.)

The above RRFS is very similar to the latest ECMWF and the ECMWF-AI forecasts. The Canadian RGEM and GDPS with “AI Spectral Nudging” models have a bit more rain closer to the city.

Waves low pressure form along this stalled front and move up into our area through Friday. Timing these wave exactly is difficult in advance, but here’s how it looks currently—

Wednesday looks to be mostly cloudy with a very low chance of light shower. A wave developing on the front will bring rain late Wednesday night into Thursday morning. Then another break in the rain is expected with just cloudy conditions Thursday afternoon.

Heavy rain and thunderstorms likely later Thursday evening and night.

Models guiding this forecast: GDPS-AI, NBM, RGEM RRFS , ECMWF-AI,

Friday may clear (?)

Originally Posted Sun 5:47 PM —Our beautiful dry weather will last through some of Tuesday. The models are in good agreement with a forecast of a cold front (1) coming through our area Tuesday late afternoon into the evening with showers and maybe a thunderstorm. The front hangs up just to our south and east as an upper low (3) closes off and a surface low sends waves of energy up along the front.

Today’s 12z ECMWF is in good agreement with the ECMWF-AI and the Canadian GDPS-AI models for a rainy Wednesday through Friday as a front hangs up near us. (Click on image for a larger view.)

We’ll have a good chance of rain from Tuesday night through Friday, likely with some breaks in the action. This is an evolving pattern and I’ll update with specifics during the week.