THIS WEEK’S WEATHER - Philadelphia Weekend Weather & Storm Forecasts

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Friday and Weekend Outlook

Posted Friday 05/01/26 @ 8:26 AM — Mostly sunny skies this morning, but a weak approaching cold front and an upper air disturbance in western PA will will bring periods of mid-level cloudiness this afternoon. The edge of the cold front and the area of clouds can be seen on visible satellite imagery—

Visible satellite at 8 AM with RAP and HRRR model parameters. **The area of mid level clouds will move in from the southwest (white arrow)** as the cold front sinks towards us.

The wind flow aloft is forecast to be anticyclonic by the time the cold front approaches this evening. Showers moving into western sections (Lancaster/Berks) are expected to dissipate as they move towards Philadelphia. They may re-form in S. Jersey later this evening.

The high pressure behind the cold front will block a southern coastal system from moving into Philadelphia over the weekend. Coastal South Jersey may see some showers and some cloudiness associated with this coastal system may extend into and just beyond Philadelphia on Saturday. Northern and western suburbs will likely see much more sun on Saturday.

06z RRFS cloud cover forecast for **Saturday** at 1 PM Blue=low clouds, maroon=mid-level clouds, tan = high cirrus clouds (Click on image for a larger view.)

Sunday looks to be quite nice and cool. Perfect for the run on Sunday.

I’ll update later today with my regular “Weekend Weather Forecast”.

Thursday -Friday-Saturday

Posted Thursday 04/30/26 @ 9:21 AM — A large sprawling area of energy will push a secondary cold front through our area this evening. The energy, captured by the GFS model as “potential vorticity”, will spawn some showers downstream to affect our area between 5 PM and 8 PM Thursday evening.

06z GFS with **red/orange** “potential vorticity’ very high in the atmosphere along with some moisture (maroon shading) will spawn scattered showers in the Philadelphia area by 5-8 PM. The dip in the red 500-1000 mb thickness line captures the dip in the jet stream.- an almost winter-like appearance. (Click on image for a larger view.)

A close up of this potential can be seen in thermal energy (theta) currently in western Pennsylvania this morning—

Area of potential thermal energy (theta) -yellow contours in white box, in western PA this morning will spawn showers later today. (Click on image for a larger view.)

The showers will be light and scattered Thursday early evening.

A similar scenario may develop Friday afternoon into evening as well.

Saturday Outlook– A low pressure system will move northeastward and brush the NJ shore during the evening. A combination of upper air energy and moisture from this system may cause light showers here on Saturday.

06z GFS forecast for 5 PM Saturday. Dip in the jet along with low pressure off the coast will cause scattered showers here. (Click on image for a larger view.)

Thu 10:54 AM —Forecast Review —We had some much-needed rainfall yesterday, but we could use another 2 inches for the month of April.

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.)

Wednesday Forecast Update

Posted Wednesday 04/29/26 @ 9:36 AM — This morning’s models continue with a late trend that started with last evening’s model runs— much of Wednesday will be rain free, but showers/rain will move into western Chester, Montgomery and Bucks counties between 5-6 PM and closer to the city by 7-8 PM.

Total Rainfall expected is shown below—

Wednesday

Posted Tuesday 04/28/26 @ 7:45 PM — A system causing severe weather in the central US will bring rain to our area Wednesday afternoon into evening. A push of cold air will bring cooler temperatures for Thursday and Friday.

Water Vapor image at 7:30 PM with superimposed RAP model MSL pressure (black contours), GFS/potential vorticity contours (violet fine contours) with MRMS Radar and surface wind barbs, (Click on image for a larger view.)

Here’s the AIGFS forecast for 8 AM Wednesday—

18z AIGFS forecast for 8 AM Wednesday. Rain advances quickly from the west.

Current models show rain moving in about 2-3 PM on Wednesday. It will be a fast mover and rain should taper off before midnight after the cold front passes through. Total rainfall about 0.40″

Tue 4:46 PM —Forecast Review —A line of very light showers/sprinkles moved through Chester County into eastern parts of Montgomery County and western sections of Philadelphia around 3-4 PM—

MRMS radar 3:26 PM Very light showers. (Click on image for a larger view.)

The 06z REFS and 06 HRRR gave a better forecast than this morning’s 12z runs.

Tuesday’s Challenging Forecast

Posted Tuesday 04/28/26 @ 11:06 AM — This morning’s RRFS and HRRR have really cut back on the showers, closer in line with the other models. They still show very light and widely scattered sprinkles about 3 PM. The two new tools I used earlier show the vertical motion outpacing the moisture; they’re not in-sync— by the time the moisture gets here, there isn’t much vertical motion. Not the best debut of a new forecasting tool. It’s a learning experience.

Posted Tuesday 04/28/26 @ 8:29 AM — Despite the sunshine right now, all models show clouds moving in between 9 and 10 AM near the city.

The big question, as alluded to yesterday, regards the chance of showers this afternoon. The low-resolution ECMWF-AI keeps any showers to our west. The low-resolution AIGFS shows some very light shower activity in western suburbs by mid to late afternoon. The Canadian high resolution HRDPS keeps us dry as does the Canadian RGEM.

Here’s the rub— the Model Blend (NBM) shows areas of showers possible mid to late afternoon. The high resolution HRRR and RRFS shows definite showers around 3PM for much of the western suburbs.

I’ve been enlisting the help of Anthropic’s Claude and OpenAI’s ChatGPT over recent months to expand my toolbox of forecast tools and methods.

Two recent additions are the addition of “Frontogenic Forcing Analysis” and “Q-Vector Analysis”. Both techniques take data I already download and apply some complicated equations to the data using Python, well beyond what I could have come up with myself. I’ve automated these approaches and I’ll be using these tools going forward.

Both of these tools are used to show high resolution ‘forcing’ and ‘convergence’ that causes air to rise, cool and create precipitation, provided there is enough moisture.

For today, both tools show low level ‘frontogenic forcing’ in an area of enhanced moisture about 3-4 PM mostly from the city westward. (There isn’t much upper level forcing coinciding with the moisture, perhaps fooling the lower resolution models.)

At 3 PM, the RRFS (and the HRRR) show a ‘couplet’ of frontogenic forcing just west of the city—

06z RRFS shows strong upward forcing at 3 PM just west of the city. Strong downward forcing to the east. These areas of rising/falling occur as couplets (conservation of momentum) (Click on image for a larger view.)

With this new tool, I’m inclined to go with a chance of light showers west of the city about 3 PM – 4 PM. The area becomes predominantly downward motion after 4 PM suggesting not much precipitation east of the city.

I guess we’ll see.

Tuesday-Wednesday Forecast Update

Posted Monday 04/27/26 @ 5:56 PM — Over the past day, there has been a back and forth between a dry forecast for Tuesday and a slight chance of very light showers during the late afternoon. Regardless, it will be mostly cloudy on Tuesday.

There’s still a low signal for some very light scattered showers mostly west of the city and specifically in Chester and Delaware counties between 3 PM and 6 PM. The Model Blend (NBM) shows a very low probability. The RRFS shows some light simulated radar images but nothing making it to the ground.

Things change for Wednesday as most models show a high probability of rain moving in from the west between 2 and 3 PM.

18z RRFS forecast (simulated radar) at 3 PM Wednesday. (Click on image for a larger view.)

Previously Posted Sun @ 4:23 PM — —I’ve entered our rainfall totals for Saturday into Sunday on my previous posting.

High pressure will nose down from the the northeast and provide sunny weather for Monday. Highs 69º-71º

An approaching system from the west will bring plenty of clouds on Tuesday. The energy with this system will move off to the far northwest and ~~we likely won’t see any rain from this system until late afternoon Wednesday.~~ Updated Sunday 04/26/26 @ 7:46 PM — This afternoon’s RRFS does show some showers moving in around 3-6 PM on Tuesday.

12z GFS forecast for Wednesday at 7 PM A dip in the jet (red and violet thickness lines) will push a cold front through Thursday. (Click on image for a larger view.)

More clouds on Wednesday and the Model Blend (NBM) showing rain moving in mid-afternoon ahead of a cold front.

A cold front will bring cooler temperatures Thursday and Friday.

Updates during the week.