#Philadelphia #weather #PAwx #PhillyWx

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—

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.

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”.

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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.

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

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.

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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—

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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.

Here’s the AIGFS forecast for 8 AM Wednesday—

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″

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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—

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.

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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.

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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.

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.

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#Philadelphia #weather #PAwx #PhillyWx

Forecast Update

Updated Saturday 04/25/26 @ 11:12 PM — Latest models now show some clearing a bit later, about 3PM.

Updated Saturday 04/25/26 @ 9:20 PM — The rain will be ending from west to east about 3 AM Sunday morning. Low clouds in the morning thin for sunshine by early afternoon, sooner north and west. Windy conditions slowly subside during the afternoon.

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Significant Forecast Change

Posted Saturday 04/25/26 @ 7:50 AM — Last night, the models were predicting about 1 inch + of rain for our area. The trend (which is in-line with our increasing drought pattern) has moved towards about 1/3 of an inch or less.

As mentioned yesterday, this is not a very moist system and the large precipitation amounts previously forecast was based on a great deal of energy causing lift.

Unfortunately, this energy is looking much lower and its influence will be in the the NYC area and northward.

It should be noted that many models were trending towards a northern track of the heavier rain over the past two days.

Some showers are right on our doorstep at 8 AM, moving into an area of supportive upward vertical motion—

While some showers are possible this morning, most of the rain will fall after 4:30 PM into the night.

Sunday should be cloudy, but dry.

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Originally Posted Fri 4:47 PM —A complex and energized system in the Midwest will push its moisture east towards us this weekend. Plenty of rain is possible with the timing now late Saturday into Sunday morning, a change from earlier forecasts.

Here are the current trends that have evolved since my posting last night. There are changes in the timing and and heavy rain banding.

• The heavier rain for Saturday is delayed until the evening and night time hours.
• The heaviest axis of rain will be a bit further north than forecast by the previous RRFS- heaviest rain in NYC
• Two periods of rain are expected on Saturday, one beginning during the morning hours that will feature light scattered showers. Following a break in the action early Saturday afternoon, heavy rain will develop towards evening and continue through the night.
• This is not an extremely moist system (Precipitable water about 1.3″). Any heavy rainfall will be the result of strong dynamics.

Saturday

A glimmer of sun early, then cloudy in the morning, light, scattered showers move in around noon. There may be a break in the action as most of the rain initially moves far north of our area. The rain restarts about 4-7 PM in our area and becomes rather heavy by midnight. The rain continues into Sunday morning. Windy and gusty.

High temperatures 54º Blue Bell, 56º Philadelphia. Higher than average uncertainty ± 3.0º

Sunday

Rain continues into Sunday morning, tapering off by early afternoon. There’s remains uncertainty at this time regarding the time the rain ends, with some models continuing it into early afternoon and some having it end mid-morning. VERY WINDY! GUSTS > 35 MPH.

High temperatures 52º Blue Bell, 52º Philadelphia. Higher than average uncertainty ± 3.3º

Another chance for rain on Wednesday next week.

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#Philadelphia #weather #PAwx #PhillyWx

Sunday Forecast Update

Posted Sunday 04/19/26 @ 11:17 AM — The latest RRFS shows the rain to have exited the area by 2 PM

Posted Sunday 04/19/26 @ 10:31 AM — The cold front went through our area before daybreak as forecast. This cold front is what is called an “anafront“. Unlike most cold fronts, where the rain precedes and accompanies the front, an anafront has the warm air rising over the cold air wedge, causing rainfall to fall significantly behind the actual frontal passage.

The rainfall is moving to the northeast as the main line of rain slowly moves eastward. The models have revised the total rainfall lower as most of the energy driving the showers moves off to the northeast before it gets here. Total rainfall is likely to be less than 0.20″ in the general Philadelphia area.

This event again captures a tendency towards drought-like conditions for our area, especially for the immediate PHL area and S. Jersey. Regular readers to this site know I’ve been talking about this developing dry pattern over the past month or two.

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Weekend Forecast Update

Posted Saturday 04/18/26 @ 10:01 AM — Low clouds lifted a few hours ahead of what had been forecast. Mostly sunny for the rest of Saturday.

For Sunday, rain begins somewhat later than previously forecast, now about 8-9 AM. Rainfall amounts will be somewhat lower than forecast yesterday. Total rainfall about 0.3″. High wind gusts around 10-11 AM!

Rain ends and clouds lift between 3 and 4 PM Sunday.

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A Quick Update

Posted Friday 04/17/26 @ 6:09 PM — Some areas saw showers Friday afternoon. Some additional activity may move into our area over the next hour or so. There is some strong support near Philadelphia and just east. We’ll see if they hold together.

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Originally Posted Fri 4:55 PM —High pressure behind the front that broke our early Spring anomalous heat wave will move off to our northeast. An easterly flow will keep temperatures cooler on Saturday. A strong cold front moves through Sunday morning with some much-needed rain.

Saturday

High pressure departs and an easterly flow of air will bring low clouds in the early morning. These dissipate by late morning and we’ll have sunshine through high cirrus clouds. An easterly wind keeps temperature east of the city quite cool.

The winds increase from the southeast as a warm front approaches later Saturday night.

High Temperatures Saturday– warmer further west from the shore—

Sunday

Cloudy with rain as a cold front moves through the area. Rain begins before daybreak. It becomes windy and gusty by late morning as the cold front moves through. There’s a chance of a thunderstorm embedded in the showers. Currently, about 0.5 inches of rain is expected. Rain ends and skies gradually clear by late afternoon.

Temperatures drop behind the cold front and remain in the 60s falling later in the day. It will be windy.

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