Philadelphia Area Weather Forecasts

THIS WEEK’S WEATHER

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Friday & the Weekend Outlook

Posted Thursday 05/28/26 @ 8:18 PM — An upper closed low near the New England coast and surface will take time to move away as another low drops down from northeastern Canada on Sunday. An elongated area of high pressure along with these upper low systems will bring north to northwesterly winds and cool temperatures.

12z ECMWF-AI shows vertically stacked upper and surface lows over northern New England on Saturday morning. A flow of cool dry air associated with an elongated area of high pressure (blue arrow) will keep things breezy and cool. The winter-time thickness lines (red violet, yellow) show how cold this pool of air is. (Click on image for a larger view.)

By Sunday, another low drops down into the same area—

12z ECMWF-AI forecat for Sunday evening. Another surface low takes form over New England (Click on image for a larger view.)

While the simplistic forecast would be just fair skies, windy and cool temperatures through Monday, these systems have a habit of surprising with instability cloudiness and even light showers (as happened on Thursday). Many of the atmospheric changes with upper lows are subtle and not well-predicted by the models. So, one shouldn’t be surprised by poor forecasts and last minute changes over the next several days. Overall, it will be dry with low humidity.

Wednesday Forecast Update

Posted Wednesday 05/27/26 @ 9:10 AM — The showers have developed along the stalled front. Here’s the current setup displayed as lines of equal potential thermal energy—

Isentropic (theta-E) lines run west to east. Most of the Philadelphia area will be along lines of lower potential energy (2). Southern Chester and Delaware counties are downstream from a center of isentropic lift (1) that may result in stronger storms during the afternoon. The entire setup also shifts southward during the day. (Click on image for a larger view.)

The storms above are expected to move into our area between 1 and 3 PM. Here is the HRDPS, showing much of the activity will be south. The latest RRFS is similar, with the storms even further south into southern Chester and Delaware counties.

06z HRDPS shows the rain and embedded storms mostly from the city and southward at 2 PM (Click on image for a larger view.)

Another line of storms may move through the entire area after midnight tonight.

Wednesday Forecast

Updated Tuesday 05/26/26 @ 9:00 PM — The stalled front to our south will try to move back north as a warm front propelled by a low pressure wave.
Showers and thunderstorms will break out after midnight tonight and scattered and interspersed with possible sunny breaks on Wednesday with several rounds of showers and storms moving through.

While storms are not expected to be very severe, there’s plenty of moisture and instability to work with so some storms could be quite strong with localized heavy rain.

18z REFS total rainfall (probability matched mean) by late afternoon Wednesday. Model placement of heaviest rain is often not literal. (Click on image for a larger view.)


The wave will then propel the front to move south again as a cold front with some additional showers towards evening. Total rainfall will range from 0.75 inches to 1.25 inches of rain

Tuesday Forecast

Posted Tuesday 05/26/26 @ 9:02 AM — The more optimistic HRDPS (regarding today’s cloud cover) was correct and this morning’s RRFS has joined its forecast for a mostly sunny day in the Philadelphia area. (To be accurate, it will be sunny though a layer of thin clouds.) Highs today 81º-83º.

06z HRDPS cloud cover at 2 PM Tuesday. (dark= clouds) The HRDPS tends to define cloudiness differently than the US models. To a weather nerd. what the HRDPS forecasts might be considered incorrect since it shows no clouds where there certainly will be some high or even medium thin clouds. Instead the HRDPS defines cloudiness more along what the average person might distinguish as cloudy skies vs sunny skies. (Click on image for a larger view.)


That wave of low pressure to our south will bump some clouds northward and maybe even a few showers/thundershowers later tonight into Wednesday morning. Then sunshine in the afternoon.

Posted Monday 05/25/26 @ 7:25 PM — There was some sun this afternoon after 3 PM. I always look at the HRDPS and compare it to the RRFS for post-frontal cloudiness.

It was hoped that there will be some sun on Tuesday, but the latest RRFS model has that same frontal boundary that gave us all that rain stalled to our south and clouds are forecast for much of Tuesday, perhaps with some bright spots. The HRDPS is more optimistic for sunshine on Tuesday, with areas north of the city in the clear.

18z RRFS shows showers to our south on Tuesday morning (including Cape May) with mid-level clouds over our immediate area. (Click on image for a larger view.)

A wave riding along the stalled front may bring some light showers on Wednesday.

Originally Posted Mon 9:10 AM —A weak cold front is passing through this morning. Scattered light showers will continue this morning and will become even more sparsely scattered early this afternoon. (Most areas will have no rain this afternoon/eve except extreme southern NJ and Delaware.)

Regarding the clearing previously expected about 3-4 PM, the latest RRFS and HRRR have backed away from that with just a few bright spots; the mid level clouds hang tough. (The latest HRDPS does show clearing from Philadelphia, north and westward, after 3-4 PM, so still a possibility. )

For Tuesday, high pressure builds in and we’ll have sunshine through thin high clouds.

For the rest of the week, this front stalls near the Carolinas and a wave of low pressure will move along the front, possibly bringing our area (especially south of the city) some showers on Wednesday.

Then cool high pressure builds in for Thursday and Friday.

For the coming weekend, an anomalous pool of cold air aloft and a closed upper low that may retrograde west and south is expected to spawn low pressure off the NJ coast! A complicated scenario and too complex to be sure of this at this time.

06z ECMWF-AI forecast for 2 PM Saturday. Anomalous upper closed low to our north spawns a surface low near the NJ coast? Too strange to believe right now. (Click on image for a larger view.)