Updated Friday 06/05/26 @ 3:42 AM —The heat is returning for Friday and Saturday with highs 92°-94°. A cold front moves through late Saturday afternoon or early evening. As described in an earlier post, the upper air configuration will work against precipitation developing near Philadelphia and at best we’ll have widely scattered showers and storms
Updated Thursday 06/04/26 @ 9:53 AM — The increasingly dry, warm weather continues as an upper level ridge tries to build in the Central US. Temperatures approaching or exceeding 90º will be likely Friday and Saturday. It appears that we have re-entered a dry pattern.
A ‘cold’ front approaching and moving through on Saturday afternoon and evening will bring an increasing chance of showers and thunderstorms to our area—
00zECMWF-AI forecast for Saturday evening. A cold front moves through with showers and thunderstorms, especially western sections. The showers may fall apart as they move eastward. (Click on image for larger view.)
Despite what looks like a lot of rain on the above ECMWF-AI graphic, the upper air situation does not support much in the way of rainfall for Philadelphia. As has happened many times in recent months, we will be in what is called the ‘right entrance region’ of a small jet streak—
ECMWF-AI 300 mb winds show Philadelphia in the area of the jet streak called the ‘right entrance region’. The right entrance region has downward vertical motion that is not supportive of rainfall. The showers will likely fall apart as they move through Philadelphia later Saturday afternoon and evening. (Click on image for larger view.)
So the immediate Philadelphia region will likely miss out on another opportunity for much-needed rainfall on Saturday.
Originally Posted Sun 9:03 AM —A relatively stagnant pattern with cold low pressure in eastern Canada and an elongated surface high pressure to our west will keep relatively cool weather and dry conditions for our area this week.
While some of the high resolution models have a disturbance wrapping around this low bringing some widely showers late Monday afternoon/evening, this would likely be the only chance of any precipitation this week.
Tuesday‘s forecast—
06z ECMWF-AI forecast for Tuesday. High pressure combined with low pressure in Eastern Canada will maintain a northerly flow of dry, cool air. (Click on image for a larger view.)
By Thursday, high pressure sinks down into the central US while a semi-tropical system forms in the western Atlantic—
06z ECMWF-AI forecast for Thursday. The somewhat tropical low will move off to the northeast, away from the coast. Dry weather with moderating temperatures for our area. (Click on image for a larger view.)
There’s a chance of more summer-like conditions returning by next weekend, along with the possibility of some rainfall.
During this relatively tranquil weather period of the coming weeks, my posting activity will be reduced from its usual level. The European and Canadian models have made some significant changes to their models in recent weeks with different servers and different parameter names.
The RRFS will be moving to the main NOAA server (called “NOMADS”) from their development location at AWS on June 9th, and over the summer, NOAA will be dropping several long-standing models such as the NAM.
As many long time visitors to this site know, I download and post-process all the model data I use for these forecasts and graphics. I’ve already been working on the updating my data workflows over the past week and I expect the process to continue for the next several weeks. If we get some active weather, I hope to post at that time.
The weather remains cool and windy for this weekend. The current stratocumulus layer is forecast to dissipate early afternoon. A stagnant upper air low in northeastern Canada will be sending disturbances down to our area along with some significant winds due to the pressure gradient that has set up.
Over the coming weeks, updates will be less frequent than usual.
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.
18zREFS 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 comingweekend, 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.)
