Sat 7:38 PM —Forecast Review — Many areas in the immediate Philadelphia area have seen little in the way of heavy rain and severe storms. Most of the activity was in Bucks county, Central and Northern NJ and NY. The current rainfall shown in the latest MRMS shows this—
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.)
The RRFS, HRDPS and HRRR all show varying degrees of divergence/storm suppression over the immediate Philadelphia area this evening into midnight. We’ll see if this comes to pass. Always listen to the NWS for official warnings.
Saturday Forecast Update
Posted Saturday 07/18/26 @ 12:58 PM — For the record, I’ve resurrected my “parameter severity table” that compares today’s storms with past storms. For consistency, I’m using today’s HRRR, although I’d like to transition to the RRFS in the future.
You can see that today’s storms are highly comparable to past severe weather outbreaks. ( FYI, These high numbers were mostly found in South Jersey southern Salem and Cape May counties.)
Severity Parameter (HRRR)
Sept 1 , 2021 Tornado Outbreak (example of highly severe)
Severity Parameters Impact: ⚑ indicates favors Severity⚐ indicates Possible Severity ↓ indicates works against Severity ⇩ Significant, but less impact
Saturday Forecast Updates
Posted Saturday 07/18/26 @ 11:22 AM — The storms that eventually developed just to our north were produced by a warm front that is moving towards NYC at this time Last night’s model’s haven’t been too helpful. Here’s the current radar—
Water Vapor (Channel 14) with superimposed RAP model wind streams, Omega and MRMS radar at 11:15 AM (Click on image for a larger view.)
New 12z models with updated upper air data from weather balloon launches may provide some better guidance for this afternoon. The latest 12z RRFS, while still not handling the current radar, does appear to offer some insights.
It has some sun breaking out around 1 PM-4 PM. This will heat up the already unstable atmosphere.
A line of strong storms develops to our west and moves northeastward about 4-5 PM. These will mostly affect northern Montco and Bucks counties. Some may be severe.
12 Z RRFS RADAR/Rain at 5 PM. (Click on image for a larger view.)
By 6-7 PM, additional storms, some likely severe, move through—
12z RRFS Radar/1 hour rain at 7 PM. (Click on image for a larger view.)
By 10 PM, the cold front approaches with more storms—
The above graphics are provided as a rough guide. No model accurately predicts the exact location and timing of severe thunderstorms.
Posted Saturday 07/18/26 @ 8:27 AM — None of last night’s models really come very close to matching the current radar—
Radar with RAP model streamlines and Omega (yellow contours) (Click on image for a larger view.)
Nonetheless, the area of storms just to our west will move into parts of Philadelphia in the next hour or so. This was partly predicted by last night’s Canadian models, but the HRRR, RRFS and NAM-NEST didn’t show any of this. Trying to nail down the timing of the storms this morning doesn’t look possible at this time as the 06z model runs still aren’t capturing reality.
That said, the period of the most severe weather remains 3 PM through 8 PM and later in S Jersey and Delaware. The models also hint at another line of showers moving through between 10 PM and midnight.
Here’s the current Model Blend that gives an idea of the difficulty in nailing down the timing—
New models based on 12z upper air measurements will be trickling in over the next few hours. I hope to update before noontime with more details on severe weather.
Severe Weather Possible Saturday Afternoon
Originally Posted Fri 8:43 PM — A series of disturbances with plenty of energy will converge on our general area on Saturday. Since this is being driven by two separate pieces of energy, there is some disagreement among the models whether the more severe weather will occur in NY State or nearer Philadelphia.
The current water vapor image shows the two disturbances—
Water Vapor image at 5 PM with superimposed RAP model MSL pressure (black contours), 500-1000mb Thickness (yellow contours) , GFSpotential vorticity contours (violet fine contours) with MRMS Radar and surface wind barbs, (Click on image for a larger view.)
I’m not sure how well the new RRFS is handling of this complex system, but its forecast is somewhat similar to the HRRR. It is favoring a more northern severe weather forecast and it has very little activity around here Saturday morning.
The ECMWF as well as the Canadian GDPS with “AI Spectral Nudging” and the GFS show thunderstorms moving through Philadelphia around 8-10 AM, followed by a break in the action. We likely will see some breaks of sunshine through high clouds and residual smoke late morning into early afternoon.
12z GDPS with AI spectral input at 10 AM Saturday. The model shows this area of storms forms as early as 8 AM. (Click on image for a larger view.)
The real action occurs between the hours of 3 and 8 PM. With sunshine earlier, very high instability, high PWAT, severe storms are likely to form in several lines moving from northwest to southeast. The Model Blend shows moderate probability (for this region) of tornadic activity around 3-8 PM.
Additional line of storms move through around midnight!
The RRFS SWEAT index shows a few areas exceeding 400, the threshold for severe weather like tornadoes at 8 PM —
SEVERE WEATHER INDEX THREAT (SWEAT) index based on RRFS model data at 8 PM. 400 is considered a major threshold for severe weather. Over the past 14 months that I’ve been calculating this index, this is first time I’ve seen values above 400.
Those areas of severe weather expand as they move eastward into NJ by 9-10 PM.
Rainfall amounts may range 1.5-2″. Some localized, heavier accumulations likely.
The Canadian HRDPS seems to capture Saturday best—
I expect that by Saturday morning, things will better clarify regarding timing.
Saturday
A stormy day, with thunderstorms possibly beginning as early as 8 AM. Severe thunderstorms and possible tornadoes are looking likely during the late afternoon and/or early evening. There may be some breaks of sun late morning or early afternoon. Very humid and becoming windy. Lines of strong storms will move through from 3 PM to 9 PM. High temperatures 85º Blue Bell, 88º Philadelphia. High uncertainty with a sd of ± 4.1º
Sunday
Becoming mostly sunny Highs (Model Blend): Blue Bell- 81 º Philadelphia 83º ±4.0º High sd and uncertainty.
Posted Friday 07/17/26 @ 8:53 AM — The dark skies this morning is predominantly due to heavy smoke, not cirrus clouds. When trying to see the sun through the smoke, the measurement that captures the dimming is the amount of particulates in the full vertical column of air.
Here’s the current full column “vertically integrated smoke particulates”—
11z HRRR smoke model full integrated column smoke at this time (9 AM) . This affects how dim the sun is. (Click on image for a larger view.)
With air quality measurement, the main interest is the smoke that you break, or as they call it, “near-ground” smoke which is really measured/calculated at a height of 8 meters—
11z HRRR near ground smoke at 9 AM (now) Friday. This affects what you breathe. (Click on image for a larger view.)
No rain today and the near ground smoke level remains high near the city even at 3PM—
06z HRRR near ground smoke at 3 PM. While the sun may brighten considerably, the near ground smoke improves mostly to our far north and northeast. (Click on image for a larger view.)
Thunderstorms are likely in Philadelphia by 8-10 AM Saturday. By 1PM Saturday. smoke is clearing out near-ground as the winds turn to the southwest—
06z HRRR near ground smoke forecast for Saturday at 1 PM. Southwesterly winds will clear out the smoke from our region. (Click on image for a larger view.)
As mentioned, Saturday looks to be an active day weather-wise. Most models have the heaviest rain to our north, in contrast to forecasts of previous days where the bull’s-eye was right over us.
Several rounds of rain and heavy thunderstorms, one in the morning, several in the afternoon on Saturday. There may be a hiatus between 6 PM and 8 PM, with more storms returning after 9 PM to midnight as a cold front moves through. Details will likely change. Stay tuned.
Friday -Sunday Outlook
Posted Thursday 07/16/26 @ 5:25 PM — The showers predicted this morning by the NAM-NEST did materialize, although they were less extensive than had been forecast. Most of the area saw no activity.
Radar loop around 4 PM Thursday with RAP model 700mb wind streams and Max Omega. (Click on image for a larger view.)
Skies finally clear of most cloudiness on Friday, but the smoke will hang tight. Here’s this afternoon’s 18z HRRR-smoke forecast for Friday afternoon. There will be some short-lived relief in northern sections—
18z HRRR smoke forecast. Near ground smoke. (Click on image for a larger view.)
Unfortunately, the model shows this northern clearing to retreat back north by Friday evening into Saturday!
As for Saturday, several energy triggers will converge over us with heavy rain.
Water Vapor image around 4 PM today with superimposed RAP model MSL pressure (black contours), 500-1000mb Thickness (yellow contours) , GFS potential vorticity contours (violet fine contours) with MRMS Radar and surface wind barbs, The violet contours represent energy triggers that will converge over us. (Click on image for a larger view.)
The latest ECMWF shows 4 inches of rain over Philadelphia; it’s too early to take this literally.
The GFS also shows many rounds of rain and storms on Saturday into Saturday night, although not 4 inches.
12z GFS forecast for Saturday at 2 PM. Plenty of rain/storms. (Click on image for a larger view.)
Clearing on Sunday. Sunday looks to be sunny.
Saturday’s weather active weather will require monitoring.
Thursday Update
Posted Thursday 07/16/26 @ 9:57 AM — The current smoke conditions are as bad as had been expected. They should moderate a bit during the day, but still in the hazardous zone. (Total smoke now is estimated to be 120-184 x 10-9 kg/m3. It’s forecast to go to 300 about 10 PM tonight. Normal is 0 – 20 )
12z HRRR Smoke Model. The smoke level is defined at 8 meters above ground. (Click on image for a larger view.)
Today’s highs—
12z Model Blend (NBM) mean high temperatures for Thursday. Philadelphia and Blue Bell have standard deviations of ± 2º (Click on image for a larger view.)
Several models have light, isolated / scattered showers moving through about 4 PM or so. Of interest is the latest NAM-NEST which shows a large cluster of strong storms moving through Montco/Bucks and part of Philadelphia between 4-6 PM. This model is currently an outlier, but its forecast should be kept in mind.
The extended range forecast- a stormy Saturday with several rounds of showers and storms from early morning (7AM) through midnight. The storms look to become more intense during the evening. Things clear out on Sunday.
Thursday through Saturday Outlook
Posted Wednesday 07/15/26 @ 7:42 PM — Today’s high was 98º at the airport. The expected 100º high temperature was moderated by an increase in haze and smoke, reducing sunshine. It felt very hot despite this due to dew points near 74º.
A wind shift line is moving through Philadelphia right now; winds will shift from the southwest to the north-northwest. Somewhat less hot air arrives for Thursday. Unfortunately, it will bring down some very smokey air into our area.
18z HRRR Smoke Model. This depicts very highlevel of smoke near ground level (8 m) Thursday morning at 7 AM. (Click on image for a larger view.)
Once the winds pickup during the morning, the extreme smoke will mix and we’ll be left with very dim sunshine further dimmed by high cloud cover. The smoke may remain with us at least through Friday.
A very stormy Saturday is shaping up, especially towards evening. Most models show showers and thunderstorms moving over the heat ridge and into our area as early as Saturday morning. Several rounds of showers and storms are expected through the day and into the late evening. Heavy rain and some severe storms are possible.
18z RRFS forecast for Saturday at 7 PM. Heavy storms and rain. (Click on image for a larger view.)
Details over the next day or so.
Wednesday Update
Posted Wednesday 07/15/26 @ 9:41 AM — The smoke/haze will reduce the expected high temperatures today, especially north and west of the city. The airport (who lives at the airport?) may still just reach 100º but I wouldn’t be surprised if it falls a degree short.
Latest NBM just available. Model Blend high temps for Wednesday Fine Contours are near-surface (2m) temperatures in 1º increments. This is a good example of why the official temperature at the airport is not representative of the rest of the area. (Click on image for a larger view.)
Here are the other changes for today:
Dew points are forecast to be higher, in the low 70s. Despite the slight decrease in temperatures due to the haze/smoke, it will feel more uncomfortable and heat indices may be even higher. The combination of heat and smoke makes this a dangerous situation.
There’s an increasing chance of a line of scattered showers and thunderstorms moving down from the north between 7 and 9 PM. They may not make it into Philadelphia.
Smoke. Increased smoke and haze is forecast during the day. The edge of the very heavy smoke reaches the city around 9-10 PM tonight—
This morning’s latest HRRR 12z smoke forecast for 10 PM Wed. This is serious air pollution! (Click on image for a larger view.)
By Thursday morning, our area is sitting in serious smoke—
12z HRRR smoke forecast for 8 AM Thursday (Click on image for a larger view.)
Big Forecast Changes
Posted Tuesday 07/14/26 @ 9:17 PM — There has been a significant change in the forecast which includes the effects of wildfire smoke from Canada. First, wildfires in Canada along with a weak cold front dropping from the north may reduce temperatures on Wednesday and Thursday from previous forecasts.
The smoke plume is impressive and will dim the sunshine, along with making it somewhat hazardous to breath outside. The effects will be most noticeable late Wednesday into at least Thursday.
Today’s 18z HRRR smoke model forecast for Wednesday evening. A significant smoke plume will dim sunshine and adversely affect air quality. The plume. which has been held at bay by the heat dome, will descend towards us later Wednesday along with somewhat less hot air.. (Click on image for a larger view.)
The GFS captures the wind-shift line (drawn as a cold front) that drops south Wednesday evening—
18z GFS forecast Wednesday evening. Wind shift line with winds shifting to northwest, bringing more smoke to our area. The edge of the heat dome also shifts southward (576 thickness) also shifts southward. (Click on image for a larger view.)
Some models are showing the possibility of showers and thundershowers Wednesday evening, mostly Bucks county and northward.
By tomorrow morning, I would not be surprised to see further forecast changes. Stay tuned…
BTW, the weekend looks stormy at this time, although the forecast horizon is currently beyond the range of the high resolution models.
All Star Game – Philadelphia
Posted Tuesday 07/14/26 @ 5:13 PM — Temperatures for the All Star Game at Citizen’s Bank Park this evening:
18z Model Blend mean temperature at 8 PM. Heat index will be 93º (Click on image for a larger view.)
At 11 PM, the temperature will have dropped to 81º with Heat index of 83º.
For Wednesday, the Model Blend high temperatures have come down a bit—
18z NBM high temperatures (mean) on Wednesday. (Click on image for a larger view.)
Tuesday-Wednesday Update
Posted Tuesday 07/14/26 @ 10:02 AM — As forecast, the edge of the heat dome is just entering our area this morning. The “576 thickness” is an excellent proxy for the edge of the heat dome—
Water Vapor image at XX with superimposed RAP model MSL pressure (black contours), 1000-500mb Thickness (yellow contours) , GFS potential vorticity contours (violet fine contours) with MRMS Radar and surface wind barbs, The 576 thickness is further notated. (Click on image for a larger view.)
You can think of the thickness as the lines you would see if slicing through an onion sagitally, in this case, the lines represent lines of slicing through the atmosphere horizontally. This “thickness” is the difference in height of a pressure 1000 mb (near ground) and a pressure of 500mb (about 18,000 feet or a half atmosphere). Higher thickness means that “more atmosphere of lower density (hot) is needed to have it be be 5760 meters in thickness. (or the 576 line). Indirectly, it translates into a proxy for average temperature in this lower half of the atmosphere. ( physics – “Ideal Gas Law”).
Once the heat arrives, we’ll see temperatures reach the 90sº today and near and over 100º on Wednesday
06z NBM highs for Tuesday (Click on image for a larger view.)12z NBM mean high temperatures for Wednesday (Click on image for a larger view.)
Monday Update
Posted Monday 07/13/26 @ 9:02 AM — The edge of the heat dome, as captured by the 576 thickness contour, continues to push eastward from the upper Midwest.
Water Vapor image at 9 AM with superimposed RAP model MSL pressure (black contours), 500-1000mb Thickness (yellow contours) , GFS potential vorticity contours (violet fine contours) with MRMS Radar. The 576 thickness is emphasized. The line of clouds to our south will continue to be displaced southward. (Click on image for a larger view.)
The models continue to predict temperatures over 100º on Wednesday. Here’s the ECMWF and the Model Blend—
00z ECMWF high temps Wednesday. (Click on image for a larger view.)06z NBM mean high temps Wednesday (Click on image for a larger view.)
During our last extreme heat wave, the ECMWF’s forecast highs were too high. The Model Blend was very close to actual highs.
These forecast numbers will likely change in the coming days.
Originally Posted Sun 8:38 PM —We had more clouds than forecast Sunday and a few small isolated, short-lived storms developed this evening. The boundary of clouds and rain that moved through Saturday was just to our south, enough to have clouds and disturbances move through. Going back through past models, this morning’s GFS showed this cloudiness and the new RRFS didn’t show it until the 2 PM run.
Hopefully, the forecast for this week will be more straightforward. Basically we have the heat dome punching through from the west (not from the south or southwest). Over the past day or so, all-time record high temperatures have been recorded in the Upper Plains.
12z ECMWF forecast for 8 AM Wednesday. Edge of the heat dome (as captured by the 576 thickness line) is moving in from the west. (Click on image for a larger view.)
The return of the heat dome will move in from the west will bring high humidity and severe heat; several models (Canadian RDPS, ECMWF, German ICON) are forecasting actual temperatures slightly above 100º in Philadelphia for Wednesday. Specifics regarding the heat and humidity will clarify with Monday’s model runs.
I have to laugh… I made a point to remember the date (July 7thevening news 11 PM, ) when one of the weather entertainers who seems to believe she’s more than a “broadcast meteorologist” (in contrast to a real meteorologist who has actual degrees in meteorology and related earth sciences) made the outlandish statement that she didn’t think we would be seeing 100º temperatures again this summer! I said to my wife at that time, “how can anyone, no less a weather person, make that kind of statement the first week in July??”
FYI….There are real meteorologists on TV, including in the Philadelphia area. Back in the 1990’s the American Meteorological Society (AMS) created a category called “BroadcastMeteorologist” which required taking courses and an exam to obtain a certificate. There are probably continuing education requirements as well. The AMS had the excellent intention of trying to assure that the weather person on TV at least knew what a cold front really is. Somehow, the designation “Broadcast” got dropped (just like grade inflation) and now everyone on TV calls themself a “meteorologist”.
It’s unfortunate because it doesn’t allow true meteorologists who know what a Lagrangian motion equation is to distinguish themselves from people who should probably still be called the Weather Girl or Weather Man. (or in my case, theweatherguy, but at least my undergraduate degree was a BS in physics.)
Sat 4:59 PM —Forecast Review — My experiment with the HRRR, despite an excellent surface parameter match with the RTMA, showed the HRRR to be wrong about the thunderstorms today. It’s reassuring that our newest model, the RRFS, while over-forecasting rainfall did much better than the HRRR, as did the older NAM-NEST and the Canadian HRDPS. I’ll have to rethink my parameter match approach. Here’s today’s MRMS rainfall estimate through 4 PM —
Posted Saturday 07/11/26 @ 5:05 PM — Another batch of storms are to our northwest, but the steering winds will be moving towards a northerly direction, keeping them away from Philadelphia.
Radar Image 4:55 PM. Storms will slide westward and southward, away from us this evening. Parts of Chester county may see some of these showers. (Click on image for a larger view.)
High pressure builds in for Sunday with fair skies.
Heat returns next week as the edge of the heat dome pushes in from the west. The 90s return on Tuesday and mid 90s on Wednesday—
12z ECMWF-AI forecast for Tuesday 8 PM. Note the 760 thickness of the heat dome is to our north. (Click on image for a larger view.)
Posted Saturday 07/11/26 @ 11:30 AM — There appeared to be no question about the showers and storms expected to develop here during the mid afternoon. However, this morning’s HRRR casts serious doubt on this forecast. The question is, which model is correct??
This morning’s 12z HRRR forecast precipitation shows no rain for much of the immediate Philadelphia area; the area of rain to our west occurs during the evening—
12z HRRR total rainfall for Saturday (much of the immediate PHL area receives none!) (Click on image for a larger view.)
Here’s the same 12z model run of the new RRFS—
12z RRFS total rainfall for Saturday. (Click on image for a larger view.)
Finally, here’s the 12z NAM-NEST forecast for Saturday—
12z NAM-NEST total rainfall for Saturday (Click on image for a larger view.)
So, this is an interesting forecast.
It was several years ago that the HRRR predicted no rain and I was on a bike ride where I got positively drenched and there was severe flooding in some areas.
So I know first-hand that the HRRR can be wrong.
In an effort to determine which model is best, I do something that I call a 3 hour parameter match with the actual conditions (RTMA), Today’s HRRR came up with the best match today.
So this is an experiment. We may get very little rain if the HRRR is correct. But three out of four models have substantial showers and rain developing mid afternoon lasting into the evening. I guess we’ll find out.
Originally Posted Fri 5:53 PM —For Friday evening/night, there are thunderstorms to our west, but a strong area of downward vertical motion (omega) over the city will cause these storms do dissipate by 8 PM. It doesn’t appear that any storms will make it into the city.
Radar at 5 PM with windstreams. Boxed area shows dashed yellow contours indicating downward vertical motion. Storms will likely not make it into the city . (Click on image for a larger view.)
High pressure pushing down from Canada will push the edge of the hot air to our south on Saturday with a backdoor cold front. Many pop-up storms will develop in our area as early as noon on Saturday. The humidity and storms will be far to our south on Sunday.
Saturday
The backdoor cold front will bring showers and storms as early as noon to 2 PM. Precipitable water will be high near the city and southward; any storms could bring heavy rains.
RRFS 2 PM Saturday Precipitable water (PWAT) Heavy rains possible in these areas (blue) (Click on image for a larger view.)
Highs (Model Blend): Blue Bell- 85 º Philadelphia 86º ±2.3º Average sd and uncertainty.
18z NBM 1 hour shower/thunderstorm coverage at 2 PM Saturday. (Click on image for a larger view.)
Sunday
Mostly sunny in the morning; Some cloudiness increases from the west in the afternoon. Highs (Model Blend): Blue Bell- 84 º Philadelphia 86º ±2.4º Average sd and uncertainty.
