Posted Saturday 07/05/25 @ 5:43 PM — Sunday looks to remain sunny, but an increase in temperatures and humidity will become apparent by early afternoon. High temperatures will approach or exceed 90º and some areas from Center City to northeast into NJ may see highs in the 92º – 93º range. Dew points will approach or exceed 70º in some areas.
The approaching tropical system moisture (described in the earlier posting) and the developing thermal instability may cause some isolated/widely scatteredshowers/thundershowers to develop between 4 PM and 9 PM. Forecasting the exact placement and timing is never possible, but areas near Wilmington DE and in Chester County are likely areas.
The REFS (experimental) has been doing pretty well with these forecasts. Here’s what it’s showing at 8 PM—
12z REFS 1 hour rain with standard deviation. Forecast hour 8 PM. (Click on image for a larger view.)
Monday is looking humid, warm and wet due to the approaching subtropical system. Stay tuned.
Originally Posted Fri 5:24 PM —You don’t need me to tell you today was a spectacular Fourth of July, weather-wise. Low humidity, comfortable near-average temperatures, and sunny skies!
We’ll have this great weather through the balance of this weekend. The noticeable difference by Sunday will be higher temperatures and somewhat higher humidity.
Saturday Forecast
Sunny. Comfortable.
NBM high temperatures: Blue Bell, PA 86.8º ± 0.6º Philadelphia, PA 88.3 ± 1.5º Average Uncertainty (based on standard deviation)
Sunday Forecast
Sunny. Slightly more humid. Somewhat breezy in the afternoon.
NBM high temperatures: Blue Bell, PA 90.2º ± 1.2º Philadelphia, PA 91.0º ± 1.7º Average Uncertainty (based on standard deviation)
Beyond this Weekend
Of interest is a developing low pressure system with tropical characteristics (a warm center low) developing near Jacksonville FL. This will move up the coast Monday into Tuesday with rain, possibly heavy. The AI models are similar, but the track inland or somewhat more eastward is still to be determined.
ECMWF-AI model expected to move up the coast Monday into Tuesday. Possible heavy rain and high humidity expected. (Click on image for a larger view.)
Posted Saturday 06/28/25 @ 5:31 PM — On the surface, tonight should offer some fairly heavy thunderstorms; there’s plenty of CAPE (exceeding of 3000 joules/kg) and plenty of precipitable water (2.1″) and an advancing cold front as a trigger.
That said, the model forecasts aren’t all that impressive with the severity of the storms. There’s little to no jet stream energy aloft (the jet flow is far north of our region). Vertical wind shear, the ingredient needed to prevent thunderstorms from collapsing on themselves, is very low. Helicity is low-range elevated in the Allentown area, not so much here.
19zRRFS simulated radar/1hour rainfall at 9 PM. The heaviest activity ins near Allentown. (Click on image for a larger view.)
The latest Canadian RGEM just became available and it has some heavy rain closer to the city. This is in contrast to the US models and the AI model—
18z RGEM total rainfall through 1 AM Sunday morning. (Click on image for a larger view.)
So expect some showers and thunderstorms. Severe is always possible, but I don’t think so. Always follow the NWS advisories for these sort of things.
Sunday continues to look sunny and dry.
Saturday Update
Posted Saturday 06/28/25 @ 9:19 AM — The warm front is just to our west and clouds are beginning to breakup in S Jersey and in central PA.
Warm front will move through later this morning. Clouds break up for some sun by 11 AM to 1 PM. No rain expected until later this evening.
Showers and thunderstorms ahead of the next front (a cold front) will move in to western sections as early as 6-8 PM and continue through midnight.
Weekend Forecast
Posted Friday 06/27/25 @ 7:36 PM — Friday‘s rain was unexpected and not forecast by many models. Let’s see if we can do better with Saturday and Sunday.
Saturday: The front that went through late Thursday giving us the cool temperatures and easterly flow on Friday will return as a warm front and pass through between 10 AM and noon. The warm front will be preceded by showers and maybe some thunder, but there’s a wide range in model guidance regarding how widespread the showers will be.
Following the front, temperatures will rise into the mid and maybe upper 80s with dew points in the highly uncomfortable range of lower 70s. Many areas will see some sun by noon or sun filtered by clouds.
We may luck out with a dry period between noon and 5 PM. The atmosphere will be highly unstable and CAPE values will be close to extremely high levels of 4000 joules/kg. Shear will be very low, so pop-up storms will collapse on themselves and be short-lived and highly scattered. Pop-up storms will become more numerous, certainly by 5 PM and afterwards.
18z GFS forecast for midnight Saturday night. Cold front moving through with showers and storms. (Click on image for a larger view.)
A line of storms will move through Saturday night around midnight into the hours before daybreak Sunday morning.
Sunday will be mostly sunny and less humid. A period of cloudiness is expected around noon, but the sun should return filtered through high cloudiness in the afternoon. Lower dew points in the mid 60s with highs in the low to mid 80s.
Friday and the Weekend Preview
Originally Posted Fri 9:29 AM —The backdoor cold front that pushed through on Thursday has become stationary to our west and our south.
Fri 10:11 AM – Sorry folks, something was clearly wrong with the data I was looking at an hour ago for this forecast. Showers have broken out just west of the city. The RRFS that I was looking at showed nothing all day! Maybe that’s why they call the RRFS experimental”
Nexrad Radar at 10:11 AM (Click on image for a larger view.)
The RRFS forecast for the same time frame shows nothing!
00z RRFS Simulated Radar forecast for 10:10 AM. It shows nothing for our area! (Click on image for a larger view.)
What’s most disturbing- the NWS announced just yesterday that they’ll be retiring several models (NAM, HREF, HIRESW, NARRE) when the RRFS becomes operational in early 2026. See my RRFS page.
There is ongoing development of the RRFS where the plan is to take it from its current FV3 core to the MPAS (“model prediction across scales”) geometry. Here’s the highly experimental RRFS MPAS forecast for today which over-forecast the rain to our south, but captured these showers-
MPAS RRFS developmental model. For this, raw data is not available to the public. This is a NOAA graphic) (Click on image for a larger view.)
It’s not clear if the first operational release will be with the FV3 core or the MPAS core.
Originally Posted Fri 9:29 AM —
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Water Vapor Imagery reveals emitted water vapor infrared TEMPERATURE, NOT REFLECTED LIGHT from CLOUDS. I’ve drawn in the approximate position of the stationary front.
As a result, Water Vapor Temperatures indirectly reveal height, called the “effective layer” of the water vapor in a region, therefore revealing the mid and upper level 3D contours of systems.. Colder temperatures (blue- grey – white) indicate higher altitude, sometimes revealing upward motion. Red- Orange indicates warmer temperatures and a lower height for this layer. There are several water vapor “channels”; the one I display here is called the mid level height channel. (Click on image for a larger view.)
For today, Friday, a cool, stable easterly flow will keep skies cloudy, but the RRFS does have some sun filtered through high and mid level clouds early afternoon. Except for a quick possible sprinkle, the Philadelphia region should remain dry today.
Clouds return late afternoon and evening as the stationary front moves northeast as a warm front.
Saturday will have clouds in the morning, possibly with some showers, sprinkles and fog, then breaking for sunshine and increased humidity and temperatures as the warm front moves north. Showers and thunderstorms possible Saturday night, especially after midnight.
Sunday becomes sunny and quite warm, humid. Details will depend on the actual position and movement of the warm front.
Despite the unsettled and changeable weather this weekend, I don’t see much in the way of widespread substantial rain around Philadelphia
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Sun 1:05 PM —Forecast Review — Not a forecast to be proud of— Both last night’s and even this morning’s models failed to forecast this “mesoscale complex” with strong upper air support dropping south along the edge of the mid level heat dome.
I was out on a hike this morning and looked at the darkening skies to our north. I downloaded new data to my iPhone remotely and and saw the radar in no way matched the model forecasts from even two hours earlier. I remotely uploaded the “Update” below at 10:43 AM.
Here’s the current radar with the hourly RAP model superimposed—
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Water Vapor Imagery reveals emitted water vapor infrared TEMPERATURE, NOT REFLECTED LIGHT from CLOUDS. Orange contours are jet stream level vertical motion. Strong UPward areas right over Philadelphia suburbs into NJ will enhance showers/storms. The blue shading is the strong upper air vertical motion of the Mesoscale Complex. (Click on image for a larger view.)
This morning’s HRRR (12z -8AM EDT) shows the a diminished mesoscale complex moving far north of us at 1 PM—
12z HRRR forecast for 1 PM Sunday. Rain stays well to our north. Completely under forecast the rain and position. (Click on image for a larger view.)
The HRRR is one of our best “CAM” models. (CAM = “Convective Allowing Model”, the sort of process occurring that this model is designed to capture. ) Clearly, there was some input upper air data problem at the very least.
Our newest CAM model, the experimental RRFS showed a bit of activity with a 14z (10 AM) model run. Still quite disappointing.
14z (10 AM EDT) hourly RRFS (experimental) did somewhat better, but still not a good forecast. (Click on image for a larger view.)
Even this morning’s ECMW-AI model, just available, didn’t get this right—
This morning’s 12z ECMWF-AI model kept the rain far north of our area. (Click on image for a larger view.)
As I’ve said as recently as last week, these modeling errors and forecast ‘surprises’ is one of the things that I find fascinating about meteorology.
Update Sun 6/22 10:43 AM — The large area of rain to our far north was forecast by the models to be further north than currently on radar. They were also forecast to dissipate. They are the result of hot humid air pushing northward in the mid levels of the atmosphere.
The latest HRRR still shows it dissipating as it approaches.
Posted Sunday 06/22/25 @ 9:14 AM — The Canadian Meteorological Center (CMC) announced their release this week of their advance into AI weather models with the release of an experimental global model based on the GDPS. (GDPS-AI) . The CMC describes this new model as “spectrally nudged” towards an AI model. (Weather models use wave-form physics (spectral methods) to recreate the atmosphere structure and dynamics.)
The GDPS-AI is run twice daily, 00z and 12z and becomes available four hours and 45 minutes after it is run. It forecasts out 240 hours. The nice thing about this AI model: it has hourly forecasts through forecast hour 84, in contrast to the European and US AI models that forecast at 6 hour intervals.
My experience with the Canadian models has found their regular models tend to bias towards the high side of temperatures in this region during the summer months.
So let’s do an ‘experiment’. Let’s see if their AI version is forecasting temperatures that are too high today?
00z GDPS AI model temperature forecast for 5 PM today, Sunday. (Click on image for a larger view.)
Here’s the latest NBM model, just available. Its highs are significantly less, but the temperatures have a wide uncertainty (standard deviation) of ± 2.2º (The NBM (“model blend”) is a weighted average of 30-40 models)
Today’s 12z NBM high temp forecast. Contour lines are 1 degree increments. The Temps are statistical means, ±2.2º (Click on image for a larger view.)
It wouldn’t surprise me if the NBM is more accurate today, assuming adding 1º-2º to to its forecast.
Originally Posted Sat 10:28 AM —The main weather feature of this weekend and much of next week will be a building upper level ridge, bringing very high temperatures and humidity to the Philadelphia region. The building ridge is visible on this morning’s water vapor image—
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Water Vapor Imagery reveals emitted water vapor infrared based on TEMPERATURE, NOT REFLECTED LIGHT from CLOUDS. As a result, Water Vapor Temperatures indirectly reveal height, called the “effective layer” of the water vapor in a region, therefore revealing the mid and upper level 3D contours of systems.. Colder temperatures (blue- grey – white) indicate higher altitude, sometimes revealing upward motion. Red- orange indicates warmer temperatures and a lower height for this layer. There are several water vapor “channels”; the one I display here is called the mid level height channel. (Click on image for a larger view.)
The expected temperatures based on the European AI model (the ECMWF-AI Single) were posted yesterday. When the temperatures here are hitting 100º, I don’t see that much value, (and only hype) in giving the heat indices.
According to the latest NBM, high temps will hit 90º today, Saturday near the city and 97º onSunday.
When the temperatures get this hot, there’s always the possibility of an isolated thundershower popping up. A trigger is often needed for these to occur. The models show some storms in far northeastern PA later this afternoon.
Yet with the upper atmosphere being so hot, the upper level heat in many ways inhibits convective rainfall.
