Posted Tuesday 08/18/26 @ 5:20 PM — Today, from the city eastward into NJ, skies were stormy looking and showers/thunderstorms have broken out as had been forecast.
A change in the short term pattern is in-store for us for the coming days into the weekend, as a large upper trough develops in the eastern US.
First, Wednesday may be one of the nicer upcoming days with lower humidity and sunny skies. High temperatures 85º (Blue Bell) 86º-87º (PHL airport) and dew points in the low to mid 60s.
Clouds increase Wednesdaynight and rain moves inThursday as an approaching cold front and low pressure moves towards us. There’s uncertainty about the exact timing of the rain on Thursday, but
12zECMWF-AI forecast for Wednesday at 8 PM. Approaching cold front and low pressure to affect our weather on Thursday. (Click on image for a larger view.)
For Thursday, light scattered showers are possible in the morning. Increased coverage of showers for Thursday afternoon with numerous showers and thunderstorms by 5 PM, according to the Model Blend and the Canadian RDPS. (It should be noted that the RRFS has us fairly rain-free during the morning and early afternoon on Thursday, possibly an outlier.)
18z RRFS forecast for 6 PM Thursday. The RRFS keeps us dry for much of the day. (The Model Blend has light showers in the morning and scattered showers early afternoon.) (Click on image for a larger view.)
For Friday, the above system exits eastward and partly to mostly sunny skies prevail before another system moves in Friday night with rain for some part of Saturday.
Following review of several models, I’d say the coming period, influenced by a new developing upper trough, is showing an unusually wide spread of forecasts. I expect changes in the forecast beyond 24 hours.
Monday Update
Posted Monday 08/17/26 @ 5:18 PM — I thought I would update with a latest RRFS moisture convergence graphic. More of a shift into Philadelphia. (compare to below)
Posted Monday 08/17/26 @ 4:42 PM — As expected, scattered storms have broken out and are heading this way. Recent RRFS model forecasts show the heaviest activity to head into southern sections of Chester and Delaware counties as well as northern Maryland. Much of the city may receive less activity and should exit in time for minimal delay of the Phillies game tonight.
Current radar—
Composite RADAR at 4:35PM with superimposed RAP model Omega (green contours) and yellow thickness contours. (Click on image for a larger view.)
Here’s the areas forecast to receive the heaviest rainfall, based on calculated “moisture convergence” of RRFS data—
Mon 4:49 PM —Forecast Review — Speaking of forecasts, today’s high temperatures and heat indices were nowhere near the high values I posted from the Model Blend. Forecast rainfall last night was MUCH more than forecast by many models. I posted 0.25″ from the RRFS. Clearly too low!
Last night’s rainfall—
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.)
Originally Posted Sun 8:02 PM — A warm front will move through our area between 11 PM and 3 AMMonday morning.
The front will be accompanied by more showers and thunderstorms. Some residual showers may last until about 7 AM Monday, but skies should clear by mid morning Monday. Currently, rainfall totals for our area do not look that impressive (less than 0.25″) with the heavier rain staying to our west and southwest.
Once the front passes through, dew points will move into the uncomfortable 70 as the edge of the heat dome moves north of us.
On Monday afternoon, weak low pressure over NJ and Long Island will rotate some disturbances through between 4 PM and 8 PM with scattered showers and thunderstorms
Today’s 18z RRFS forecast shows some high clouds at 2 PM with showers/storms developing and moving southeastward towards us on Monday. Note the 576 thickness line is north of us, indicating tropical type humidity. Also note that the showers seem to hang close to the 576 line due a sharp thermal/moisture gradient. (Click on image for a larger view.)
While temperatures will likely be just short of 90º in Philadelphia, heat indices will be quiet high.
18z NBM heat index max on Monday afternoon 4 PM (Click on image for a larger view.)
Not everyone will see a storm late Monday, as they will be scattered.
Updated Saturday 08/15/26 @ 8:38 PM — The models have showers moving in from the southwest between noon and 2 PM on Sunday. It would not surprise me if some scattered showers move in earlier – late morning.
Heavier widespread rain is likely by 4-6 PM. There’s a large spread in forecast total rainfall from 0.63” to over 2.1”.
Originally Posted Fri 9:51 PM — Very warm humid air just to our south will be blocked from advancing northward on Saturday by high pressure.
A low pressure system, currently developing in the Midwest will move east to be over our area late Sunday.
Water Vapor image at 4:51 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, Low pressure will organize and move towards us on Sunday. (Click on image for a larger view.)
Saturday
Mostly sunny, although sunshine will be a bit dimmed by considerable wildfire smoke. Highs (Model Blend): Blue Bell- 87º Philadelphia 89º -90º ±2.7º Slightly above average standard deviation and uncertainty.
Sunday
Low pressure moves directly over our area.
Cloudy. Showers and thunderstorms move in from the west, as early as noontime and as late 3 PM Highs (Model Blend): Blue Bell- 87 º Philadelphia 88º ±2.0º Average sd and uncertainty.
The models continue to predict hot weather for Monday and Tuesday. Highs in the low 90s Monday and mid 90s Tuesday!.
Posted Thursday 08/13/26 @ 7:39 PM — I haven’t posted much over the past two days. We’ve had a much-needed break in the extreme humidity.
The front that moved through Tuesday night will return as a warm front over the weekend, and certainly by Sunday. (I’d swear I heard on the TV weather earlier this week say that the weekend looked great. I’ve been scratching my head about this, since the AI GFS and ECMWF AI models have been forecasting rain for Sunday for several days with the returning warm front.
Two factors may come together for heavy rain on Sunday—
Philadelphia will be in a region called the Right Entrance Region of the jet streak. This is an area that has upward movement which enhances rainfall.
The edge of the heat dome, approximated by the 576 thickness line, will moving towards us. This area has a gradient of moisture, also lending itself to moisture convergence and heavy rainfall.
ECMWF-AI jet stream winds and the position of the right entrance region, right over Philadelphia. (Click on image for a larger view.)
Here’s the position of the 576 thickness line on Sunday afternoon—
ECMWF-AI forecast of rain and 576 thickness line on Sunday afternoon This area has high gradient of temperature and moisture and resulting convergence of moisture in upward motion. (Click on image for a larger view.)
So Sunday looks like a rainy day, possibly very rainy
My favorite model (of sorts) is the RRFS (Rapid Refresh Forecast System). The RRFS is moving from “experimental” (hosted on Amazon Web Services) to “parallel operational” status on the main NOAA server, “NOMADS“. As of Tuesday, it’s no longer on AWS and is only partially available on NOMADS. It’s not yet in the format that lends itself to reasonably sized downloads. This transition has been postponed twice. I’m sure the NOAA meteorologists and and data scientists are working hard to bring to life. But, I’m forecasting with limited to no access right now.
Tuesday Evening Storms
Posted Tuesday 08/11/26 @ 5:03 PM — A disturbance in western PA will move towards us tonight. There’s a chance of scattered showers and thunderstorms over the next 12 hours. Scattered clusters of storms are predicted, but the models are not showing a distinct pattern or timing.
Water Vapor image at 5 PM with superimposed RAP model MSL pressure (black contours), 500-1000mb Thickness (yellow contours) , GFS potential vorticity contours (violet fine contours) with surface wind barbs, (Click on image for a larger view.)
Many areas will see some rain, some may not. I often try to localize thunderstorm activity, but tonight’s setup doesn’t lend itself to that.
Skies clear for less humid weather Wednesday morning and Wednesday should be a more comfortable day.
Monday Storms Update
Posted Monday 08/10/26 @ 4:17 PM — The forecast north-south split in the line of storms is indeed occurring.
Radar with RAP omega overlay at 4:10 PM. North-South split of the energy is occurring as forecast. Some areas will receive very little rain. The Omega fields suggest Delaware, Chester counties and South NJ. may receive the strongest storms. (Click on image for a larger view.)
Update
Posted Monday 08/10/26 @ 10:43 AM — In my rush to post the severity parameters this morning, I violated rule #1 of forecasting: Always check the satellite and radar images first. At this time, an area of showers is moving into western sections.
The HRRR from 2 hours ago shows this first impulse weaker and totally falling apart before it makes it much further east. We’ll learn first hand if the models are doing their job today.
Here’s the current radar (10:35 AM) entering an area of very strong doward motion (dashed Omega lines)
Composite RADAR at 10:35 AM in an area of downward motion (dotted lines) The disturbance should fall apart. (Click on image for a larger view.)
Monday Thunderstorm Potential Update
Posted Monday 08/10/26 @ 9:24 AM —The models continue to change their forecast over last night and this morning. Here are the current trends—
The arrival time for the storms near Philadelphia is now expected to be between 4 PM and 6 PM.
A majority of models have the energy splitting as it approaches, with heavier rain far north of the city and heavier rain in Delaware and Chester counties and southward.
Some of the severity parameters have decreased significantly from my last posting (e.g., Lifted Index went from -10 to -5. More negative is more severe.)
There have been significant model changes from the 06z to the 11z hourly model runs, suggesting further forecast changes are possible.
The 11z RRFS captures the trends—
11z RRFS simulated radar/1 hour previous rain at 5 PM. Split energy just north of the city. Based on other tools, I’m not overly confident in this split. (Click on image for a larger view.)
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
Originally Posted Sun 8:49 PM —This week promises to bring a change in air masses by the end of the week. A series of disturbances will move through both Monday and again Tuesday.
A disturbance (called a short wave) will approach Monday afternoon. It’s visible on water vapor imagery and the fine purple lines behind the disturbance represent ‘potential vorticity’ from the RAP model suggesting there’s much support for this system.
Water Vapor image at XX 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, (Click on image for a larger view.)
A preliminary look at some of the severity parameters suggests that severe storms are possible Mondayafternoon. The models are showing scattered storms moving in from the northwest as early as2 PM-3 PM. More widespread activity between 5 PM and 8 PM (We saw as recently as this past Saturday that timing these things accurately can be tricky.)
As a best guess, heaviest rains will be west of the city and then south of the city.
Heavy rains, strong gusty winds are looking likely. I’ll update Monday morning before 10 AM with the latest severity parameters from the 12z HRRR.
