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Friday and Weekend Outlook
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.
Here’s the position of the 576 thickness line on Sunday afternoon—
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.
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.
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)
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—
| Severity Parameter (HRRR) | Sept 1 , 2021 Tornado Outbreak (example of highly severe) | 4-1-23 Tornadoes (Example of severe) | 08-07-23 18z HRRR Bow Echo | Today 08-10-26 12z HRRR | Today 08-10-26 11z RRFS |
| CAPE Joules/kg | 3500-4200 ⚑⚑⚑⚑ | 2100 ⚑⚑ | 2600 ⚑⚑ | 2200 ⚑⚑ | 2600 ⚑⚑ S Jersey |
| Helicity m^2/s^2 | 1350 ⚑⚑⚑⚑⚑ | 655 ⚑⚑ | 726 ⚑⚑ | 350 ⚑ | 322 ⚑ |
| Vertical Shear 1/sec | 40-46 ⚑⚑⚑ | 40-45 ⚑⚑⚑ | 30 ⚑⚑ | 21 ⚑ | 31 ⚑⚑⚑ Green Lane |
| Precipitable Water | 2.7″ ⚑⚑⚑⚑ | 0.83” ↓ | 2.1″ ⚑⚑ | 2.6″ ⚑⚑⚑⚑ Delaware county | 2.5″ ⚑⚑ Chester County |
| Lifted Index º K | minus 6º ⚑⚑ | minus 9.3º ⚑⚑⚑ | minus 9.9º ⚑⚑⚑⚑ | Minus 6.7º ⚑⚑ | Minus 8,7 ⚑ |
| Peak Wind Gusts mph | 40-50 ⚑⚑⚑ | 40-50 ⚑⚑⚑ | 65mph ⚑⚑⚑⚑ | 46-62 mph ⚑⚑⚑⚑ Delaware County/ S Jersey | 59 mph ⚑⚑⚑ |
| Storm Motion Shear Vector Alignment | Aligned ⚑⚑⚑ | Almost aligned ⚑⚑ | Aligned ⚑⚑⚑ | Aligned ⚑⚑⚑ | Aligned ⚑⚑⚑ |
| 250 mb Jet Stream Wind mph | 63 ⚑ | 135 ⚑⚑⚑⚑ | 74 ⚑⚑ | 50 ⚑ | 91 ⚑⚑⚑ |
| CWASP (NBM Model) | 54 ⚑ far northwest | 54 ⚑ far northwest | |||
| SWEAT Index (RRFS) | 316 ⚑⚑ | 349 ⚑⚑ Kennett Square | |||
| K Index (RDPS only) | 41 ⚑⚑⚑ | 41 ⚑⚑⚑ |
↓ 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.
A preliminary look at some of the severity parameters suggests that severe storms are possible Monday afternoon. The models are showing scattered storms moving in from the northwest as early as 2 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.
