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Better Weather on the Weekend?
Posted Friday 08/21/26 @ 9:18 AM — Last night’s models showed a significant (potential) change in the rainy forecast for the weekend.
Basically, low pressure spawned by the sharp dip in the jet was supposed to bring rain for our area. However, last night’s models show high pressure to the northeast hanging tight, with rain suppressed to areas south of Philadelphia airport and in the case of the NAM-NEST, much further south of the airport.
(The NAM, an older model to be retired in October 2026, and the newer NAM-NEST, is very keen to this sort of northward blocking.)
So, instead of this forecast from yesterday:
Today’s 06z Forecast—
The NAM-NEST has the precipitation even more suppressed to our south—
Unfortunately, today’s models from 12z onward will be influenced by reduced upper air measurements due to NWS budget and staffing cuts—
So while we would ordinarily wait for clarification with newer model runs, today’s models from 12z on may be less reliable.
I’ll update later this afternoon with my regular Weekend Weather Forecast.
Friday and Weekend Outlook
Posted Thursday 08/20/26 @ 7:57 PM — The system that gave us the heavy rain and strong storms is moving away and high pressure will build in from the northeast for Friday. Just some high cirrus clouds in the morning along with sunshine and occasional clouds for Friday afternoon. It will be windy and gusty in the morning, with winds reducing somewhat by the afternoon.
A very amplified upper level (jet stream) trough will develop over the weekend spawning low pressure that will bring rain on Saturday, possibly lingering into part of Sunday.
As for rainfall on Thursday, here’s the 24 hour rainfall through 7 PM Thursday. (It’s still raining in NJ at this time.)
Severe Weather Update
Posted Thursday 08/20/26 @ 5:39 PM — The NBM has had a “Tornadic Probability” parameter for about 4-5 years. Here’s the current 18z NBM—
Posted Thursday 08/20/26 @ 4:59 PM — Slightly earlier than prior forecasts, severe thunderstorms are right on the door step of western Chester/Montgomery/Bucks Counties at 4:50 PM—
Here’s the latest Severity Table from the 18z RRFS model just available. Some values have increased; some have decreased — Southern sections of NJ and Chester/Delaware county seem to have the most threat—
| 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 08-20-26 18z RRFS |
| CAPE Joules/kg (available energy) | 3500-4200 ⚑⚑⚑⚑ | 2100 ⚑⚑ | 2600 ⚑⚑ | 2100 ⚑⚑ S Chester County Cape May County |
| Helicity m^2/s^2 (possible tornadoes) | 1350 ⚑⚑⚑⚑⚑ | 655 ⚑⚑ | 726 ⚑⚑ | 667 ⚑⚑ Various in NJ |
| Vertical Shear 1/sec (Maintains storms wind damage) | 40-46 ⚑⚑⚑⚑ | 40-45 ⚑⚑⚑⚑ | 30 ⚑ | 35.5 ⚑⚑⚑ Montgomery Co |
| Precipitable Water (Flooding) | 2.7″ ⚑⚑⚑⚑ | 0.83” ↓ | 2.1″ ⚑⚑ | 2.4″-2.6″ ⚑⚑⚑⚑ S Jersey S Chester Co |
| Lifted Index º K (instability) | minus 6º ⚑⚑ | minus 9.3º ⚑⚑⚑⚑ | minus 9.9º ⚑⚑⚑⚑ | Minus 6.4 ⚑⚑ S Chester Co Cape May |
| Peak Wind Gusts mph | 40-50 ⚑⚑⚑ | 40-50 ⚑⚑⚑ | 65mph ⚑⚑⚑⚑ | 54mph ⚑⚑⚑ Salem Vineland NJ |
| 250 mb Jet Stream Wind mph (Energy) | 63 ⚑ | 135 ⚑⚑⚑⚑ | 74 ⚑⚑ | 90 ⚑⚑⚑ Upper Bucks Upper Montco |
| SWEAT Index (RRFS) (severity >400 is extremely severe) | 387 ⚑⚑⚑ Near Delaware | |||
| K Index (RDPS only) (instability) | 41 ⚑⚑⚑⚑ South Jersey | |||
| Max Updraft Helicity (RRFS) | 544 ⚑⚑⚑⚑ |
↓ indicates works against Severity ⇩ Significant, but less impact
The SWEAT Index captures much of the story—
Thursday Severe Weather Update
Posted Thursday 08/20/26 @ 9:19 AM — After reviewing the latest RRFS 09z, HRRR 11z models and the Canadian HRDPS RDPS, here are the trends I’m seeing—
- While widely scattered showers storms are possible any time after 2 PM, the main line of severe weather moves in from the west somewhat later than forecast yesterday. 6-7 PM western suburbs, 7-8 PM in the city, and exiting the Jersey shore about 11 PM.
- Total rainfall is somewhat less than forecast yesterday. Flooding is possible in some areas with almost 3 inches falling in a short period of time. (see graphic below)
- Still a chance of tornadic activity, still low but possible. Lower chance than yesterday.
- As mentioned yesterday, any increase in cloud cover today without periods of sun will decrease severity. The current HRRR is more on-target with more clouds than the RRFS.
Here’s my latest Severity Table, comparing today’s parameters to past severe outbreaks:
| 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 08-20-26 12z HRRR |
| CAPE Joules/kg (available energy) | 3500-4200 ⚑⚑⚑⚑ | 2100 ⚑⚑ | 2600 ⚑⚑ | 1300-2200 ⚑⚑ Chester County |
| Helicity m^2/s^2 (possible tornadoes) | 1350 ⚑⚑⚑⚑⚑ | 655 ⚑⚑ | 726 ⚑⚑ | 757 ⚑⚑ Quakertown |
| Vertical Shear 1/sec (Maintains storms wind damage) | 40-46 ⚑⚑⚑⚑ | 40-45 ⚑⚑⚑⚑ | 30 ⚑ | 35 ⚑⚑⚑ Berwyn Collegeville |
| Precipitable Water (Flooding) | 2.7″ ⚑⚑⚑⚑ | 0.83” ↓ | 2.1″ ⚑⚑ | 2.5″-2.6″ ⚑⚑⚑⚑ Entire Area |
| Lifted Index º K (instability) | minus 6º ⚑⚑ | minus 9.3º ⚑⚑⚑⚑ | minus 9.9º ⚑⚑⚑⚑ | Minus 6.0 ⚑⚑ |
| Peak Wind Gusts mph | 40-50 ⚑⚑⚑ | 40-50 ⚑⚑⚑ | 65mph ⚑⚑⚑⚑ | 56 mph ⚑⚑⚑ |
| 250 mb Jet Stream Wind mph (Energy) | 63 ⚑ | 135 ⚑⚑⚑⚑ | 74 ⚑⚑ | 66 ⚑⚑ Green Lane |
| SWEAT Index (RRFS) (severity >400 is extremely severe) | 301 ⚑⚑ Near Delaware | |||
| K Index (RDPS only) (instability) | 41 ⚑⚑⚑⚑ South Jersey | |||
| Max Updraft Helicity (RRFS) | 208 ⚑⚑ |
↓ indicates works against Severity ⇩ Significant, but less impact
I’ve switched back to the HRRR for the above table. Overall, the above severity table parameters have decreased in intensity from yesterday, likely due to the increased cloud cover and lower thermal energy. Heavy rainfall and wind gusts remain the main threats.
Thursday Severe Weather Possible
Posted Wednesday 08/19/26 @ 5:26 PM — As mentioned this morning, a deepening upper level trough and energy along a boundary to our west is expected to bring strong, possibly severe storms to our area on Thursday.
The timing has changed and the latest RRFS and RDPS have a line of very strong storms taking shape about 3-5 PM Thursday afternoon and continuing as a severe line of storms into NJ from 5 to 8 PM.
A few light showers are possible Thursday early morning during the rush hour. Following those showers, any sunshine that breaks out in the late morning and early afternoon will add fuel to the fire.
If Thursday is mostly cloudy, the chance of severe weather will be less.
Here’s the latest RRFS model forecast for 5 PM—
Expect localized very heavy rain, strong gusty winds and a low probability but possible tornadic activity.
| 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 08-19-26 18z RRFS |
| CAPE Joules/kg (available energy) | 3500-4200 ⚑⚑⚑⚑ | 2100 ⚑⚑ | 2600 ⚑⚑ | 2500 ⚑⚑ S Jersey |
| Helicity m^2/s^2 (possible tornadoes) | 1350 ⚑⚑⚑⚑⚑ | 655 ⚑⚑ | 726 ⚑⚑ | 870 ⚑⚑⚑ north Jersey |
| Vertical Shear 1/sec (Maintains storms wind damage) | 40-46 ⚑⚑⚑⚑ | 40-45 ⚑⚑⚑⚑ | 30 ⚑ | 35 ⚑⚑⚑ Trenton |
| Precipitable Water (Flooding) | 2.7″ ⚑⚑⚑⚑ | 0.83” ↓ | 2.1″ ⚑⚑ | 2.5″-2.8″ ⚑⚑⚑⚑ Chester County |
| Lifted Index º K (instability) | minus 6º ⚑⚑ | minus 9.3º ⚑⚑⚑⚑ | minus 9.9º ⚑⚑⚑⚑ | Minus 6.8 ⚑⚑ |
| Peak Wind Gusts mph | 40-50 ⚑⚑⚑ | 40-50 ⚑⚑⚑ | 65mph ⚑⚑⚑⚑ | 50 mph ⚑⚑⚑ |
| 250 mb Jet Stream Wind mph (Energy) | 63 ⚑ | 135 ⚑⚑⚑⚑ | 74 ⚑⚑ | 76 ⚑⚑ |
| SWEAT Index (RRFS) (severity >400 extremely severe) | 345 ⚑⚑ Bucks/Montco Co. | |||
| K Index (RDPS only) (instability) | 41 ⚑⚑⚑ | |||
| Max Updraft Helicity (RRFS) | 280 ⚑⚑⚑ |
↓ indicates works against Severity ⇩ Significant, but less impact
Thursday Outlook
Posted Wednesday 08/19/26 @ 8:39 AM — Today, Wednesday, continues to look like a very nice day. Highs 87º Blue Bell, 89º Philadelphia airport. Dew points only in the low 60s.
Thursday increasingly looks to have potential for severe weather. Timing of precipitation has clarified with more models showing some scattered showers and thundershowers as early as 11 AM to 2 PM. The main disturbance, now in the Midwest, will move through later Thursday afternoon and evening. Severe thunderstorms and possible tornadic activity is currently looking possible, especially for southern Chester/Delaware counties, southern NJ and Maryland/Delaware.
The disturbance expected to create this severe weather is still to our west.
I’ll update later today and tomorrow morning with a severity table.
Wednesday – Friday Outlook
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 Wednesday night and rain moves in Thursday as an approaching cold front and low pressure moves towards us. There’s uncertainty about the exact timing of the rain on Thursday, but
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.)
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—
Here’s the areas forecast to receive the heaviest rainfall, based on calculated “moisture convergence” of RRFS data—
Last night’s rainfall—
Originally Posted Sun 8:02 PM — A warm front will move through our area between 11 PM and 3 AM Monday 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
While temperatures will likely be just short of 90º in Philadelphia, heat indices will be quiet high.
Not everyone will see a storm late Monday, as they will be scattered.
