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Friday

Posted Friday 09/04/26 @ 9:34 AM — Today will be mostly sunny. The RRFS shows scattered showers developing about 3-4 PM and continuing as a stream of scattered showers into the early evening. This may be over-done. (The NBM and new HRRR shows just a few isolated showers after midnight.)

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Thursday Evening Storms

Posted Thursday 09/03/26 @ 5:53 PM — At 5:45 PM, the leading edge of the thunderstorm complex has entered western Pennsylvania.

Looking over the latest HRRR, RRFS and NAM-NEST models, all have these storms intensifying and reaching the Philadelphia area as early as 11 PM (NAM-NEST) and 1-2 AM (HRRR, RRFS)

The NAM-NEST is unusually impressive and fast with its forecast precipitation—

The RRFS has the storms approaching as two clusters that come together over Philadelphia by 2 AM—

Here’s my Severity Parameter Table for tonight—

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	09-02-26 21z RRFS	Today 09-03-26 18z RRFS
CAPE Joules/kg ( energy)	3500-4200 ⚑⚑⚑⚑⚑	2100 ⚑⚑	2600 ⚑⚑⚑	540-1200 ⚑⚑	1600- 2000 ⚑⚑
Helicity m^2/s^2 (tornadic energy)	1350 ⚑⚑⚑⚑⚑	655 ⚑⚑	726 ⚑⚑⚑	506 ⚑⚑ Trenton	612 ⚑⚑
Vertical Shear 1/sec (storm strength)	40-46 ⚑⚑⚑	40-45 ⚑⚑⚑⚑	30 ⚑⚑	39 ⚑⚑⚑⚑ Chester Co	28.7 ⚑⚑
Precipitable- Water (flooding)	2.7″ ⚑⚑⚑⚑	0.83” ↓	2.1″ ⚑⚑	2.5″ ⚑⚑⚑⚑ Chester Co Delaware	2.6″ ⚑⚑⚑⚑
Lifted Index º K (Instabilty)	minus 6º ⚑⚑	minus 9.3º ⚑⚑⚑⚑	minus 9.9º ⚑⚑⚑⚑	minus 7.7-8.6º ⚑⚑⚑	minus 7.2-8.3º ⚑⚑⚑
Peak Wind Gusts mph (damage)	40-50 ⚑⚑⚑	40-50 ⚑⚑⚑	65mph ⚑⚑⚑⚑	20 mph ⚑	61 mph ⚑⚑⚑⚑
250 mb Jet Stream Wind mph (energy)	63 ⚑	135 ⚑⚑⚑⚑	74 ⚑⚑	45 ⚑	81-95  ⚑⚑⚑
SWEAT Index (RRFS) (severity)				454 ⚑⚑⚑⚑ S. Chester Co Delaware	393 ⚑⚑⚑ NJ Delaware
K Index (severity)				46.4 ⚑⚑⚑⚑ Kennett Square	44-46 ⚑⚑⚑⚑ Allentown & NJ
Max Upward Helicity (storm strenght)				139 ⚑⚑ Chester Co	60.8 ⚑

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Thursday Update

Posted Thursday 09/03/26 @ 10:06 AM — The thunderstorm cluster came through last night between 1 AM and 2 AM. As expected, the heaviest rainfall was in Chester county.

This morning, there is thunderstorm activity in central and southern PA that is expected to die out. It will be hot and humid. The models have no rain here until the late evening hours.

Another ‘mesoscale complex’ is expected to move through between 11 PM and 2 AM tonight.

I’ll update later this afternoon with with any change in timing and severity.

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Wednesday Night Storms Update

Posted Wednesday 09/02/26 @ 8:04 PM — The model forecasts have been anything but reliable beyond 18-24 hours. Today’s rain wasn’t predicted last night and the rain was moderate to heavy in far western sections.

Tonight, about 12 midnight to 2 AM, another cluster of storms ( a “meso-scale complex”) will enter our area from the northwest, driven by the return of the edge of the heat dome. Here’s the latest 21z RRFS forecast—

I’ve been expanding my toolbox with new methods to forecast and visualize the mechanisms that cause rain/snow/thunderstorms in an attempt to identify more accurately where the most severe weather will occur with these systems.

Tonight’s storms will be driven by a bunch of “forcing mechanisms”; one of interest is called frontogenic forcing. Here’s the location of high level frontogenic forcing at 1 AM—

Another frontogenic forcing tool also includes “potential energy” from latent heat in water vapor, referred to as theta-e. Here’s the theta-e depiction of the energy maxima, showing that areas of Lancaster and Chester counties will be most impacted—

Unfortunately, these depictions of frontogenesis are not invariant and change from model run to model run. Put another way, they’re only as good and consistent as the model they are calculated from.

Here’s today’s Severe Weather Table, with comparisons to yesterday. The storms are coming at a lower energy time of the day—

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	Yesterday 09-01-26 18z RRFS	Today 09-02-26 21z RRFS
CAPE Joules/kg	3500-4200 ⚑⚑⚑⚑⚑	2100 ⚑⚑	2600 ⚑⚑	3300 ⚑⚑⚑⚑ North west	540-1200 ⚑⚑
Helicity m^2/s^2	1350 ⚑⚑⚑⚑⚑	655 ⚑⚑	726 ⚑⚑	546 ⚑⚑⚑ S Jersey	506 ⚑⚑⚑ Trenton
Vertical Shear 1/sec	40-46 ⚑⚑⚑	40-45 ⚑⚑⚑⚑	30 ⚑	21 ⚑ Many areas	39 ⚑⚑⚑⚑ Chester Co
Precipitable- Water	2.7″ ⚑⚑⚑⚑	0.83” ↓	2.1″ ⚑⚑	2.5″ ⚑⚑⚑⚑ Many Areas	2.5″ ⚑⚑⚑⚑ Chester Co Delaware
Lifted Index º K	minus 6º ⚑⚑	minus 9.3º ⚑⚑⚑⚑	minus 9.9º ⚑⚑⚑⚑	Minus 9.1º ⚑⚑⚑⚑	Minus 7.7-8.6º ⚑⚑⚑
Peak Wind Gusts mph	40-50 ⚑⚑⚑	40-50 ⚑⚑⚑	65mph ⚑⚑⚑⚑	57 mph ⚑⚑⚑⚑	20 mph ⚑
250 mb Jet Stream Wind mph	63 ⚑	135 ⚑⚑⚑⚑	74 ⚑⚑	82 ⚑⚑⚑	45 ⚑
CWASP (NBM Model only)				44 ⚐	42 ⚐
SWEAT Index (RRFS)				437 ⚑⚑⚑⚑ Souderton	454 ⚑⚑⚑⚑ S. Chester Co Delaware
K Index				47 ⚑⚑⚑⚑ Souderton	46.4 ⚑⚑⚑⚑ Kennett Square
Max Upward Helicity				249 ⚑⚑⚑⚑ Northern Bucks Co	139 ⚑⚑ Chester Co

Wednesday- More Storms Late Evening

Posted Wednesday 09/02/26 @ 10:56 AM — UPDATE
The previous posting was based on the 2 AM (06z) models. The morning HRRR model data (12z) has become available and there are significant changes.

The latest HRRR shows a cluster of storms moving towards Harrisburg. Current radar shows these storms further south and east than forecast with a bow echo—

HRRR forecast for 11 AM—

2 PM—

So my earlier forecast of just clouds ( 06z RRFS) during the daytime today doesn’t appear to be correct.

While these storms will hit a stable easterly flow as they move eastward towards us, the HRRR has much of it hanging together in Chester county and southwestward by about 2- 3 PM.

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Posted Wednesday 09/02/26 @ 10:08 AM — The storms came through yesterday with quite a punch in some areas (mostly strong straight line winds and heavy rain in some spots). The cluster was fast moving and more impressive on radar than the actual rainfall that resulted. Here’s the MRMS rain totals from last night—

Today, Wednesday, is looking very cloudy with an easterly moist wind flow. The RRFS shows another cluster of storms moves into our area from the northwest between 10 PM and midnight—

With the edge of the heat dome moving northward tonight, Thursday will again be hot and very humid.

I’ll update later this afternoon with a severity index.

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Tuesday Severe Weather Update

Posted Tuesday 09/01/26 @ 4:36 PM — The models continue to forecast a cluster of showers and thunderstorms, some severe, this evening between 6 PM (north and northwest) and 11 PM.

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	08-10-26 12z HRRR	Today 09-01-26 18z RRFS
CAPE Joules/kg	3500-4200 ⚑⚑⚑⚑⚑	2100 ⚑⚑	2600 ⚑⚑	2200 ⚑⚑	3300 ⚑⚑⚑⚑ North west
Helicity m^2/s^2	1350 ⚑⚑⚑⚑⚑	655 ⚑⚑	726 ⚑⚑	350 ⚑	546 ⚑⚑⚑ S Jersey
Vertical Shear 1/sec	40-46 ⚑⚑⚑	40-45 ⚑⚑⚑⚑	30 ⚑	21 ⚑	21 ⚑ Many areas
Precipitable Water	2.7″ ⚑⚑⚑⚑	0.83” ↓	2.1″ ⚑⚑	2.6″ ⚑⚑⚑⚑ Delaware county	2.5″ ⚑⚑⚑⚑ Many Areas
Lifted Index º K	minus 6º ⚑⚑	minus 9.3º ⚑⚑⚑⚑	minus 9.9º ⚑⚑⚑⚑	Minus 6.7º ⚑⚑	Minus 9.1º ⚑⚑⚑⚑
Peak Wind Gusts mph	40-50 ⚑⚑⚑	40-50 ⚑⚑⚑	65mph ⚑⚑⚑⚑	46-62 mph ⚑⚑⚑⚑ Delaware County/ S Jersey	57 mph ⚑⚑⚑⚑
Storm Motion Shear Vector Alignment	Aligned ⚑⚑⚑	Almost aligned ⚑⚑	Aligned ⚑⚑⚑	Aligned ⚑⚑⚑	Not Aligned ⚑
250 mb Jet Stream Wind mph	63 ⚑	135 ⚑⚑⚑⚑	74 ⚑⚑	50 ⚑	82 ⚑⚑⚑
CWASP (NBM Model only)				54 ⚑ far northwest	44 ⚐
SWEAT Index (RRFS)				316 ⚑⚑	437 ⚑⚑⚑⚑ Souderton
K Index				41 ⚑⚑⚑	47 ⚑⚑⚑⚑ Souderton
Max Upward Helicity					249 ⚑⚑⚑⚑ Northern Bucks Co

Based on the above table, there are many ingredients present for severe weather. On radar at about 5 PM, nothing much is showing—

With all the ingredients lined up, I would be very surprised if more significant storms didn’t develop. Then again, the models have been terrible lately.

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Severe Storms Possible Tuesday Evening

Posted Tuesday 09/01/26 @ 1:26 PM — A quick update. Severe storms are expected to develop in the immediate Philadelphia area this evening between 7 PM and 10 PM. Many models are on-board with this forecast. Here’s this morning’s 13z RRFS—

This weather is the result of a backdoor cold front pushing the edge of the heat dome to our south. The storms will moved down from the north-northwest. I’ll have a severe weather table hopefully by around 5 PM.

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Tuesday Outlook

Posted Monday 08/31/26 @ 7:36 PM — The heat dome in the central US is expanding in our direction and we’ll be in heat and humidity on Tuesday as the 760 thickness line moves north of our area—

During the daytime hours, a few isolated to scattered storms are possible; most areas will not see anything.

By 5-6 PM Tuesday, showers and thunderstorms move into our area, according to the Canadian High Resolution model—

Our new REFS shows extensive coverage of rain and storms by midnight into Wednesday early morning hours—

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Monday Update

Posted Monday 08/31/26 @ 10:21 AM — This morning’s HRRR shows the amount of rainfall during the daytime to be minimum here.

Posted Monday 08/31/26 @ 9:19 AM — While the models last night had forecast our area to be without showers today, the latest radar/water vapor shows an area of showers headed towards Philadelphia.

The 06z RRFS showed nothing like this and the 09z (below) does forecast this, but shows this area of showers dissipating as it approaches Philadelphia around noontime.

However, based on Q-vectors for the same model, it would not surprise me if this area of showers hangs together as it approaches Philadelphia at noon. (Yes, another experiment.)

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Originally Posted Sun 8:19 PM — Showers and thunderstorms expected around 6-8 PM tonight are just beginning to get going. The latest models have scattered showers and thunderstorms moving in around 9 PM and continuing past midnight.

Tonight’s showers should end by morning. Monday looks to have a mix of clouds and sun. Additional scattered showers are possible again Monday night.

By Tuesday, the approximate edge of the heat dome will be north of us. We’ll have hot temperatures and high humidity.

In recent days, I’ve seen the fine structure of the forecasts change almost daily. This week, I expect having high confidence in a forecast beyond 24 hours will be difficult; there will likely be significant changes daily.

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