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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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Friday Update

Posted Friday 08/28/26 @ 11:19 AM — A quick update- A few isolated showers are still expected to pop up around the 3-5 PM time frame with most of the heavier activity in NJ.

As for rainfall totals yesterday, here’s the final MRMS (2 Pass validation) —

My regular Weekend Weather Forecast follows later today.

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Thursday Update – Friday Forecast

Posted Thursday 08/27/26 @ 7:35 PM — Incredible levels of rainfall were received today in some areas, with much falling outside of the areas forecast by this morning’s models.

Here’s the preliminary totals as of 6 PM this evening. Much more has fallen and will fall since this MRMS measurement—

The rain is expected to end around daybreak Friday but a secondary cold front along with an assist from a favorable jet streak location will cause more scattered showers and thunderstorms from the city into NJ and the shore from early afternoon into the evening.

Here’s the REFS 1 hour rainfall snapshot at 4 PM. Additional areas will see showers develop before and after 4 PM —

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Latest Update

Posted Thursday 08/27/26 @ 2:58 PM — The new RRFS has an hourly model. The latest available is 1 PM (17z)
Here’s the 17z updated total rainfall forecast with flooding rains near Haddonfield and Brownsville NJ—

Can we believe this, when the morning model had the rain barely making it into NJ?? (see below)

I guess that’s why the NWS forecasts don’t even attempt to localize the heaviest rain bands or flooding locations in advance.

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One more thing…

Posted Thursday 08/27/26 @ 11:01 AM — The biggest takeaway from this is the heaviest activity will be north and west of the immediate Philadelphia area—

Even more surprising is this morning’s RRFS which just became available—

This will be a good test of the new high resolution RRFS compared to a well-established high resolution model, the HRRR. Even more interesting- the RRFS jet streak is different than the HRRR (as reviewed below). (The RRFS has us in neither the right or left regions. )

Thursday Update 2

Posted Thursday 08/27/26 @ 9:59 AM — The HRRR 12z has become available. The 12z models includes new direct upper air measurements via weather balloons.

Severity Parameter	Sept 1 , 2021 Tornado Outbreak (example of highly severe)	4-1-23 Tornadoes (Example of severe)	08-07-23 18z HRRR Bow Echo	Last week 08-20-26 18z RRFS	Today 08-27-26 12z HRRR
CAPE Joules/kg (available energy)	3500-4200 ⚑⚑⚑⚑	2100 ⚑⚑	2600 ⚑⚑	2100 ⚑⚑ S Chester County Cape May County	2800 ⚑⚑⚑ Western Suburbs NJ Shore
Helicity m^2/s^2 (possible tornadoes)	1350 ⚑⚑⚑⚑⚑	655 ⚑⚑	726 ⚑⚑	667 ⚑⚑ Various in NJ	209 ⚐ Western Suburbs
Vertical Shear 1/sec (Maintains storms wind damage)	40-46 ⚑⚑⚑⚑	40-45 ⚑⚑⚑⚑	30 ⚑	35.5 ⚑⚑⚑ Montgomery Co	19 ⚑ Trenton Area
Precipitable Water (Flooding)	2.7″ ⚑⚑⚑⚑	0.83” ↓	2.1″ ⚑⚑	2.4″-2.6″ ⚑⚑⚑⚑ S Jersey S Chester Co	2.1″ ⚑⚑⚑ Spotty Areas
Lifted Index º K (instability)	minus 6º ⚑⚑	minus 9.3º ⚑⚑⚑⚑	minus 9.9º ⚑⚑⚑⚑	Minus 6.4º ⚑⚑ S Chester Co Cape May	Minus 7.4º ⚑⚑⚑
Peak Wind Gusts mph	40-50 ⚑⚑⚑	40-50 ⚑⚑⚑	65mph ⚑⚑⚑⚑	54mph ⚑⚑⚑ Salem Vineland NJ	51 mph ⚑⚑⚑ Isolated Trenton
250 mb Jet Stream Wind mph (Energy)	63 ⚑	135 ⚑⚑⚑⚑	74 ⚑⚑	90 ⚑⚑⚑ Upper Bucks Upper Montco	92 ⚑⚑⚑
SWEAT Index (RRFS) (severity >400 is extremely severe)				387 ⚑⚑⚑ Near Delaware	309 ⚑⚑ Scattered areas
K Index (RDPS only) (instability)				41 ⚑⚑⚑⚑ South Jersey	41 ⚑⚑⚑⚑ Southern Lancaster and Chester
Max Updraft Helicity (RRFS)				544 ⚑⚑⚑⚑	106 ⚑⚑

So what does all this mean?

• There is plenty of energy for any storms that do develop. Very high CAPE values.
• Storms that develop will be scattered and less organized. Difficult to time
• Lower chance of tornadoes compared to other stormy days this summer.
• Highest chances of any clustered storms will be this morning and again about mid-afternoon. Exact times not possible today. Another maxima seems to be in the 2-4 PM time period. Storm activity this evening will remain mostly north and west.
• This is the first potential storm day this year that has a strong influence of a jet streak nearby. (“jet streak”- localized increase in winds of the jet flow, similar to a garden hose nozzle, where restriction causes increase in force/speed.)
• From the HRRR, we will be in an unfavorable position for storm enhancement by the evening based on jet stream/streak position.

Here’s the current radar. The overall forecast suggests that today’s potential strong storms will be where the current precipitation is falling right now, except for the 2-4 PM time period—

Thursday Update

Posted Thursday 08/27/26 @ 9:06 AM — As mentioned last night, there are two issues with today’s forecast:

• Wide spread and lack of agreement in the model forecasts, suggesting higher than average uncertainty regarding timing of any precipitation and amounts
• A disconnect between the forecast for heavy rains in our area and the model forecasts suggesting the heaviest activity remains west and north of our area.

I’ve been trying to piece this together into a coherent forecast and I’m going to use my “severity table” to come to a likely forecast. This table is based on the 09z RRFS (“off hour” forecast)—

Active Weather Day – Thursday

Posted Wednesday 08/26/26 @ 7:41 PM — An interesting and active weather day is in store for us on Thursday. Here’s the current setup—

A warm front moves through between 5 AM and 9 AM Thursday morning. This will be accompanied by showers and thunderstorms.

Additional scattered storms are possible throughout the day with high humidity and dew points. (The RRFS shows some storms, somewhat heavy north of our area between 12 PM and 2 PM. ) A cold front moves through sometime Thursday night preceded by more showers and thunderstorms about 6 PM – 9 PM. It’s unclear whether the majority of the stronger storms will move to our northwest. Here’s the RRFS-based SWEAT index —

There’s significant differences among the models with the timing of the storms tomorrow with the NAM-NEST quite different than the RRFS.

I’ll update Thursday morning, likely before 10 AM after the 12z HRRR becomes available.

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Wednesday -Thursday

Posted Tuesday 08/25/26 @ 5:00 PM — The degree of cloudiness today was more than expected and forecast. Here’s yesterdays RRFS cloud forecast for today, Tuesday—

Looking back at yesterday’s RRFS, it appears that its cloud algorithm doesn’t include very low stratocumulus cloudiness. Indirectly, we can infer stratocumulus cloudiness based on high relative humidity at a height of 925 mb (2500 feet above ground). Looking at this level, we see the cloudiness we had—

Unfortunately, cloud physics is much more complicated than just relative humidity, but an upper cyclonic curvature flow that we have today combined with high RH at this level gives the right forecast at least for today.

So what about Wednesday? Wednesday appears to have more sun early and more sun late afternoon with a period of low clouds between 11 AM and 2 PM, but not as dark as today—

The warm front is expected Thursday with a cold front Thursday night. Thursday looks to be cloudy, but with breaks of sun possible. Rain will be intermittent and widely scattered, but a more organized area of showers and thunderstorms is expected between 2 PM and 5 PM. There may be enough sunshine and energy for strong storms.

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Tuesday through Thursday Outlook

Posted Monday 08/24/26 @ 8:07 PM — Tuesday and Wednesday should be delightful with plenty of sunshine both days and somewhat warmer temperatures on Wednesday.

There may be some increase in cloudiness on Wednesday afternoon or evening as a warm front approaches from the south.

Considerable rain is expected with this warm front/cold front on Thursday with some areas receiving more than 1 inch by Friday morning.

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About this Morning’s Showers

Originally Posted Mon 10:49 AM — Today we began a day forecast to be sunny with some showers and clouds in Chester and Montco. What gives?

This is the sort of stuff about weather that fascinates me. None of the models showed this in advance. So, I needed to do some detective work to understand what was happening.

Here’s the water vapor image—

Here’s the RRFS based calculated moisture convergence and relative humidity at 10,000 feet, something that would cause these showers—

Finally we have a trigger, upward motion as shown by something called frontogenic forcing, which when upstream from a band of high relative humidity at 10,000 feet, will also cause showers—

So we have multiple ingredients that caused the showers this morning, despite the fact that the RRFS model (and the HRRR and RAP models) showed no precipitation from all of these contributors. Interesting!

The balance of the day should be sunny and quite nice!

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