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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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