#Philadelphia #weather #PAwx #PhillyWx #wx

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.

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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—

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.

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#Philadelphia #weather #PAwx #PhillyWx #wx

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.

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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.

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Monday Storms Update

Posted Monday 08/10/26 @ 4:17 PM — The forecast north-south split in the line of storms is indeed occurring.

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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)

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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 ⚑⚑⚑

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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.

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#Philadelphia #weather #PAwx #PhillyWx

Friday & The Weekend Outlook

Posted Thursday 07/30/26 @ 8:01 PM —The scattered showers forecast for this evening are moving through and should dissipate later tonight. Friday looks to be mostly sunny, although some periods of cloudiness will develop mid to late afternoon Friday, especially in NJ. Scattered showers are possible again, mostly at the Jersey shore.

For the weekend, Saturday will be mostly sunny through high thin cirrus clouds. Towards the late afternoon, an increase in cloudiness is expected as a warm front moves towards us.

For Sunday. there’s some uncertainty regarding cloud cover and the northern movement of the very warm humid air behind the warm front. Most models keep the rain to our far west (Lancaster and Berks counties) and some have some have a few showers near Cape May county in NJ. It will be noticeably warmer and certainly more humid.

We’ll have to wait another day or so for the Sunday forecast to clarify.

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Thursday Weather Update

Posted Thursday 07/30/26 @ 11:50 AM — The upper air low continues to rotate close enough to cause some cloudiness and showers. (Some areas saw late day showers yesterday; many areas did not.) Here’s the current water vapor image—

This morning’s RRFS shows considerable shower coverage this evening about 6 PM. (Some isolated showers may develop earlier this afternoon as well.)

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Folks, my posts have been less frequent over recent days. A recent MacOS update decided that my model post-processing scripts (python-based) were malware and both quarantined and inactivated them., Apparently my python certificate had also expired. Diagnosing the issues and repairing them has taken the better part of almost 3 days. During that time, my model downloads were reduced significantly. I’m now back with 4 computers working again.

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Wednesday Update

Posted Wednesday 07/29/26 @ 9:56 AM — The system has exited our area, but there is an upper low with cold air aloft in New York State may cause instability showers and thunderstorms to develop later today. The latest RRFS has scattered showers and thunderstorms north of our area during the afternoon and some of this activity may develop in the immediate Philadelphia area around 5 -8 PM—

Here’s the rainfall totals, MRMS-based for Tuesday—

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Storm Review

Posted Tuesday 07/28/26 @ 8:35 PM — Today’s heavy rain has begun to enter the Philadelphia area. Very little with today’s weather has gone as forecast yesterday or even this morning. Much of the activity went to our north and west much of the day. Here’s the current radar—

I don’t see much in the way of very severe weather. Nonetheless, the NWS has issued a tornado watch for some of the area. Additionally, much of the immediate Philadelphia area hasn’t received anywhere near the heavy rain that had been forecast.

The back end of the rain is already visible on radar and it should be ending around midnight. As for the forecast for Wednesday, it appears now that there won’t be as much in the way of shower activity as had been forecast. Wednesday may be a decent day weather-wise with just a few scattered storms.

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Comparison Table- Severe Weather

Posted Tuesday 07/28/26 @ 11:58 AM — With the sun breaking out through clouds (and smoke) additional energy for storms is available.

Severity Parameter (HRRR)	Sept 1 , 2021 Tornado Outbreak (example of highly severe)	Recent 4-1-23 Tornadoes (Example of severe)	Recent 08-07-23 18z HRRR Bow Echo	08-10-23 Today’s 06z HRRR & Impact	Today 07-28-26 HRRR
CAPE Joules/kg	3500-4200 ⚑⚑⚑⚑	2100 ⚑⚑	2600 ⚑⚑	1200 ⚐ ⚐	2800 ⚑⚑
Helicity m^2/s^2	1350 ⚑⚑⚑⚑⚑	655 ⚑⚑	726 ⚑⚑	560 ⚑	583 ⚑ Bucks county
Vertical Shear 1/sec	40-46 ⚑⚑⚑⚑	40-45 ⚑⚑⚑	30 ⚑	32 ⚑⚑	21 ⚑
Precipitable Water	2.7″ ⚑⚑⚑⚑	0.83” ↓	2.1″ ⚑⚑	2.3″ ⚑⚑⚑	2.3″ ⚑⚑⚑ Delaware County S Jersey
Lifted Index º K	minus 6º ⚑⚑	minus 9.3º ⚑⚑⚑	minus 9.9º ⚑⚑⚑⚑	Minus 4.4º ⚐	Minus 6.7 ⚑⚑
HRRR Hail inches	1.9 ⚑⚑	1.4 ⚑	1.8 ⚑⚑	1.6 ⚑	1.8″ ⚑⚑
Peak Wind Gusts mph	40-50 ⚑⚑⚑	40-50 ⚑⚑⚑	65mph ⚑⚑⚑⚑	32mph ⚐	36 mph ⚑
Storm Motion Shear Vector Alignment	Aligned ⚑⚑⚑	Almost aligned ⚑⚑	Aligned ⚑⚑⚑	Somewhat aligned ⚐	Variable ⚐
250 mb Jet Stream Wind mph	63 ⚑	135 ⚑⚑⚑⚑	74 ⚑⚑	95-104 ⚑⚑⚑	42 ⚐
CWASP (NBM Model)					70 ⚑⚑⚑⚑ Chester Co
SWEAT Index (RRFS)					302 ⚑⚑
K Index (RDPS)					40.2 ⚑⚑⚑⚑ Philadelphia

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Heavy Rain Update

Posted Tuesday 07/28/26 @ 11:32 AM —Many of the 12z models are in. There have been changes in the forecast with a later start and the current appearance of the precipitation on radar.

• The HRRR differs somewhat from the RRFS with the HRRR having most of the rain and storms expected this afternoon storms move north of the immediate Philadelphia area. It delays the heavy rains near the city until about 10 PM. The RRFS has scattered activity near the city late afternoon and evening, again now with the heavy activity mid to late evening about 9 PM – 1 AM Wednesday.
• Severe weather (tornadoes, hail,) is still not expected with these storms but wind gusts and heavy rain still in the picture.
• None of the models capture the current radar at all. This suggests that this is a low confidence forecast in terms of timing.

I’ll be posting the severe weather comparison table over the next hour.

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Heavy Rain

Posted Tuesday 07/28/26 @ 9:11 AM — The morning models (12z) haven’t been completed yet. But based on the 06z models (2 AM EDT), here’s the trends I’m seeing.

• Heavy rain will be the primary concern with this system. Precipitable water (PWAT) is ‘only’ in the 2 inch range (last severe system we had had PWATs close to 2.5″. However, the expected longer duration of the system and development of a surface low (and upper air low) is expected to cause strong dynamics any precipitation accumulate and be significant.
• The storms move in between 12 PM and 2 PM. Heaviest activity is between 4 PM and 10 PM .
• Severe parameters are not overly impressive. I don’t expect tornadic activity with this system. Wind gusts may reach 30 mph in S. NJ.
• First look suggests heaviest rain from just west of Philadelphia into NJ.

I’m short on time right now. I’ll update with a severity table and new model data before noon today.

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Tuesday Weather

Posted Monday 07/27/26 @ 7:57 PM — We had quite the rain-maker develop this afternoon. In addition to the triggered storms (see below), many other cells developed in-place and moved very little causing significant rainfall and flooding. Here’s the MRMS summary of rainfall through 5 PM; incredible totals considering the rain lasted not much more than 2-3 hours—

For tonight (Monday), some additional widely scattered storms are still possible. The real action begins on Tuesday morning (10-11 AM) where showers and thunderstorms will develop and move in from the northwest. —

Rain continues through Tuesday night and tapers early Wednesday. But an upper low associated with this system lingers and additional showers and storms are expected Wednesday afternoon.

As much as an additional 1-3 inches of rain is expected by Wednesday evening. I’ll update on expected severity parameters tomorrow morning.

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A Quick Update

Posted Monday 07/27/26 @ 1:33 PM — As mentioned predicting these popups is an exercise in utility. But I’m trying. Current RRFS shows and area of thermal potential energy (theta-e) and an area of moisture convergence at the same level—

The latest water vapor image with hourly RAP model data shows thunderstorms moving into an area that the RAP model feels is favorable for storm strengthening—

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Monday Forecast Update

Posted Monday 07/27/26 @ 10:33 AM —This morning’s 12z HRRR just became available. Today’s weather will have some isolated to scattered showers and thunderstorms. The triggers for today’s weather will be weak upper air disturbances with some help from thermal/dew point gradients. Severity parameters are in the low to moderate range with shear values less than 15 and helicity less than 200.

The triggers for today’s storms are visible on water vapor imagery—

At 5 PM, the RRFS shows scattered storms with the triggers—

That translates into moderate to low level storms where they pop up. That said, expect the NWS to issue some severe thunderstorm alerts.

Attempting to forecast today’s storms location is an exercise in futility, but that doesn’t keep me from trying. Based on this morning’s RRFS and the latest HRRR, storms will first form around 12-3 PM in NJ, along the coastal areas. Storms closer to the immediate Philadelphia area will likely form closer to 4-7 PM.

As shown in the water vapor image, there’s a strong impulse far northwest that will give us heavy storms on Tuesday. Stay tuned.

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Originally Posted Sun 8:08 PM —With the weather so perfect over the past few days, I’ve taken a break from forecasting. With the coming week, we have some active weather expected from Monday through part of Wednesday.

Warm humid air is moving northward towards us as a disturbance riding over the heat dome will descend towards us —

With the gradient in temperature moving in, showers and thunderstorms will develop during the afternoon and evening on Monday. The RRFS has most of the activity to our north by late afternoon, but several models are forecasting showers and thunderstorms as early as 2PM in our immediate area.

The ensemble model, the REFS, captures this, perhaps better than the RRFS—

The Canadian GDPS with “AI spectral nudging” captures the forecast for Tuesday into Wednesday. A surface low forms over Pennsylvania with a nearby upper level low. Periods of rain and thunderstorms likely on Tuesday with additional showers developing again Wednesday.

The models have been predicting 1-3 inches of rain from this multi-day system. Exact placement can’t be predicted in advance and the total rainfall may change from the forecast.

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