Category Archives: Severe Weather

THIS WEEK’S WEATHER

Tue 7:12 PM —Forecast Review — The storms that came through this evening were impressive and stronger than I would have guessed based on forecast severity parameters which were high in CAPE and Precipitable Water (PWAT) and Lifted Index. and somewhat middle of the road in Vertical Shear and Helicity. (All based on the HRRR forecast values.) As mentioned yesterday, the trigger was a “potent” short wave.

However, my update (below) posted at 3:51 PM showed very impressive vertical motion ahead of the line of storms to our west, and the greater strength of the storms was less of a surprise when they moved in.

Several tornadic signatures showed on radar. Here’s one that was impressive in upper Montgomery county—

This PHL Terminal Doppler Radar (courtesy of Weathertap.com) shows a likely tornado in the white box, based on radar signature. (Click on image for a larger view.)

Another tornadic signature in eastern Montgomery county may or may not be an actual tornado.—

This PHL Terminal Doppler Radar (courtesy of Weathertap.com) shows a possible tornado in the white box, based on radar signature. (Click on image for a larger view.)

What was really impressive was the experimental REFS model which was accurate with its timing of the lead storms in Philadelphia and the subsequent storms that are still lingering. The HRRR, our current high resolution operational model, really got the timing of today’s storms wrong.

The REFS is scheduled to become operational in early 2026, if NOAA is allowed to do its wonderful job. The REFS and the RRFS will be replacing several older, less-skillful “legacy” models such as the NAM and HREF and HIRESW-ARW models.

More about this on my RRFS page.

“Tracking” Storms?? What does that really mean??

As I’ve said here before, I avoid the using the expression “tracking” the storms. The TV people use this all the time. Storms, and especially thunderstorms, are dynamic processes. They can form at a point almost instantly. They’re not a train that arrives at a scheduled time. There’s nothing to “track”. Case in point are the storms that just formed over Center City after 4 PM.

PHL Airport Terminal Doppler Radar at 4:10 PM Click to animate (Click on image for a larger view.)

The appearance of these storms between 4-5PM suggests the REFS is doing a great job with thunderstorms.


Tuesday’s Storms Update

Posted Tuesday 07/08/25 @ 3:51 PM — At 3:45 PM, radar and hourly updated RAP model shows a line of storms far west of the city with significant upward vertical motion (vertical velocity pressure, Omega) ahead of the line. This signifies likelihood of significant strengthening of storms as they approach our area—

Radar 3:45 PM with RAP model Omega overlay. Significant vertical upward motion predicted ahead of the storms (X) suggests likelihood of intensification. (Click on image for a larger view.)

The latest REFS (experimental) shows some storms develop here as early as 5 PM, but the main batch of storms arrive near the city between 6 and 8 PM.

12z REFS 1 hour rain and standard deviation (contours) at 5 PM Not sure if this is still forecasting a bit early. (Click on image for a larger view.)

For comparison I’ve resurrected an old table with some significant storm histories. The highlighted column is today’s severity parameters and allows you (and me) to compare with today’s values.

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
07-08-25
Today’s
18z
HRRR
&
Impact

Notes
CAPE
Joules/kg
3500-4200
⚑⚑⚑⚑
2100
⚑⚑
2600
⚑⚑
2680
⚑⚑
Helicity
m^2/s^2
1350
⚑⚑⚑⚑⚑
655
⚑⚑
726
⚑⚑
403
Vertical Shear
1/sec
40-46
⚑⚑⚑
40-45
⚑⚑⚑
30
20
Precipitable Water 2.7″
⚑⚑⚑⚑
0.83”
2.1″
⚑⚑

2.6″
⚑⚑⚑⚑
Heavy rain potential
Lifted Index
º K
minus 6º
⚑⚑
minus 9.3º
⚑⚑⚑
minus 9.9º
⚑⚑⚑
Minus 8.3º
⚑⚑
HRRR Hail
inches
1.9
⚑⚑
1.4
1.8
⚑⚑

1.8
⚑⚑
Peak Wind Gusts
mph
40-50
⚑⚑
40-50
⚑⚑
65mph
⚑⚑
51.6mph
⚑⚑
NJ area
Storm Motion Shear Vector
Alignment
Aligned – ~ 90º
⚑⚑
Almost aligned
⚑⚑
aligned
⚑⚑
Somewhat aligned
250 mb
Jet Stream Wind
mph
63
135
⚑⚑
74
41
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity
↓ indicates works against Severity ⇩ Significant, but less impact

I just want to add that the HRRR shows the heaviest storms moving north and south of the immediate PHL area. Not true of the REFS and RRFS. Something to watch.

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Due to the highly unbalanced political environment on X, I’ve considerably cut back auto-posting there. To receive all of my forecast postings, I suggest following me on mastodon.social or bluesky.social


Tuesday’s Storms Update

One More Thing: the latest Canadian GDPS “with AI nudging” also suggests earlier timing, as early as 4PM, similar to the REFS below.

Posted Tuesday 07/08/25 @ 7:28 AM — A potent short wave and front will move through Tuesday evening. Ahead and along with this disturbance will be strong dynamics that promises more wide spread heavy rainfall and the potential for somewhat more severe storms than Monday.

In terms of timing, the REFS (experimental) still shows some showers/storms as early as 3-5 PM in the western suburbs, then moving eastward. The latest RRFS (experimental) continues to be about two hours later, with storms approaching the immediate PHL area around 6-7 PM.


The latest HRRR shows storms approaching the immediate PHL area around 6-8 PM, somewhat earlier in western suburbs.

06z HRRR forecast “simulated radar” at 8 PM Tuesday. (Click on image for a larger view.)

In terms of severity, greatest likelihood is in western sections about 5-7 PM. Heavy rain possible throughout the area.


Tuesday Storms

Posted Monday 07/07/25 @ 7:59 PM — Just wanted to add that the GFS shows some storms in Harrisburg moving through our area around midnight. Not shown in REFS or RRFS forecasts for tonight (Monday).

Posted Monday 07/07/25 @ 7:33 PM — The storms developed in many (but not all) areas around Philadelphia this afternoon. Here’s the MRMS rainfall totals—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

Tuesday should be mostly sunny hot and VERY humid again. A slight dip in the jet stream and a potent ‘short wave’ will approach, causing the development of widespread showers and thunderstorms beginning about 4 PM. Additionally, we’ll be near the right entrance region of a jet streak, enhancing available energy.

The latest GFS shows this potent area of upward motion around this short wave.

18z GFS forecast for 5PM showing strong concentration of upward vertical velocity pressure. Strong, possibly severe storms are possible. (Click on image for a larger view.)

The REFS did well today and I’ll stick with its forecast for Tuesday.

12z REFS forecast rain with standard deviation (contours) at 6 PM Tuesday. (Click on image for a larger view.)

The REFS has showers breaking out west of the city as early as 3 PM, but widespread showers and thunderstorms are expected by 4-6 PM.

(It should be mentioned the the latest RRFS has the storms moving in about 2 hours later than the REFS shown above. )

Several hours of widespread rainfall are expected with storms moving into NJ by 5-6 PM. Localized areas of 1-3″ of rain expected. Unlike today’s storms which were mostly heavy rain, Tuesday’s storms will have somewhat higher vertical wind shear and areas of elevated helicity. Some severe storms possible, especially into South Jersey and Delaware.


Monday Update

Posted Monday 07/07/25 @ 10:10 AM — The moisture associated with the remnants of tropical depression Chantal have started to move across our area today. Any sunshine will induce instability and thunderstorm formation. Here’s the latest REFS hourly rain forecast at 1 PM, 3 PM and 5 PM today—

06z REFS hourly rainfall with standard deviation (contours) 1 3 5 PM today. (Click on image for a larger view.)

Most of the activity will be in the afternoon hours and taper off by 9 PM.

Showers will be scattered, but any storm will be slow moving and capable of dropping heavy rainfall. Precipitable water forecasts are as high as they get around here, 2-3″ – 2.5″ in some areas. Flooding type rains are possible.

RRFS forecast precipitable water at 4 PM. As high as they get outside of the tropics. (Click on image for a larger view.)

Originally Posted Sun 5:37 PM —The remnants of tropical depression Chantal are located in North Carolina and the moisture and energy of this weak system is expected to move over our area on Monday.

5 PM Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Water Vapor Imagery reveals emitted water vapor infrared TEMPERATURE, NOT REFLECTED LIGHT from CLOUDS. Current position of CHANTAL is indicated as {L} (Click on image for a larger view.)

Some showers are breaking out in Delaware at this time (Sunday evening) and some showers may make into Delaware County and just south of the city by 8 PM.

More numerous showers and thunderstorms will develop on Monday. Shear and helicity values are weak. The main issue with these showers and storms will be the possibility heavy, slow moving rainfall. The showers will be scattered.

For Monday, a few scattered showers are possible in the morning, but the real deal will be during the afternoon where daytime heating will induce these showers and storms. 2PM to 8 PM will be the likely time slot with showers peaking around 4PM – 5 PM Monday. It will be warm (mid 80s) and VERY humid.

Here’s the latest REFS forecast for 5 PM

12z REFS forecast for 5 PM Monday. Previous 1 hour precipitation with standard deviation (contours)(Click on image for a larger view.)

Daytime hours on Tuesday look to be dry until about 4 to 8 PM, when showers and storms move in from the southwest. It will be very warm and VERY humid on Tuesday


THIS WEEK’S WEATHER

Interesting Weather for Monday

Forecast Review Mon @ 7:26 PM — So, in my book, these were typical thunderstorms with typical outflow wind gusts. Strong, yes. Severe, hardly. (I’m sure there will be some TV footage of a tree down somewhere.)

As mentioned the HRRR, HIRESW and the NAM-NEST did very well with timing and the handling of the front. The RAP was less impressive. Here are a few takeaways for the lack of severity—

•Temperatures were in the 70s, not the 80s or 90s
•Shear values were in the regular range one sees before thunderstorms. They weren’t over 30.
•Helicity values were relatively low. My software settings don’t even show helicity contours until they reach 400+.
•Shear and storm motion vectors were not aligned.
•Barometric pressure tendencies were flat before the frontal passage
• Jet streak position was west of the front and PHL was not in the entrance or exit regions to enhance the storms.

Radar loop showing storms diminishing as they proceeded eastward.

Update Mon @ 5:46 PM — Well, some severe strong activity has popped in upper Montgomery County. The NAM-NEST looks to be correct, as did the earlier HRRR. Expect some strong storms about 6-7PM


Update Mon @ 4:57 PM — The latest HRRR model has really backed off on the extent of [severe] storms coming. I can understand how it’s difficult for TV weather and the NWS to back off on a [hyped] forecast, especially when people’s safety might be at risk.

Nor did I want to stick my neck out too far with my post earlier today with a forecast saying these are going to be just the usual thunderstorms. But with my commentary on the overuse of word ‘severe’, it was my way of hinting at the possibility.

Anyhow, the latest NAM-NEST is still predicting a significant line of storms about 6-7PM . The latest HRRR not so much.

Any barometer fans out there? Maybe you’ve noticed the barometric pressure is already rising flattened? The front has already close to moving through!


Update Mon @ 3:01 PM — Newest models have the front moving through earlier- 4-5:30 PM (RAP Model) 5-7 PM (HRRR model)

Greatest shear is in northern NJ, not in this area.

CAPE is in the 1500-2200 range which is reasonably high. Latest HRRR has helicity in the 100-160 range in some areas.

There are differences in the two short range models. The RAP has several areas of storms moving through this afternoon into this evening The latest HRRR has storms ahead of the front with the main storm line moving through 5:30-7:30 PM (Philadelphia).

Updated Severity Parameter Table: (17z = 1 PM EDT)

Last September‘s TornadoesToday’s Weather
HRRR 06z
Today’s HRRR 12zToday’s HRRR 17zToday’s RAP 17z
CAPE 3500-4200 Joules/kgCAPE 1500-2400 Joules/kg20001500-20002200
Helicity 800-1100*Helicity 150-250*100100-160130
Vertical Shear 35-45Vertical Shear 15-2510-1525-3015-20
Precipitable Water 2.0″Precipitable Water 1.6″1.5″1.6″1.6″
Lifted Index 7-9 (minus)Lifted index (minus) 6-7 minus 6-7minus 6-7minus 6-7
Peak Wind Gusts 40-50Peak Wind Gusts 3535 mph35 mph25-30

Plenty of ingredients available for strong storms. However, shear vectors and storm motion vectors not aligned, suggesting difficulty in creating/maintaining severe storm cells. I guess we’ll see.


Update Mon @ 11:57 AM — Some storms have broken out ahead of the main front, as mentioned previously.

Of interest is the current water vapor image with superimposed NAM and GFS potential vorticity. There’s poor line up of these features, suggesting that these current models aren’t capturing reality that well.

11:30 AM Water Vapor with superimposed potential vorticity contours.

The RAP model has a better conformity to current water vapor imagery. The RAP has significant showers/thunderstorms during the afternoon, breaking out ahead of the main front that moves through at 6 PM—

14z RAP model shows better conformity to water vapor image. The RAP has significant showers moving in during the afternoon, ahead of the main front.

This morning’s HRRR and HIRESW show the main area of thunderstorms moving through between 4-6 PM. Here’s the 12z HRRR at 6 PM—

HRRR 12z showing simulated radar and cloud levels at 6 PM today. (Click on image for a larger view.)

I’ve also updated the table below with the latest HRRR representative values. Notice that the severe parameters have decreased, especially shear values.

Last September‘s TornadoesToday’s Weather
HRRR 06z
Today’s HRRR 12z
CAPE 3500-4200 Joules/kgCAPE 1500-2400 Joules/kg2000
Helicity 800-1100*Helicity 150-250*100
Vertical Shear 35-45Vertical Shear 15-2510-15
Precipitable Water 2.0″Precipitable Water 1.6″1.5″
Lifted Index 7-9 (minus)Lifted index 6-7 (minus)6-7
Peak Wind Gusts 40-50Peak Wind Gusts 3535 mph

One parameter that stands out today is the hail parameter, unique to the HRRR model. It’s a parameter that can’t be always taken literally, but today’s hail parameters show the possibility of large size hail—

HRRR hail forecast for 6 PM. Black is likely hail greater than 1.6″ diameter possible (Click on image for a larger view.)

Update Mon @ 8:37 AM — Just a few thoughts about today’s weather. Can anyone remember an outbreak of thunderstorms where the term ‘severe’ wasn’t used?

From a liability point of view, I think the NWS and TV weather people use the term just a bit too frequently. (But who’s not going to say when there’s any lightning and wind involved that somewhere, it might be severe?) I think you get my point. Let me be more specific regarding today’s forecast.

I see a high probability of strong storms and maybe even ‘severe’ weather today. But let’s compare a few severity parameters to those from the relatively recent tornadic outbreak of this past September 1st.

Last September‘s TornadoesToday’s Weather
CAPE 3500-4200 Joules/kgCAPE 1500-2400 Joules/kg
Helicity 800-1100*Helicity 150-250*
Vertical Shear 35-45Vertical Shear 15-25
Precipitable Water 2.0″Precipitable Water 1.6″
Lifted Index 7-9 (minus)Lifted index 6-7 (minus)
Peak Wind Gusts 40-50Peak Wind Gusts 35

As you can see, there’s ‘severe’ and then there’s SEVERE of last September. Today will be different.

It’s sorta like the radio world where all children are ‘above average’ and all thunderstorms are severe.

I’m sure there’s going to be some big storms today. But I think it’s important to keep it all in perspective. While anything is possible with thunderstorms, based on today’s current forecast parameters, I don’t think there will be any tornadoes. This will likely just the first of many summertime ‘severe’ thunderstorm days. I’m glad I got this off my chest. 🙂


Updated Mon 7:36 AM —Unstable air and a cold front will combine to form strong thunderstorms today, Monday.

Additionally, low pressure systems are expected to form ahead and along the front, enhancing the duration of the rain. While the main front is expected to pass through about 5:30 – 6:00 PM, low pressure will develop causing thunderstorms to break out as early as 1-2 PM; additional rain may last until 8 PM. (This event will not be a single thin line of storms, based on current models.)

The morning high resolution shorter range models 24 hour forecasts will available shortly after 10:30 AM. I’ll update this post later this morning.

STRONG STORMS TONIGHT

Thu 12:05 PM Forecast Review — The thunderstorms ahead of the front fell apart just as they approached Philadelphia about 1 AM, as the energy got transferred to the developing low pressure system that formed along the front.

Last night’s early models that prompted my “Huge Forecast Change” update, the HRRR and RAP, were a little too far west with the rain that developed this morning. The later NAM and HIRESW models did a better job with the westward extent of the rain.

I was off-duty by the time these came out.😉

UPDATE 9:50 PM – HUGE FORECAST CHANGE: Tonight’s 00z models are becoming available. (The 00z models is one of the two model daily runs that include direct upper air measurements done with weather balloons [radiosondes].)

If you’ve been following the forecast, the timing of the frontal passage has been pushed further up with each model run. The latest HRRR and RAP 00z models shows the front significantly stalling as it tries to move through the Philadelphia area tonight.

The latest: low pressure is expected to develop along this stalled front and move up over Philadelphia and southern NJ overnight and through tomorrow. Most of the thunderstorm activity may stay to our west overnight or just make it into Philadelphia about 1-3AM.

Additional rain expected to develop with this low pressure system over our area in the early morning hours and through much of tomorrow. The forecast has changed significantly. Too early to put it all together. But the RAP and HRRR area forecasting rain for much of early Thursday! (This afternoon’s GFS had hinted at this change.)

00z HRRR Radar/ surface pressure forecast for 11 AM Thursday (Click on image for a larger view.)

The higher resolution HRRR keeps the heavy storms just to our west tonight. The RAP (which uses GFS input data), has the storms making it to PHL, followed by significant rain on Thursday.


Update Wed @ 3:53 PM — Latest HRRR 18z available. Forecast still on track. A couple of isolated storms late this afternoon. Rain and thunderstorms, possibly severe, move into the immediate PHL area around 11PM – 2 AM, earlier north and west, in line with this morning’s NAM and HIRESW models. Rain and storms last through the night. Helicity and shear values lower than previous model runs, making the chance of tornadoes in our area unlikely. Still strong/severe storms possible with total rain amounts somewhat higher, near 1 inch, possibly 2 inches.


Strong storms are expected tonight. Most of the forecast remains unchanged from last night’s post. As usual, there are differences in the models from this morning.

To make things easier, I’m going to do a straight HRRR model forecast and then indicate where things don’t all agree.

Most of the thunderstorm activity breaks out between 9 PM and 1 AM with the maxima around 11-1 AM. (The latest HRRR does not show much activity in the afternoon, BUT the HIRESW models continue to show scattered storms this afternoon after 3 PM.)

The difference in forecasts between having strong/severe thunderstorms and extreme events like tornadoes are physical dynamics that are parameterized as helicity and vertical shear in numerical weather prediction models.

During the period between 9 PM and 2 AM, the HRRR has notable areas of moderately high helicity and high wind vertical shear. (As per my post last night, the helicity values are 350 m2/s2 compared to last week’s 1000 m2/s2 values; these are significantly less.) My personal feeling is that strong/severe thunderstorms are likely, but tornadoes are possible but not likely.

HRRR forecast relatively helicity (contours in red box) at 10 PM, although MUCH LESS than last week’s storms. Orange shading = CAPE (Click on image for a larger view.)

As for wind shear, there is a concerning level of high vertical shear about midnight in areas north of the city. This would translate into severe thunderstorms with high straight line wind gusts, hail and heavy rain.

While I have boxed in the area of highest shear, strong storms could form in other areas.

HRRR vertical wind shear at 12 AM midnight. Severe thunderstorms likely in the red box. (Click on image for a larger view.)

Heaviest rain areas—

HRRR total rain forecast predicted rain maxima. (Click on image for a larger view.)

It should be noted that the NAM and HIRESW version models don’t agree on the above HRRR forecast. The principle differences for these models is that they have the heaviest rain and storms staying mostly to our north and west and have weaker showers/thunderstorms moving in after midnight and continuing until about 3-4 AM in the PHL area. They also have some scattered storms in the late afternoon today.

Stay tuned.