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Thunderstorm Update

Update Sun 7/02 11:05 PM — A quick late update. Tonight’s NAM-NEST and HRRR have some thunderstorms moving through Philadelphia about 1-2 AM. This is a change from earlier models.

Additionally, storms expected Monday afternoon and evening. Another change — Tuesday may also have storms, as the cold front hangs up near here. Updates tomorrow.

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Thunderstorm Update

Updated Sun 07/02 @ 7:42 PM — The TV meteorologists are doing a fine job this evening covering the severe weather outbreak in far western suburbs. Here’s a recent radar with the few severe storms that have developed—

As has been the case both last night and again this afternoon, the models do not have the storms above coalescing into a line that will enter Philadelphia.

On the contrary, the HRRR model runs, supported by the Canadian models, have forecast that these storms will move to our north and to our south this evening, largely missing the immediate Philadelphia area.

The reason for the storms not entering Philadelphia— a strong area of negative vorticity (sinking motion) will be over Philadelphia, dampening out the storms—

We never know in advance if the models are correct about exact placement of severe weather, but that’s the current model forecast. And I imagine that’s why you visit this blog.

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Live Thunderstorm Blog 07-02-23

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Sunday Thunderstorm Outlook

Updated Sun 07/02 @ 9:26 AM — As forecast last night, the heavy showers and thunderstorms last night stayed far northwest of the city, with most areas in the Delaware Valley getting 0-0.02″ of sprinkles.

Last night’s early models had today’s potential storms strangely bypassing the immediate Philadelphia area, but the 06z models now show Philadelphia in the path of some strong storms today.

The latest HRRR and RAP model runs have just become available (12z runs) and show thunderstorms developing in place and moving in between 4:30 PM and 8 PM. Peak is 6-7 PM.

Despite very high “precipitable water” (PWAT ) of over 2 inches, the models are placing heavy rain accumulation north of the city. Here’s the forecast total precipitation, which is low considering the PWAT, suggesting that the storms won’t linger—

Comparing today’s forecast severity parameters to recent severe events, the potential severe storms with damaging winds today is high—

Today’s HRRR Forecast Severity Table—

⇣	Sept 1 , 2021 Tornado Outbreak (example of highly severe)	Recent April 1st 2023 Tornadoes (Example of severe)	Today’s 12z HRRR Forecast	Today’s Impact	Today‘s Notes
CAPE Joules/kg	3500-4200	2100	2500-3204	⚑⚑
Helicity m^2/s^2 (tornados)	1350	655	208 (420)	↔ ⚐	High southern Chester County
Vertical Shear 1/sec	40-46	40-45	18	↓ ↓
Precipitable Water	2.7″	0.83”	2.1″	⚑⚑
Lifted Index º K	minus 6º	minus 9.3º	minus 6.2º	⚐	average
HRRR Hail inches	1.9	1.4	1.6	⚑⚑
Thunderstorm Echo Tops	46,000 ft	32,000 ft	45,000 ft	⚑⚑	very high
Peak Wind Gusts mph	40-50	40-50	55	⚑⚑
Storm Motion Shear Vector Alignment	Aligned – ~ 90º	Almost aligned	aligned-curved direction	⚑
250 mb Jet Stream Wind mph	63	135	46	↓

Sunday Update

Updated Sat 7/01 10:40 PM — With the backdrop of having had a cluster of storms move through Cape May County this morning, unpredicted by any model, I’m going to attempt to forecast the showers and storms expected on Sunday.

A warm front will move through the area tonight into Sunday morning with showers and thunderstorms. Most likely time is from 2 AM to 5 AM. The models are keeping the storms mostly far northwest of the city, although some may make it in here.

Sunday morning will start cloudy, but sunshine should break through high thin clouds by noon.

The warm front hangs near us on Sunday. High moisture and instability will cause more storms to develop late afternoon and evening, with increased chances between 4 PM – 10 PM. Strangely, tonight’s models have most of the activity moving northwest and south of the city, somewhat missing the immediate Philadelphia area.

It’s always difficult to know if the model placement of the storms will be accurate.

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Updated Sat 07/01 @ 9:25 AM — According to models, the clouds over us this morning should dissipate by noon. The slowly approaching warm front seems to be moving a bit faster than forecast yesterday. Some thunderstorm activity expected moving into Philadelphia as early as 8 PM this evening (HRDPS) but the latest HRRR keeps it moving in until after midnight.

This morning’s water vapor image with superimposed RAP model jet stream level wind streamlines. I’ve drawn the warm front in —

Some “interesting” weather expected Sunday.

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Updated Fri 6/30 @9:41 PM — I’ve revised the Sunday forecast below

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Previously Posted Fri 8:19 PM —

A stationary boundary running generally west-east to our south will move north as a warm front Saturday night into Sunday morning. The exact timing varies from model to model, but somewhere between daybreak and noon on Sunday is the general trend, although some models have it hanging up in our area. —

Before the warm front, showers develop, but all the models keep the showers and thunderstorms from moving eastward during the daytime on Saturday . Western Chester county and areas west may see some showers and thunderstorms Saturday evening.

A cold front comes through later Monday with showers and thunderstorms.

Tuesday, the 4th of July, looks very nice.

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Sunday Forecast Update

Updated Sun 06/25 @ 9:03 AM — The HRDPS and latest RAP model have thunderstorms forming dynamically about noontime today. Most of the other models are forecasting storms developing around 2-6 PM.

So which forecast, if any, can we hang our hat on? The first clue is to look at the current conditions, specifically the current satellite water vapor image (Channel 9). Is there any disturbance visible that will move in about 3 to 4 hours?

In the above image, we see an area of colder (whiter) effective layer, suggestive of the spark for thunderstorm development. This area is right over Chesapeake Bay, about 2-4 hours away by jet stream winds, currently at 30 mph in this region. (The one thing not in favor of this forecast: the upper air area is not as developed as it might be. The blue areas in the lower right corner are examples of much more developed upper air disturbances.)

So the noontime to1 PM forecast may be correct. Let’s see. This is the fun of weather forecasting.

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Sunday Forecast Update

Updated Sat 06/24 @ 10:53 PM — We had the majority of the rain late morning and early afternoon. The forecast for a 3-5 PM peak in thunderstorm activity didn’t occur around Philadelphia, despite almost every model having predicted it. The NBM thunderstorm prediction was all wrong.

If we go with today’s relative ‘winner’ in the forecast timing of the today’s showers, it would be the HRDPS. So let’s go with it for Sunday.

The latest HRDPS shows showers beginning to pop up around noon Sunday with a relative peak between 1 PM and 4 PM. The showers will be scattered and like today, there will be periods of sunshine interspersed. It will be much warmer Sunday with highs around 85º but with a larger than usual uncertainty of about ± 3º.

Here’s the HRDPS forecast for 2 PM Sunday—

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Saturday Forecast Update

Updated Sat 06/24 @ 10:55 AM — It’s pretty clear, based on current radar and this morning’s models, that showers and thundershowers are forming now (late morning) and more numerous showers and storms are expected over the next few hours.

I guess last night’s HRDPS was correct with its forecast of earlier storms. None of these are expected to be severe (low shear low helicity)

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Saturday Forecast Update

Updated Sat 06/24 @ 8:40 AM —The main area of rain from last night has departed to our northeast. Another area of enhanced moisture is in West Virginia and will rotate into our area this afternoon. Combined with the cyclonic flow and some breaks of sun, thunderstorms are expected to form this afternoon.

Rainfall will not be as heavy as it was in some areas yesterday. (PWATS yesterday were in the 2.0-2.1″ range) Today’s PWATs will be closer to 1.6″

While we’ve seen it’s impossible to predict exact timing and placement of the rain, it appears that showers and thundershowers begin popping up about 2-4 PM today as the area of enhanced moisture moves in, based on the HREF. The Canadian HRDPS has some showers popping up earlier, just before noontime.

The NBM 4.1 model has a new parameter, thunderstorm coverage, which attempts to predict the areas/timing of greatest thunderstorm density. It pegs the period 3-5 PM as most active with the greatest areal coverage of thunderstorm activity as shown below—

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Update Fri 6/23 11:39 PM — Additional high resolution models just available have showers and thunderstorms during the early afternoon as well as into the evening..

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Update Fri 06/23 @ 9:59 PM — Tonight’s HRRR just available has considerable sunshine with clouds for us mid day and afternoon. Showers don’t move in until after 6 PM Saturday.

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Previously Posted Fri 9:09 PM —

The closed upper low pressure system will move closer to us on Saturday through Sunday. As you can see from the water vapor image below, the ‘action’ with an upper low is often a certain radius away from its center; as it moves eastward over Pennsylvania on Saturday, we’ll still be in an unstable air mass, but much of the active moisture plume will move to our east. Nonetheless, instability will lead to scattered thunderstorm development.

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