Strong Thunderstorms Thurs Afternoon into Evening

Update Thu @ 5:13 PM — Thunderstorms have broken out as expected with one or two severe warnings in effect. Most of the activity has been typical thunderstorms. The latest satellite imagery shows another impulse approaching from the southwest and may trigger more storms, especially from Philadelphia, south and east.

Update Thu @ 9:24 AM — Reviewing last night’s models and the new models coming available this morning. A cold front moving down from the northwest will develop a wave along it; low pressure directly over our area (or just to our south according to the latest models) will enhance rainfall and thunderstorm potential.

HRRR: Thunderstorms develop 2-5 PM far northwest areas and move southeastward, reaching Philadelphia suburbs and Philadelphia between 4:30 and 7 PM. Additional thunderstorms and showers develop dynamically in-place due to developing low pressure lasting through midnight.

RAP: Scattered storms break out as early as 3 PM in our immediate northwest suburbs. Main activity here 4-6 PM. Additional showers/thundershowers through midnight.

Current severity expectations is “marginally severe possible.”

HRRR forecast showing low pressure over New Jersey at 7 PM (Click on image for a larger view.)
HRRR Shear forecast at 7 PM tonight. Peak shear is around 30 (darker orange) (Click on image for a larger view.)
9/1/21 Tornadoes
(example of severe)
Today‘s Forecast
HRRR 15z
CAPE 3500-4200 Joules/kg1300-1600
Helicity 800-1100 m^2/s^2250
Vertical Shear 35-45 25 (34)
Precipitable Water 2.0″1.9-2”
Lifted Index (minus) 7-9 minus 5
Peak Wind Gusts 40-5020 (40)
Aligned Storm Motion and Shear Vectors- YESNo
Jet stream- Highly CyclonicFlat wavy**
Jet Stream Speed – Highlow-moderate
500mb – Highly Cyclonicflat wavy
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity
↓ indicates works against Severity

Update Wed @ 8:30 PM — The front made it to Lancaster county during the daytime and barely moved back eastward. Around Philadelphia, an easterly flow kept things cool. Thunderstorms developed northern Chester County (Phoenixville area) this afternoon and again this evening. The evening storms were very slow moving and based on radar, they dumped a lot of rain in a small region. They attempted to move eastward, but ran into high CIN and fell apart.

Additional storms in the Allentown area may move through here around midnight.

Thursday is shaping up to be an interesting day weather-wise as most models have consistently forecast the development of a low pressure system directly over us late afternoon. An approaching front and low pressure could result in heavy rains. Right now scattered storms look to begin about 2 PM in western areas and move into the Philadelphia region by 4-6 PM. I’ll post something about intensity of these storms tomorrow morning.

NOAA is doing a live test of their new supercomputers for the next 24 hours. All models today are coming off of this new system. I’ve never seen their servers so fast! Another live test in two weeks and then it becomes fully operational on June 28th. 

So where are all these models on the web?  Here on the NOMADS site.  (Warning: The NOMADS site is somewhat intimidating and designed for extreme weather nerds and professionals only.) 

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Update Sun @ 8:50 PM — Monday forecast updated below with new NBM model temperatures

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Sunshine through high cirrus clouds (in the morning) and very warm. More humid. Dew points in the low to mid 60s. High 88.8º ± 2.0º NBM model, Blue Bell



Update Fri @ 5:04 PM — Not much in the way of strong storms or severe weather in our immediate area. (Although severe weather was picked up on radar in NJ.) The amount of rain so far has been less than forecast. More showers expected later this evening and around/after midnight.

Update Fri @ 9:44 AM — Latest HRRR and RAP have converged on a peak time for storms to start: about 3 PM, earlier in western areas.

Update Fri @ 9:10 AM — Reviewing the latest hourly models, the previous forecast time frame for showers and thunderstorms has remained unchanged in our immediate area: a large range of time, from 2 PM to well after midnight.

Based on helicity values and storm motion/shear vectors, the strongest storms appear to be most likely this evening, after 5:30 PM, although CAPE values are already high at 2 PM.

Different severe parameters and factors peak at different times. There’s a strong jet stream influence later in the evening after midnight.

The severe parameter values are similar to those shown in my severe parameter table from last night.

Last September‘s Tornadoes (9/1/21)Thursday’s
RAP 21z Model
RAP 09z Model
CAPE 3500-4200 Joules/kg12001200
Helicity 800-1100 m^2/s^2350250
Vertical Shear 35-45 16-2025
Precipitable Water 2.0″1.8″1.9″
Lifted Index minus 7-9 minus 5minus 5
Peak Wind Gusts 40-503532
Aligned Storm Motion and Shear Vectors- YESYES (early)YES
6 -9 PM
Jet stream- Highly CyclonicFlat to anticyclonicMore Cyclonic
Jet Stream Speed – HighModerateMod-High
500mb – Highly CyclonicAntiCyclonicCyclonic
Updated Update Fri @ 8:44 AM

As was my forecast last night, these storms can be strong. I don’t expect severe weather, but I’m sure the NWS will issue severe storm watches and warnings. Base your plans on the NWS.

As for this weekend’s weather, this has been an especially challenging forecast, especially for Saturday, with models having inconsistent and shifting forecasts from day to day. The upper low has been hard to pin down. The ICON, whose forecast had been totally dry for Saturday has added showers and even some thundershowers for mid-day Saturday. Indeed, several models have a mix of sun, clouds and convective showers (thundershowers) during the morning and early afternoon.

I’ll post the weekend forecast this evening.

Updated Thu 11:30 PM — Tonight’s models continue with forecast from the earlier RAP— showers and thunderstorms develop late morning/early afternoon Friday and continue through at least midnight.

Several batches of storms during this period— Some late morning activity possible to start things off —then early afternoon (2PM) and some potentially stronger storms 6-9 PM. Another batch around midnight. Some models have some showers even Saturday morning before the major clearing trend starts for the balance of the Memorial Day weekend.

Updated Thu 7:38 PM — Prior to what’s shaping up to be a rather nice weekend, we’ll have to get through Friday.

An approaching upper air trough will trigger the development of showers and thunderstorms on Friday. My table of severity parameters below compares tomorrow model forecast (RAP) to what has become my standard of extremes- last year’s (9/1/21) tornadic storms.

Last September‘s Tornadoes (9/1/21)Fridays Storms
RAP 21z Model
CAPE 3500-4200 Joules/kg1200
Helicity 800-1100 m^2/s^2350
Vertical Shear 35-45 16-20
Precipitable Water 2.0″1.8″
Lifted Index (minus) 7-9 minus 5
Peak Wind Gusts 40-5035
Aligned Storm Motion and Shear Vectors- YESYES (early)
Jet stream- Highly CyclonicFlat to anticyclonic
Jet Stream Speed – HighModerate
500mb – Highly CyclonicAntiCyclonic

As you can see, the current severe parameters are in the middle range. I expect some strong storms tomorrow. I’ll use the disclaimer that severe weather is always possible with thunderstorms, but I don’t expect much in the way of severe.

What appears to be the most important aspect of tomorrow’s storms will be the extended time frame that scattered storms develop and move through. Some early activity late morning, the bulk of the activity from about 2 PM to midnight.

This afternoon’s 21 RAP model radar/accumulated rain forecast at 2 PM Friday. Large area of rain (1) for the afternoon. A second batch of rain (2) catches up and moves in around midnight. (Click on image for a larger view.)

I’ll keep an eye on things. Stay tuned.

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For the weekend, there’s a lack of agreement between the GFS, ECMWF, ICON, and CMC regarding the upper trough and closed upper level low. The GFS has a slow, closed low forecast which would adversely affect this coming weekend’s weather, while the other models open up the low and move it off the coast. It’s too early to tell which is correct and we’ll have to follow the model trends for the next few days.

The most optimistic forecast right now: Friday looks to be rainy, sunny Saturday, Sunday into Monday—

Today’s 12z ECMWF optimistic forecast for early Saturday morning. (No closed upper level low) (Click on image for a larger view.)

Unfortunately, the latest GFS maintains that closed low through the weekend with plenty of showers. Again too early to hang our hats on the pessimistic GFS—

Today’s 18z GFS pessimistic Sunday forecast. Closed low over Pennsylvania Plenty of rain and clouds. (Click on image for a larger view.)