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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