A quasi-stationary boundary was in place over southwest Pennsylvania on May 24th. Taking advantage of an abundance of moisture and upper shortwave support, showers and thunderstorms developed and tracked along the boundary.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS PBZ wrote it up.
A quasi-stationary boundary was in place over southwest Pennsylvania on May 24th. Taking advantage of an abundance of moisture and upper shortwave support, showers and thunderstorms developed and tracked along the boundary. A couple of flooding events occurred in the urbanized areas of Allegheny County.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Pittsburgh, PA wrote its last area forecast discussion before the first event at May 24, 4:42 AM EST, 2 hours 34 minutes earlier.
Area Forecast Discussion National Weather Service Pittsburgh PA 542 AM EDT Mon May 24 2021 .SYNOPSIS... Scattered showers and storms are expected through this evening along a stationary front. More widespread showers and storms are expected with a Wednesday cold front. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Sfc analysis shows a quasi-stationary front across OH and PA. Water vapor satellite and near term models indicate another shortwave should increase scattered shower coverage again this morning. Made a few adjustments to early morning POPs based on radar and near term models. Diurnal destabilization in addition to shortwave support in vicinity of the front should also result in scattered thunderstorm development this afternoon, though limited instability and weak wind flow should preclude a severe weather potential. Highs should average around 5 degrees above seasonable levels. && .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/... Scattered showers and thunderstorms should diminish this evening with waning instability and exiting shortwave support. Residual instability E of PIT could result in a few showers/isolated storms E of PIT Tuesday, though mainly dry weather is expected under a building shortwave ridge. Dry weather should continue as the ridge slides east Tuesday night. Shower and thunderstorm chances should escalate Wednesday with a crossing shortwave, its associated sfc cold front, and a prefrontal trough. A few strong wind gusts are possible in thunderstorms with increasing shear, though cloud cover and limited instability could be a limiting factor. Precip should end Wednesday night as the front exits and high pressure begins to build in. Temperatures should continue to be 10 to 15 degrees above average through most of the period. && .LONG TERM /THURSDAY THROUGH SUNDAY/... Ridging, ahead of a Central CONUS trough, should maintain dry weather Thursday. Rain with cooler temperatures are likely Friday as the trough approaches and a sfc low crosses the OH Vly region. Sct showers should continue Saturday as the trough completes its passage. Dry weather should return Sunday as ridging builds behind the exiting trough. && .AVIATION /10Z MONDAY THROUGH FRIDAY/... VFR should deteriorate to MVFR by early morning as cigs lower in scattered showers with a crossing shortwave and quasi-stationary sfc front across OH and PA. Patchy MVFR fog is also expected erly this morning especially for ports that saw showers overnight. Additional sct tstms are expected to develop by aftn as diurnal destabilization occurs in the vicinity of the sfc front. These should diminish by eve as instability wanes, though patchy MVFR cigs wl remain possible, especially east of I 79. .Outlook... Restrictions are again likely on Wednesday with crossing low pres. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. &&
Hazardous Weather Outlook National Weather Service Pittsburgh PA 347 AM EDT Mon May 24 2021 OHZ039>041-048>050-057>059-068-069-PAZ007>009-013>016-020>023-029- 031-073>076-WVZ001>004-012-021-509>514-250800- Tuscarawas-Carroll-Columbiana-Coshocton-Harrison-Jefferson OH- Muskingum-Guernsey-Belmont-Noble-Monroe-Mercer-Venango-Forest- Lawrence-Butler-Clarion-Jefferson PA-Beaver-Allegheny-Armstrong- Indiana-Washington-Greene-Westmoreland- Higher Elevations of Westmoreland-Fayette- Higher Elevations of Fayette-Hancock-Brooke-Ohio-Marshall-Wetzel- Marion-Monongalia- Ridges of Eastern Monongalia and Northwestern Preston-Preston- Eastern Preston-Western Tucker-Eastern Tucker- 347 AM EDT Mon May 24 2021 This Hazardous Weather Outlook is for east central Ohio, northwest Pennsylvania, southwest Pennsylvania, western Pennsylvania, northern West Virginia and the northern panhandle of West Virginia. .DAY ONE...Today and tonight. No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday. Isolated strong wind gusts are possible in thunderstorms Wednesday afternoon and early evening. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time.
Special Weather Statement National Weather Service Pittsburgh PA 724 AM EDT Mon May 24 2021 PAZ020-021-241200- Beaver-Allegheny- 724 AM EDT Mon May 24 2021 ...A STRONG THUNDERSTORM CAPABLE OF PRODUCING SMALL HAIL AND HEAVY RAIN WILL AFFECT SOUTHEASTERN BEAVER AND NORTH CENTRAL ALLEGHENY COUNTIES... At 724 AM EDT, a strong thunderstorm was located over Franklin Park, moving east at 15 mph. Small hail is possible with this storm. Locations impacted include... Pittsburgh... Ross Township... McCandless Township... Franklin Park... Shaler Township... North Side Pittsburgh... Hampton Township... Economy... O'hara Township... Bellevue... Wexford... West View... This includes the following highways... Pennsylvania Turnpike between mile markers 32 and 44. Interstate 79 in Pennsylvania between mile markers 67 and 76. Interstate 376 in Pennsylvania between mile markers 71 and 74. Torrential rainfall also is occurring with this storm and may cause localized flooding. Do not drive your vehicle through flooded roadways. This storm may intensify, so monitor local radio or television stations for additional information and possible warnings from the National Weather Service. LAT...LON 4057 8021 4065 8016 4066 7983 4040 7994 TIME...MOT...LOC 1124Z 281DEG 11KT 4058 8011 DS
Special Weather Statement
National Weather Service Pittsburgh PA
806 AM EDT Mon May 24 2021
PAZ021-073-241245-
Westmoreland-Allegheny-
806 AM EDT Mon May 24 2021
...A STRONG THUNDERSTORM CAPABLE OF PRODUCING SMALL HAIL AND HEAVY
RAIN WILL AFFECT SOUTHEASTERN ALLEGHENY AND SOUTHWESTERN WESTMORELAND
COUNTIES...
At 805 AM EDT, a strong thunderstorm was located over Swissvale, or
near Pittsburgh, moving southeast at 20 mph.
Small hail is possible with this storm.
Locations impacted include...
Pittsburgh... Penn Hills...
Monroeville... Greensburg...
McKeesport... Jeannette...
West Mifflin... Baldwin...
Murrysville... Wilkinsburg...
Munhall... Jefferson Hills...
This includes the following highways...
Interstate 70 in Pennsylvania between mile markers 47 and 55.
Pennsylvania Turnpike between mile markers 56 and 74.
Interstate 376 in Pennsylvania between mile markers 73 and 84.
Torrential rainfall also is occurring with this storm and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.
This storm may intensify, so monitor local radio or television
stations for additional information and possible warnings from the
National Weather Service.
LAT...LON 4034 7949 4013 7974 4013 7976 4042 7999
4051 7989
TIME...MOT...LOC 1205Z 306DEG 18KT 4042 7990
DSThe Pittsburgh radar (KPBZ), 14 km away, was scanning. The volume nearest the first event began at 12:15 UTC.
2 flash flood. Local standard time, as filed.
A female was rescued after her car became stuck in high water near the Verizon Building at 590 Union Avenue.
Flooding occurred at the intersection of Shady Lane and Delafield Road in Fox Chapel.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Pennsylvania in 2021Every state in 2021This day in weather historyWarnings in Pennsylvania, May 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 957746, 957747.