Several rounds of scattered thunderstorms developed in a low shear, moderate CAPE, high DCAPE environment. The first round developed as scattered convection in southwest PA on the previous night and crossed into central PA by 11 AM EDT on Aug 10.
Every event with a place to put it. Tap one for its entry below.
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 CTP wrote it up.
Several rounds of scattered thunderstorms developed in a low shear, moderate CAPE, high DCAPE environment. The first round developed as scattered convection in southwest PA on the previous night and crossed into central PA by 11 AM EDT on Aug 10. Convection took on a linear mode with bowing characteristics as it approached Dauphin County by 2 PM EDT and ultimately produced damaging winds. Additional rounds of convection developed upstream later in the afternoon and again took on linear and bowing characteristics as it approached the Lower Susquehanna Valley by the evening hours. This line of storms converged with another line of storms moving northeastward from Virginia and Maryland. Although wind shear was weak, DCAPE values of near 1000 J/kg and storm motion near 15 kts allowed for damaging straight line wind to develop. There were no reports of hail.
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 State College, PA wrote its last area forecast discussion before the first event at Aug 10, 12:34 PM EST, 51 minutes earlier.
Area Forecast Discussion National Weather Service State College PA 134 PM EDT Tue Aug 10 2021 .SYNOPSIS... A steamy and sweltering late-summer heat wave will impact much of Central PA through Friday. The most intense heat and humidity combination will be felt across south central and southeastern parts of the area with widespread highs in the low to mid 90s and heat index values reaching the triple digits. The hot and humid conditions will also set the stage for daily rounds of scattered strong to marginally severe thunderstorms. Expect relief from the heat to arrive by the weekend. && .NEAR TERM /THROUGH TONIGHT/... Noon update: Making modifications to the PoPs/Wx/QPF as we watch the cluster of storms move into the s-cent mtns. These should likely get stronger as they hit the higher CAPEs in the SE where they've been cooking a little longer. New SWO from SPC mentions that svr threat does not appear to be widespread enough for a watch, and we agree. However, there could still be a few cells grow to make high gusts as this cluster moves thru the SE and as the next line from OH comes thru later this aftn. 725AM/1125Z: Leading line of showers fizzled out for the most part as it moved east of the Alleghenies, although recent trends indicate a very modest uptick in shower coverage over the far southeast zones. Second batch of showers/embedded TS has been on a weakening trend heading toward KDUJ/KFIG. We were keen to modify hourly POP trends to reflect radar and future extrapolation into midday. Temps and dewpts are on target so no further changes needed at this time. Previous Discussion Issued: 440 AM EDT Tue Aug 10 2021 Area of light showers moving across the western portion of the CWA at 06-08Z appears to handled best by the 10/00Z HiResW-FV3 model. CAMs are split regarding the sustainability of this activity which should not amount to more than a brief downpour in spots through daybreak. An uptick in shower activity has been observed across southwest PA, while a low stratus deck as formed over the eastern half of the lower Susquehanna Valley. The majority of the hires models are signaling another round of convection evolving eastward across the area later today. Although large scale forcing will be weak, dewpoints 65-70F range will support ample instability to support t-storm development. Any storms/clusters that can grow upscale may be able to produce strong to marginally severe wind gusts due to the combination of instability and steep low level lapse rates. Storm coverage and intensity fades into tonight with the best odds for a stray shower or t-storm over the western 1/3 of the CWA. Heat and humidity will continue ramping up with highs +5 to +10 degrees F above early August climo (mid 80s to low 90s) but not close to record territory. Factoring in the humidity/dewpoints will push max heat indices into the upper 90s toward century mark across the south central valleys. Very warm and muggy tonight with minT departures from normal on the order of +10-15F in upper 60s to mid 70s. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... Noon Update: Going to put a heat advy out with the update, covering IPT and areas south on Wed, and add in UNV/AOO for Thurs. Excessive heat looks possible Thurs (105+) in the lower Susq, but confidence is still below 50pct threshold to post a watch at this point. Will go with an advy for both days, and adjustments can be made (likely additions/upgrades) as we get closer in time. Prev... Sweltering heat and humidity continues through midweek with dangerous heat indices >104F possible across parts of the lower Susquehanna Valley. Heat and humidity are forecast to peak on Thursday as Bermuda high pressure migrates farther west into the SE US and SW return flow advects deep tropical moisture from the GOMEX into Central PA. PW values will rise to 1.75-2.00 and dewpoints into the low to mid 70s for Wed and Thu, both in the 90+ pctl based on NAEFS guidance and climo. Our latest forecast has max heat index values reaching 100-104F advisory range on Wednesday with excessive heat warning criteria possible on Thursday. Heat headlines are likely to be issued by the dayshift. Daily convection should stay scattered enough as to not provide significant and widespread relief from the heat. In addition to the hot days, overnight lows will be uncomfortable and not offer much of a reprieve especially in the southeast urban corridors where minTs are forecast to be in the mid 70s. A few strong to marginally severe storms with isolated damaging winds will remain a threat both days within a convective environment characterized by weak shear, moderate instability, and steep low level lapse rates. It has been a rather dry past 2 weeks particularly across the southwest 1/3 of CPA where some areas have seen less than 25% of normal precip. That said, WPC has introduced a low (MRGL) risk of excessive rain for D3/Thu covering the northwest half of the area. && .LONG TERM /FRIDAY THROUGH MONDAY/... A long-wave trough finally breaks down the ridge late in the week with Friday perhaps a couple of degs cooler, but still above 90 for most places south of I-80 and east of I-99. Models differ with the timing of the sfc cold fropa in the Fri-Sat time frame. The ECMWF deterministic run pushes it through with numerous showers and tstms late in the day / early evening Fri, while the slower GFS holds the cold fropa until the more stable Fri overnight hours, with less convection. Showers may linger across the south Saturday as the cold front slows down through Maryland and Virginia. Otherwise, the weekend looks pleasant - cooler and much less humid as high pressure builds in from the north and west. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... Several areas of convection across PA this afternoon. One area moving through central PA between KUNV and KMDT. More convection across western portions of the area. Elsewhere, VFR is predominant with hazy skies across the region. Convection will wind down after sunset with VFR, but hazy conditions overnight and first half of tomorrow. More afternoon convection possible next few days as region in the middle of a very warm and humid weather pattern. Outlook... Wed...AM low cigs possible NW Mtns. Scattered PM tsra impacts possible. Thu-Sat...Scattered PM tsra impacts possible. && .CTP WATCHES/WARNINGS/ADVISORIES... Heat Advisory from noon to 7 PM EDT Wednesday for PAZ028-036- 046-049>053-056>059-063>066. Heat Advisory from noon to 7 PM EDT Thursday for PAZ019- 025>028-036-045-046-049>053-056>059-063>066. && SYNOPSIS...Steinbugl NEAR TERM...Dangelo SHORT TERM...Steinbugl LONG TERM...Lambert/Colbert AVIATION...Ross
Hazardous Weather Outlook National Weather Service State College PA 1258 PM EDT Tue Aug 10 2021 PAZ004>006-010>012-017-018-024-033>035-037-041-042-111700- Warren-McKean-Potter-Elk-Cameron-Northern Clinton-Clearfield- Northern Centre-Cambria-Somerset-Bedford-Fulton-Tioga- Northern Lycoming-Sullivan- 1258 PM EDT Tue Aug 10 2021 This Hazardous Weather Outlook is for central Pennsylvania. .DAY ONE...This afternoon and tonight. Strong to severe thunderstorms with gusty winds are possible this afternoon and early evening. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. Strong to severe thunderstorms with gusty winds are possible both Wednesday and Thursday afternoon and evening. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report significant hazardous weather. PAZ028-036-046-049>053-056>059-063>066-111700- Juniata-Franklin-Southern Lycoming-Union-Snyder-Montour- Northumberland-Columbia-Perry-Dauphin-Schuylkill-Lebanon-Cumberland- Adams-York-Lancaster- 1258 PM EDT Tue Aug 10 2021 ...HEAT ADVISORY IN EFFECT FROM NOON TO 7 PM EDT WEDNESDAY... ...HEAT ADVISORY IN EFFECT FROM NOON TO 7 PM EDT THURSDAY... This Hazardous Weather Outlook is for central Pennsylvania. .DAY ONE...This afternoon and tonight. Strong to severe thunderstorms with gusty winds are possible this afternoon and evening. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. Please listen to NOAA Weather Radio or go to weather.gov/StateCollege on the internet for more information about the following hazards. Heat Advisory. Strong to severe thunderstorms with gusty winds are possible both Wednesday and Thursday afternoon and evening. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report significant hazardous weather. PAZ019-025>027-045-111700- Southern Centre-Blair-Huntingdon-Mifflin-Southern Clinton- 1258 PM EDT Tue Aug 10 2021 ...HEAT ADVISORY IN EFFECT FROM NOON TO 7 PM EDT THURSDAY... This Hazardous Weather Outlook is for central Pennsylvania. .DAY ONE...This afternoon and tonight. Strong to severe thunderstorms with gusty winds are possible this afternoon and evening. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. Please listen to NOAA Weather Radio or go to weather.gov/StateCollege on the internet for more information about the following hazards. Heat Advisory. Strong to severe thunderstorms with gusty winds are possible both Wednesday and Thursday afternoon and evening. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report significant hazardous weather.
Special Weather Statement
National Weather Service State College PA
221 PM EDT Tue Aug 10 2021
PAZ057-059-101845-
Lebanon PA-Dauphin PA-
221 PM EDT Tue Aug 10 2021
...A strong thunderstorm will impact portions of central Dauphin and
western Lebanon Counties through 245 PM EDT...
At 221 PM EDT, Doppler radar was tracking a strong thunderstorm near
Dehart Dam, moving east at 30 mph.
HAZARD...Wind gusts of 50 to 55 mph.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects.
Locations impacted include...
Lebanon, Hershey, Palmyra, Linglestown, Annville, Skyline View,
Campbelltown, Pleasant Hill, Sand Hill, Cleona, Elizabethville,
Fredericksburg, Timber Hills, Harper Tavern, Lickdale, Fort
Indiantown Gap, Penn National Race Course, Dehart Dam, Jonestown and
Dauphin.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4024 7648 4038 7694 4056 7687 4051 7646
4047 7639
TIME...MOT...LOC 1821Z 285DEG 25KT 4045 7681
MAX HAIL SIZE...0.00 IN
MAX WIND GUST...55 MPH
DangeloThe State College radar (KCCX), 108 km away, was scanning. The volume nearest the first event began at 18:23 UTC.
9 thunderstorm wind. Local standard time, as filed.
A severe storm producing winds estimated near 70 mph uprooted and snapped half a dozen to a dozen large trees on and near a property off Stricker Lane. The storm also ripped shingles off a shed on the property.
A severe thunderstorm producing winds estimated near 60 mph downed trees near Clarks Valley Road.
A severe thunderstorm producing winds estimated near 60 mph knocked down a few large trees on a property in Spring Grove, PA. Winds from this storm also resulted in the partial collapse of a garage.
A severe thunderstorm producing wind estimated near 60 mph downed trees on Stock St and on McCosh St in Hanover, PA.
A severe thunderstorm producing winds estimated near 60 mph knocked down powerlines in West York, PA.
A severe thunderstorm producing winds estimated near 60 mph knocked down powerlines in York, PA.
A severe thunderstorm producing winds estimated near 60 mph brought down powerlines 1 mile SE of Lititz.
A severe thunderstorm produced a 59 mph wind gust, recorded at Lancaster Airport.
A severe thunderstorm producing wind estimated near 60 mph brought down numerous utility wires across the area.
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, August 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: 965320, 965313, 965319, 965315, 965317, 965318, 965482, 965484, 965483.