A weak frontal boundary and copious moisture in place was the trigger for significant flash flooding across Lebanon, Lancaster and York Counties on Friday August 31st.
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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 CTP wrote it up.
A weak frontal boundary and copious moisture in place was the trigger for significant flash flooding across Lebanon, Lancaster and York Counties on Friday August 31st.
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 31, 5:22 AM EST, 4 hours 3 minutes earlier.
Area Forecast Discussion National Weather Service State College PA 622 AM EDT Fri Aug 31 2018 .SYNOPSIS... Stalled out frontal boundary near the Mason-Dixon line will meander a bit north and south over the upcoming 24 to 36 hours bringing periods of showers and isolated thunderstorms. The greatest chance for thunderstorms and locally heavy rain will be across the southeast third to half of the state. High pressure will ridge in from the northeast later Saturday through early next week bringing less of a chance for showers and thunderstorms. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A well defined low-mid level theta-e ridge (best depicted within the 850-700 mb layer) was responsible for being the source region for waves of showers and tsra that were moving to the NE with the mean flow (accounting for another 100 mb or so above this layer). The best theta-e advection, low-level moisture, close proximity of the llvl quasi-stnry front and low to mid level isent upglide favors the convection forming near the I-81 corridor in southern PA and northern VA to move northeast across the Susq Valley for the rest of this morning. HREFV2's 24 hour ens localized probability matched-mean QPF indicates basin average rainfall of less than 0.50" over the bulk of Central and Ern PA today into early Sat. However, Pwats around 2 inches, combined with the aforementioned low-mid level forcing and convective parameters create the potential for some pinpoint very heavy rainfall amounts of greater than 3 inches. Amounts this heavy would likely cause some localized flooding problems of small streams and poor drainage areas. WPC upgraded their DAY 1 ERO to the Slight Risk category over our SE zones, However, the consensus of adjacent WFOs is to hold off on a Flash Flood Watch for now. The best potential and target area for this second round of convection with locally very heavy rain this afternoon will be over the Mid to Lower Susq Valley and Western Poconos. Elsewhere across the northwestern third to half of the CWA, expect just scattered, mainly morning showers, mainly to the east of route 219. The downslope on the western edge of the CWA Friday will probably keep it really nice just a few miles to the west of our CWA. This cloud/sun boundary could help to fire up a shra/tsra as the sun stirs things up, mixes in some of the moisture and gets some lift from those higher elevations. Max temps today will vary from the mid 70s across the northern and western high terrain, to around 80F in the southern valleys. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... The westerly flow will likely take any showers to the east into the lower Susq valley later tonight. There could be a heavy downpour out of some of the showers overnight, especially across the south and east (and perhaps near the RT219 corridor over the Laurels) if thunder occurs. The E-SE flow in the low levels will transport moisture from the Atlantic and what may be put down from any showers tonight up into the mountains of Central PA. The upslope flow is usually a strong signal for a solid overcast and perhaps some drizzle in the mountains. The far nrn tier and SErn cities will have the best conditions around sunrise, but the SE may still be dealing with sct shra. Mins will stay mild and in the 60s thanks to the extensive and rather thick low level clouds. as the clouds move in. && .LONG TERM /SATURDAY THROUGH THURSDAY/... High pressure sitting off shore will continue to affect the incoming system. This will keep a lingering easterly flow through the eastern half of the state. This could keep Saturday cloudy with light precipitation possible. The further east you go the winds get a more southerly direction which could bring some possible afternoon brightening over the far western counties. The best chance of showers on Saturday will be along the Mason Dixon and adjacent counties where the PWAT anomalies are the highest. Anomalous PWATS spread northward on Sunday. Moist south to southwesterly flow across Central PA will give a better opportunity for convection. Medium range guidance indicates a return to hot and humid conditions Sunday through Wednesday, as a subtropical ridge over the western Atlantic retrogrades westward to the Mid Atlantic coast. The warm up should begin Sunday, as stalled frontal boundary lifts north across Pa, causing low level flow to veer to the southwest. Under the influence of upper ridge, expect well above normal temperatures, high humidity and scattered diurnally-driven convection each day. && .AVIATION /10Z FRIDAY THROUGH TUESDAY/... A moist easterly flow will result in developing stratus across most of central Pa today. An examination of latest SREF/HREF suggest tempo IFR cigs are possible this morning over the central mountains from KJST/KAOO to KUNV. Across the Susquehanna Valley, model soundings indicate MVFR cigs will become likely around 13Z. The most favorable conditions today may be over the northwest part of the state, where KBFD will likely be on the western edge of the MVFR cigs. In addition to lowering cigs, an area of low pressure tracking southeast of Pa will produce scattered showers across the southeast half of the state. Some of these showers could be heavy enough to produce brief dips to IFR visibility. Persistent southeast flow, combined with overnight cooling, is likely to result in deteriorating cigs tonight. Model soundings support the idea of widespread IFR/LIFR cigs late tonight with occasional light rain/drizzle. .Outlook... Sat...AM low cigs/drizzle likely. Sun...AM low cigs likely. Isold PM tsra impacts poss western Pa. Mon-Tue...Patchy AM fog possible. Isold PM tsra impacts. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Dangelo/Lambert LONG TERM...Fitzgerald/Ceru AVIATION...Fitzgerald
Hazardous Weather Outlook National Weather Service State College PA 522 AM EDT Fri Aug 31 2018 PAZ004>006-010-011-037-010930- Warren-McKean-Potter-Elk-Cameron-Tioga- 522 AM EDT Fri Aug 31 2018 This Hazardous Weather Outlook is for central Pennsylvania. .DAY ONE...Today and Tonight. The probability for widespread hazardous weather is low. .DAYS TWO THROUGH SEVEN...Saturday through Thursday. The probability for widespread hazardous weather is low. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. PAZ012-017>019-024>028-033>035-041-042-045-046-049-050-010930- Northern Clinton-Clearfield-Northern Centre-Southern Centre-Cambria- Blair-Huntingdon-Mifflin-Juniata-Somerset-Bedford-Fulton- Northern Lycoming-Sullivan-Southern Clinton-Southern Lycoming-Union- Snyder- 522 AM EDT Fri Aug 31 2018 This Hazardous Weather Outlook is for central Pennsylvania. .DAY ONE...Today and Tonight. Slow moving showers and isolated thunderstorms will produce localized heavy rainfall amounts of over one inch in just 30 to 60 minutes. Ponding of water in poor drainage areas and rapid rises on streams and creeks will be possible. The greatest threat for the heavy rain will be during the mid to late morning hours across the Central Mountains, and through the mid afternoon closer to the Susquehanna Valley. .DAYS TWO THROUGH SEVEN...Saturday through Thursday. The probability for widespread hazardous weather is low. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report significant hazardous weather. PAZ036-051>053-056>059-063>066-010930- Franklin-Montour-Northumberland-Columbia-Perry-Dauphin-Schuylkill- Lebanon-Cumberland-Adams-York-Lancaster- 522 AM EDT Fri Aug 31 2018 This Hazardous Weather Outlook is for central Pennsylvania. .DAY ONE...Today and Tonight. Slow moving showers and scattered thunderstorms will combine with a nearly stationary frontal boundary and a very moist airmass to bring the chance for highly localized, very heavy rainfall in excess of 2 inches within just an hour or two. The heavy rain could lead to rapid rises on small streams, along with flooding of poor drainage areas in cities and towns. .DAYS TWO THROUGH SEVEN...Saturday through Thursday. The probability for widespread hazardous weather is low. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report significant hazardous weather.
The State College radar (KCCX), 136 km away, was scanning. The volume nearest the first event began at 14:25 UTC.
4 flash flood. Local standard time, as filed.
Flash flooding on Quarry Road between Swatara Road and Sweetbrier Lane southeast of Jonestown. Cars were reported submerged in water in Jonestown.|Airport Road in Fredericksburg suffered major damage.
A nearly stationary thunderstorm brought over 10 inches of rain in several hours between Manheim Borough and Mount Joy Borough. This prompted severe flash flooding in this area, with numerous water rescues and one home that was evacuated. PA Route 283 was closed in both directions at PA 772.
Flooding was reported along Blouse Road and Zimmerman Road.
Klines Run Road was washed out near Trinity Church Road.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Pennsylvania in 2018Every state in 2018This day in weather historyWarnings in Pennsylvania, August 2018
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: 771839, 782189, 781165, 781169.