August 24, 1998, Squall Line/DerechoA weak derecho/squall line (weak in relative terms, when considering how strong it could be) moved across Ohio, into western Pennsylvania during the morning hours of August 24, 1998 at incredible speeds -- 50-60 mph! There was a few small bow echoes embedded within the line of thunderstorms/derecho, which is where most of the damage came from. The damage was oriented in swaths... one of the criteria for a derecho. The northern most bow echo, which tracked across northwest PA, was the most destructive, causing widespread damage in Warren county, PA. Dry air at around 10,000 feet (700mb) helped enhance the downdraft/downburst potential. More stable air over central and eastern PA was a major role-player in the dissipation of the line of thunderstorms when it reached this area. However, the NWS did not know that the line of severe thunderstorms would fall apart, so SPC outlooked much of PA and surrounding areas in a moderate risk. Moderate risks are relatively rare, and imply that severe thunderstorms may be violent while affecting more than small areas. See Severe Weather Outlooks issued by local NWS Offices. SPC (located in Norman, Oklahoma) issued a severe thunderstorm watch for western and central PA, and southwest NY around 8:30 am EDT. (you can also read CTP's redefining statement). Local NWS offices (Pittsburgh, State College) issued many severe thunderstorm warnings and statements, which you can see here for your viewing pleasure. Local NWS offices didn't do such a good job at forecasting that the line of storms would fall apart rapidly in central PA, but then again, that would be nearly impossible for anyone. Thunderstorms can develop and fall apart very quickly, without warning. Credit must be given to the NWS for their excellent job at issuing timely warnings... which likely saved at least a few lives. The severe thunderstorm warning for Warren county was issued just before 9 am, while the severe storms didn't enter the county until around 9:20 am. This gave more than 20 minutes lead time for people in the path of the dangerous severe thunderstorms to take cover in a substantial building before they hit... greatly reducing deaths and injuries. I am proud to say that there were no reported injuries with this event! The NWS in State College should be proud of themselves, and even more so, the people who took the appropriate actions to protect their lives. With widespread severe thunderstorm activity, it is hard to not lose lives without advanced warning. Advanced warning systems are critical to save lives. It's our only defense... Ohio damage--Lucas, Wood, Lake, Ashtabula, Huron, Seneca, Medina, Richland, and Wayne counties: Numerous trees were reported down across Lake county, with numerous large limbs down across Ashtabula and Richland counties. Trees were also down in Lodi, and in northern sectiosn of Wayne county. A garage roof was blown off in Homerville by thunderstorm winds. A 63 mph thunderstorm wind gust was recorded at Fairport Harbor. Dime size hail was reported in Waterville, Tontogany, across Huron county, and at some spots in Seneca county. Pennsylvania damage--Crawford, Erie, Mercer, Lawrence, Beaver, Venango, Clarion, Armstrong, Allegheny counties: Large trees were downed in Greenville, Polk, Sugarcreek, Cranberry, Franklin, Oil City, Wilkinsburg, West Kittanning, Furnace Run, Schenley, Cadogan, Vowinckel, and Wampum. Many trees were downed across Crawford county, including in Conneautville. Power lines were downed in a number of areas also. A 60 mph thunderstorm wind gust was recorded in Edinboro. Quarter (1") size hail fell in Greenville, while nickel size hail fell in Cannelton. Excessive lightning was also produced with these storms... at times... exceeding 300 strikes per minute over western PA and eastern OH! Take a look at this lightning map ... showing the time period from 9:45-10:45 am EDT. As a result, at least two houses and one school were struck and damaged by lightning in Venango and Armstrong counties alone - in Clintonville, Dempseytown, and Ford City. That is a lot... considering that a bolt of lightning is only as wide as a pencil. Warren, McKean, Elk, Jefferson, and Potter counties: Trees were downed in mainly the northeastern part of Jefferson county... between Brockway and Crenshaw. In Elk county, trees were down around Saint Marys, including along route 120. A tree fell on a house in Brockport. Trees were downed around Shinglehouse, in Potter county, along route 44. Mckean county had some power lines down in Bradford. In Warren county, damage was isolated in nature across much of the county, with a few trees downed from Sheffield to Warren to Youngsville. Worth of note, nearly all of these trees fell in the same direction, an indicator of downburst winds. The most widespread damage was in northwestern Warren county. Between Sugar Grove and Lottsville, there was small areas of intense tree damage. One spot, just south of route 957 between Sugar Grove and Lottsville, had an area a few hundred feet long where every tree was snapped about 12 feet off the ground. This is where a thunderstorm produced a microburst... as the definition of a downburst (microburst or macroburst) is, "damaging winds on or near the ground". In this case, it was near the ground, not on it. Other pockets of damage just to the west included more damaged trees, wires, and poles. The most extensive damage took place in and just outside of Lottsville. At least 7 houses were moderately to severely damaged, most by falling trees, with others damaged to a lesser extent... but still requiring repairs. A shelter was opened at the Freehold township building, as a few houses were unlivable. Several grain trailers were overturned, and a grocery store had its siding ripped off. Lightning struck and ruptured gas lines, causing a fire in a shed. In the town, many trees were uprooted or snapped. Just west of town (a few blocks), there was small pockets of tree damage... which included rows of snapped and uprooted trees. Several utility poles were also snapped. Just south of town, along Sherman Hill Road, is where a large area of damaged trees was observed. Every tree was damaged in some way, with about 80% of them being snapped or uprooted! Throughout Warren county, in total, several hundred trees were downed. In the areas with the most intense tree damage (away from houses), thunderstorm wind gusts could have easily been in excess of 85 mph. See NWS statements for some damage info. I apologize the links to radar below are not working. I was linking to another website, and those images were lost during a hard drive crash they had. The Base Reflectivity radar image at 9:21 am showed the line of severe thunderstorms about to enter Warren county. The wind velocity image showed the strong winds associated with the thunderstorms. A few minutes later, the reflectivity data shows that the line of thunderstorms has entered the northwestern part of Warren county. Five minutes later (9:31 am), the entire line had entered Warren county, and it is about now when the damage was occurring in the Lottsville area. Note the weaker radar returns (oranges and yellows) surging forward just behind the line in northwestern Warren county. This is a good indicator that the damaging winds were plunging down out of the thunderstorms and reaching the ground. As stated at the top of this page, the line of severe thunderstorms quickly fell apart when moving into central PA. You can see a four hour radar loop of the line here. (note: this loop is best viewed using Netscape Navigator, and a relatively high speed modem). Notice how the reds, oranges, and yellows gradually fade to mainly greens... showing that the thunderstorms are dissipating. A complete listing of all images/text available for this event can be found here.
Email: BowEchoCPA@suscom.net |
