JULY 7-8, 1991
DERECHO
"The Southern Great Lakes Derecho of 1991"
Figure 1. Area affected by the July 7-8, 1991 derecho (outlined in blue). Approximate hourly positions of the derecho "gust front" are indicated by black curved lines. Measured wind gusts in mph are plotted (red numbers).
During the early daylight hours of Sunday, July 7, 1991 a bow echo developed over southeastern South Dakota and began racing eastward producing strong damaging winds. This was the start of a long-lived derecho event that lasted 17 hours and affected areas from the Great Plains region to western New York and Pennsylvania (Fig. 1). Wind gusts reached 80 to 100 mph in some places. The strongest gust, 103 mph, was measured at Sioux Center, Iowa about mid-morning (Fig. 1) and the roof was blown off a school in Orange City which is just southeast of Sioux Center.
As the derecho storm system progressed across northern Iowa considerable damage occurred. Many thousands of acres of corn and soybean plants were flattened resulting in crop losses of 60 million 1991 dollars. Many farm buildings were damaged or destroyed. A gust to 71 mph was measured at the Mason City Airport and there was much damage in and near the city. In McIntosh Woods State Park, which is a few miles west of the Mason City Airport, a women was killed when a tree fell on her van and crushed it.
As the storm system entered Wisconsin early on the afternoon of July 7th, the original bow echo that caused all the damage west of the Mississippi River died out and a new bow echo quickly developed to the south of the one that was dying. This bow echo increased in size rapidly and by the time it reached Madison the derecho was producing severe and extensive damage once again. At Madison's Dane County Regional Airport a gust to 83 mph was recorded and 20 buildings and hangers were damaged and several airplanes were damaged or destroyed. In the Madison area 60,000 customers lost electrical power and damage totaled 7 million in 1991 dollars.
In a corridor from the Madison area to Lake Michigan just north of Milwaukee very strong winds and intense damage continued to take place. At the Sullivan-Milwaukee NWS Forecast Office and at the Waukesha Airport gusts to 81 mph were recorded. About 30 farm buildings were destroyed and over 250 were damaged along the corridor. Many homes and businesses were damaged, hundreds of trees were blown down, and many thousands of customers lost electrical power. Four people were injured. Besides Madison, some towns that received severe damage include Ixonia, Watertown, Milford, Waukesha, and Cedarburg. In Waukeska the historic 300 year old Dunbar Oak Tree was toppled and at the airport two planes were destroyed and hangers were damaged. The Federal Emergency Management Association declared the counties of Dane, Jefferson, Waukesha, Washington, and Ozaukee as part of a Federal Disaster Area. The July 7-8, 1991 derecho caused a total of about 20 million 1991 dollars worth of damage in Wisconsin most of which was in the Federal Disaster Area.
The derecho storm system crossed Lake Michigan and entered southwestern Lower Michigan between 6 and 7 PM EDT on Sunday evening. During the next three hours it would race across the state at a speed of 60 mph causing much damage to buildings and crops, blowing down numerous trees, and cutting off electric power to 853,000 customers. This would be the largest number of customers in Michigan to lose electrical power from the same storm system up until this time. For some customers it would be more than a week before electrical power was restored.
Some of the strongest winds occurred in Kent County and northwestern Ionia County where a gust to 84 mph was measured in Grand Rapids and a gust to 85 mph was recorded at Belding. Some of the worst damage in southwestern Lower Michigan occurred in these counties also. Two homes were destroyed and over 250 others were damaged. About four dozen barns were also damaged or destroyed and many fruit growers lost more than half their orchards.
As the storm system continued east across the state a semi-trailer truck was blown over on I-69 east of Battle Creek. Some of the most severe damage in southeastern Lower Michigan occurred from Shiawassee County eastward into the Flint metropolitan area. In New Lothrop 10 mobile homes were blown over with one being blown into a nearby lake. In Durand numerous camper trailers were blown over and one person was injured. Quite a few homes and cars were damaged by falling trees in Owosso. A roof was blown off an elementary school in Swartz Creek and on the west side on Flint near I-75 roofs were blown off two apartment buildings.
Wind gusts from 75 to 85 mph were common in the Ann Arbor, Detroit, and Pontiac areas. Many homes and cars were damaged in Rochester and Tecumseh and trees and power lines were blown down throughout the region.
After crossing Lower Michigan, the derecho entered southwestern Ontario during the late evening hours on July 7th. There were many trees and electrical lines blown down and a number of roofs damaged or blown off. Some of the worst affected towns included Harrow in Essex County, Chatham and Ridgetown in Kent County, and Strathroy in Middlesex County. The bow echo system and derecho winds crossed Lake Erie and affected portions of northwestern Pennsylvania and adjacent states before ending about 2 AM EDT on Monday, July 8, 1991.
In summary, the "Southern Great Lakes Derecho of 1991" traveled about 1000 miles, caused about 125 million 1991 U.S. dollars worth of damage, cut off electrical power to almost one million customers, killed one person, and injured about a dozen others.
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Scientific data and other information concerning July 7-8, 1991 derecho in Ontario were provided by Environment Canada meteorologists Phil Chadwick and Mike Leduc.
References....Storm Data for July 1991.
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