"The I-94 Derecho"

Figure 1. Area affected by the July 19, 1983 derecho (outlined in blue). Curved black lines represent the locations of squall line (based on radar reflectivity imagery) at three-hourly intervals. The "black dots" denote personal injuries due to derecho winds; the flag symbols measured wind gusts, with the direction of the wind from the flags toward the lower end of the staffs. The maximum wind gusts (red numbers) are in mph.

Around dawn on the morning of Tuesday, July 19, 1983, well north of warm/stationary front over South Dakota (SD) and northern Iowa (IA), a bow echo moved out of northeast Montana and began producing damaging winds in northwest North Dakota (ND). The airport weather stations at Williston and Minot, North Dakota reported wind gusts to 58 and 75 mph, respectively (Figures 1 and 2). This would be the beginning of a noteworthy progressive derecho event that would move across the northern Great Plains and upper Mississippi Valley and reach the Chicago metropolitan area by late evening.

Figure 2. Hourly positions of the squall line associated with the "I-94 Derecho." Initially, the convective system appears as a single bow echo, but later it expands into a longer line with a few embedded bows. The line of storms that forms on the north edge of the squall line and perpendicular to it is a feature common to many progressive derecho bow echo systems and is called a "warm advection wing" (Smith 1990). Such features typically produce heavy rain rather than strong winds.

As with many significant derechos, the initiating convection --- in this case, in the northeast Montana / northwest North Dakota bow echo --- was elevated. By "elevated" we mean that the storms primarily were fed not by air in contact with the surface, but by a warm and moist layer located aloft, above the frontal surface associated with the South Dakota warm front (Figure 3). Despite being elevated above the relatively cool, stable air near the surface, the bow echo nevertheless continued to produce occasional damage as it moved east-southeast across North Dakota through the morning.

From Corfidi (2003)

Figure 3. Manual surface analysis for part of the north central United States and adjacent Canada, valid 7:00 a.m. CDT (1200 UTC) July 19, 1983. Conventional frontal symbols and station plots, with temperature and dewpoint (°F) to left of station circle, pressure (mb) to right. Wind direction and speed given by lines and barbs; full barb = 10 knots (1 knot = 1.15 mph). Isobar interval 2 mb. Thunderstorm cold pool "bubble high" ("B") associated with the incipient derecho-producing convective system appears in northwest North Dakota.

As the convective system's cold pool continued to deepen and elongate east-southeastward with the mean cloud-layer flow, it ultimately reached the warm front as that boundary advanced slowly north across eastern South Dakota and southern Minnesota (MN). This meeting occurred during the early afternoon (CDT) over west central Minnesota, and likely accounts for the appreciable increase in storm strength observed around that time as the convection became surface-based (Figure 2). At this time the storm system also expanded in scale, evolving into a squall line with two and sometimes three bow echo segments as it continued across Minnesota and later Wisconsin (WI), with Interstate 94 near its central axis.

Winds over 100 mph were recorded at the airport in Alexandria, Minnesota, Minnesota, where planes and hangers were damaged and destroyed. The storm continued to produce much damage as it moved east-southeast across south central and southeast Minnesota; approximately 250,000 customers lost electrical power in the Minneapolis/St. Paul area.

Trees were blown over and buildings damaged as the derecho raced through Wisconsin. In Madison, (red "M" in Figure 2), Dr. Robert Schlesinger, a research meteorologist at the University of Wisconsin, observed its approach from the southeastern shore of Lake Mendota. He photographed the laminar, layered "shelf cloud" and gust front approaching from the northwest (Figures 4 and 5). As the strong derecho winds came ashore, several piers were destroyed. On the University of Wisconsin campus (partially visible in Figure 5), several buildings were damaged and windows were blown out of the second and third stories of the Memorial Library. Tiles from the library roof were blown off and landed several blocks to the southeast. The strong winds produced waves roughly four feet high on Lake Mendota; even well after the strong winds had diminished, unusually large "whitecaps" continued on the agitated lake (Figure 6).

Thirty-four people were injured in Minnesota and Wisconsin from this storm. Of these injuries, 12 were from mobile homes being blown over, and eight were from falling trees.

Photo by Robert Schlesinger

Figure 4. Photograph taken just before sunset on Tuesday, July 19, 1983, looking west-northwest from the southeast shore of Lake Mendota (near the north end of Carroll Street) in Madison, Wisconsin. The laminar, layered shelf cloud (or arcus) associated with the derecho gust front is approaching rapidly from the northwest. Several sailboats are visible on the lake.

Photo by Robert Schlesinger

Figure 5. Photograph taken from the same location as Figure 4 and a few minutes after Figure 4, looking west as the derecho gust front and shelf cloud cross Lake Mendota toward the University of Wisconsin campus.

Photo by Robert Schlesinger

Figure 6. View of Lake Mendota looking northwest from near the same location as Figure 4 and approximately 45 minutes after the derecho gust front had passed. Note the large waves with white caps still present well after the strongest winds had passed.

The bow echo system and derecho began moving into northeast Illinois (IL) between 9 and 10 p.m. CDT. National Weather Service meteorologist Richard Koeneman experienced this derecho and described his experiences in a weather diary. He noted that the evening of July 19, 1983 was "warm and humid," and that the wind was "dead calm." From his home in Lindenhurst, Illinois (red "L" on Fig. 2 near the Wisconsin border north of Chicago), he had a good view of the approaching storm as there was a large lake behind his yard. He noted lightning visible on the horizon to the north and northwest of his home around 9:30 p.m.  At 9:40 p.m., with the wind being calm in his backyard, he could hear the "rushing" noise of strong winds approaching from the northwest. Three minutes later the gust front hit his house and within a minute or two he estimated that the wind was gusting to 70 mph. The house shuddered, and Richard was worried that it might start flying apart. The house lost electrical power and he noted that the temperature dropped from 83 °F at 9:30 p.m. to 68 °F by 9:50 p.m.  He said that it was the first time he had ever been able to hear the noise associated with an approaching gust front. In this case the gust front already had traveled hundreds of miles but was still quite intense as it passed Koeneman's home.

The derecho winds remained intense as they moved into the northwest side of the Chicago metropolitan area. A gust to 69 mph was recorded at O'Hare International Airport at 10:10 p.m. CDT. After this, the bow echo system began to weaken; the wind gusted to only 55 mph as the system passed Midway Airport on Chicago's south side. The derecho finally dissipated over northwest Indiana (IN) near midnight July 19-20th, after having traveled 1000 miles in 18 hours with an average speed of 56 mph.

Like some other noteworthy progressive derechos over the north central states, the July 19, 1983 event occurred on the northern fringe of a strong upper level high pressure area (Figure 7) associated with an expansive heat wave affecting the mid Mississipppi Valley and central Plains. Deep, seasonably strong west-northwest flow on the north side of the high carried an elevated mixed layer plume (see Derechos and Heat Waves) eastward from the Rockies to the upper Mississippi Valley. These winds also encouraged thunderstorm cold pools to elongate east-southeastward, allowing for very rapid development of new storms along the downwind-moving part of the gust front given the very warm and moist air present ahead of it.

Figure 7. 500 mb (approximately 18,000 foot) upper air analysis for part of the United States and adjacent Canada valid 7:00 am CDT (1200 UTC) July 19, 1983. Black lines are contours of the 500 mb pressure surface, labeled in feet (e.g., "19500" = 19,500 feet). Wind flow is parallel to the contours and depicted at radiosonde observation sites in the form of "barbs" and "flags;" full barb = 10 kts (1.15 mph); flag = 50 kts (58 mph). Thin dashed lines are isotherms (°C; e.g., -10 = minus 10 °C). High pressure area over Kansas was associated with an expansive heat wave that was gripping the southern half of the Plains and the mid-Mississippi Valley at the time of the derecho.


Additional information:
Corfidi 2003
Johns and Hirt, 1985
Storm Data, July 1983

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