Corn Belt derecho

The Corn Belt derecho was a progressive derecho which affected a large area of the central United States on June 29, 1998. In the morning, thunderstorms, including a supercell, developed over South Dakota and tracked into central Iowa. As the thunderstorms reached central Iowa, a strong rear-inflow jet developed which caused the thunderstorm to take on a different characteristic, becoming a derecho. It traveled more than 600 miles in about ten hours, causing more than $125 million worth of widespread damage destruction, especially to crops, and was responsible for power outages to nearly a half a million people.

Meteorological synopsis
At 1200 UTC (7:00 am. CDT), a stationary front extended from South Dakota to Southern Michigan, bringing warm and humid air to its South. The temperature was around 25 C at this early hour and the dew point was at 23 C. At 850 mb, the southwesterly flow was maintaining this situation while at higher levels the flow was turning to the northwest, bringing drier and colder air. Daytime heating would increase the instability of the airmass and the CAPE was expected to reach a strong 3,689 J/kg. At the same time, there was relatively weak synoptic-scale forcing, the surface flow being a barometric col.

Along the front in South Dakota, an unorganized area of thunderstorms formed by 9:00 a.m. CDT. They rapidly organized and spread along the front, moving east-southeast into northeast Nebraska. By midday, the storms reached northwestern and north central Iowa, supercells among them, while forming a west–east band and assuming a bow echo shape.

In the early afternoon, a second area of thunderstorms formed west of Des Moines and merged with the original bow echo line which then accelerated east-southeast into Illinois by 4:00 p.m. The line evolved into a classic large scale bow echo, showing a "book end vortex" on its northern end, becoming a progressive derecho. Damage, especially to crops and trees, became continuous from the Iowa border into Indiana as most of the damage was produced by strong straight-line winds on the leading edge of the gust front. Some embedded supercells, showing smaller-scale vortices on radars, produced narrower corridors of more intense damage, with measured wind gusts up to at least 110 mph.

The derecho crossed central and southern Indiana during the early to mid evening while its highest wind gusts decreased somewhat compared with those observed earlier in the day. The system became a roughly west–east arc and turned more southward as it moved into Kentucky by late evening, dissipating gradually

Impact
By the end of the morning, the thunderstorms produced hail up to the size of hen's eggs and locally damaging wind in Nebraska. By mid-day, supercells along the bow echo in Iowa began to produce very strong winds, up to tennis ball-sized hail, and several mostly short-lived tornadoes. On Doppler weather radar, a large fast-moving mesocyclone associated of the track of a supercell was nearly in contact with the ground as it moved from southwest Boone County east-southeast across the northern and eastern parts of the Des Moines metro area.

Over the Davenport, Iowa NWS Weather Office area of responsibility, numerous reports of wind gusts ranging from 80 to 100 mph were received. The highest measured wind gust of 123 mph was reported in Washington, Iowa near coordinates 41.3°N, -91.7°W. This was the highest unofficial recorded wind gust in the history of the state of Iowa until the August 2020 Midwest derecho. At the same moment, the area of green in the radar display to the right shows the velocities toward the weather radar. The lighter shade, beneath the orange arrow, represented Doppler-estimated mean wind speeds in excess of 64 knot all along the gust front, and the yellow circle are mesocyclone detections.

In Illinois, railroad cars were toppled, steel power transmission towers were bent, and many buildings were seriously damaged during the afternoon. By the evening into Indiana, hundreds of trees were uprooted in the Bedford and Indianapolis areas, two semi-trailer trucks were blown off Interstate 65 near Columbus. By late evening, damage into Kentucky was minimal, mostly limited to toppled trees and several roofs blown off buildings and houses.

Along with the long-lived derecho, 20 tornadoes were reported, one of which was an F2 tornado, injuring 85 people in central Iowa. Over eight states, the derecho and associated tornadoes killed one person and injured 174.