June 2012 North American derecho
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| Composite radar image as the storm moved from Indiana to Virginia | |
| Date(s) | June 29–30, 2012 |
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| Duration | ~18 hours (10:00 AM-4:00 AM) |
| Track length | ~800 Miles |
| Peak wind gust | 91 mph (Fort Wayne, Indiana) |
| Largest hail | 2.75 Inches (Bismarck, Illinois) |
| Tornadoes caused | None confirmed |
| Fatalities | 22 total |
| Damage | $2.9 billion |
| Areas affected | United States Midwest, United States Mid-Atlantic |
| Part of the Summer heat wave of 2012 derecho series | |
The June 2012 Mid-Atlantic and Midwest derecho was one of the most destructive and deadly fast-moving severe thunderstorm complexes in North American history. The progressive derecho tracked across a large section of the Midwestern United States and across the central Appalachians into the mid-Atlantic states on the afternoon and evening of June 29, 2012, and into the early morning of June 30, 2012. It resulted in a total of 22 deaths, millions of power outages across the entire affected region, and a damage total of US $2.9 billion which exceeded that of all but the top 25 Atlantic tropical cyclones. The storm prompted the issuance of four separate severe thunderstorm watches by the Storm Prediction Center. A second storm in the late afternoon caused another watch to be issued across Iowa and Illinois.
The storm started as a small thunderstorm cell in central Iowa and continued into Illinois, developing into a mesoscale convective system (MCS) while mostly below severe limits at the time. Due to an extremely hot and highly unstable atmosphere with CAPE values in excess of 5,000 J/kg and temperatures on the south side of a stationary front well in excess of 100°F (37.8°C) prompting widespread excessive heat warnings, the storm complex strengthened and produced isolated instances of severe weather as it crossed over the Chicago metropolitan area late in the morning of June 29 for the Chicago area and much of northern Indiana by early afternoon. The first severe thunderstorm watch of the afternoon was issued at 11:50 a.m. CDT (1650 UTC) just before the storm crossed over Chicago.
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