Point of correction, we had derecho 2.0 in December of 2021. This would be derecho 3.0.
December 15, 2021 was an unprecedented and historic event for the state of Iowa. It featured the first derecho in December anywhere in the United States and the first
Moderate Risk (Level 4 or 5) of severe thunderstorms issued by the NWS Storm Prediction Center in December in Iowa. Officially, it set the new record for most tornadoes in Iowa and the most EF-2/F-2 or stronger tornadoes in a single day in Iowa since 1950. Finally, it will break the all-time December record high temperature for Iowa.
The main feature was a powerful low pressure that rapidly moved from the Colorado Rockies into Siouxland and eventually over western Lake Superior on December 15, 2021. Gusty, environmental winds preceded a line of severe thunderstorms, which produced numerous tornadoes and thunderstorm wind gusts across the state and region. There were 63 confirmed tornadoes that occurred in Iowa with 21 of those tornadoes being rated EF-2. Sixty three is officially the most tornadoes in Iowa in a single day with the previous being 35 tornadoes on August 31, 2014 (
2014 Iowa Tornadoes |
Journal Article). Further, 21 EF-2 tornadoes is officially the most EF-2/F-2 or stronger tornadoes in a single day with the previous being 16 tornadoes on June 7, 1984 (
Tornado Tracks |
NOAA Technical Memo). Prior to this event, a total of 5 tornadoes had occurred in Iowa in December since 1950 with all of them in southeastern Iowa. To have over 10 times that many tornadoes in a single day is unprecedented in any month, let alone the month of December!
Numerous straight line wind gusts were also associated with these thunderstorms, some in excess of 80 mph. During the event, nearly the entire state of Iowa was under a Severe Thunderstorm Warning at some point. Audubon reported the highest gust in Iowa at 88 mph. This storm also met the criteria for a “derecho'', which makes it the first derecho on record in the month of December anywhere in the United States. Finally, we had an unofficial gust of 108 mph recorded by a mesonet station near Sheffield, Iowa associated with one of the tornadic circulations.
After the line of storms had passed, numerous strong, non-thunderstorm wind gusts over 70 mph occurred overnight. The top three non-thunderstorm wind gusts were 83 mph in Decatur City, 81 mph in Marshalltown, and 80 mph in Johnston. The Des Moines International Airport recorded a 74 mph non-thunderstorm wind gust, which is the highest non-thunderstorm wind gust at this location since 1970.
In addition to the severe weather and strong non-thunderstorm winds, the temperatures were remarkable and record setting. Numerous record daily highs were broken by many degrees. In addition and preliminarily, the all-time December record high temperature for Iowa of 74 degrees that was set in Thurman, IA on December 6, 1939 was broken with four sites reaching 75 degrees (Oskaloosa, Muscatine, Iowa City, and Ottumwa).
