The December 15, 2021 Derecho & Tornado Outbreak

The December 15, 2021 severe weather outbreak was the first recorded derecho in the month of December in the United States. A derecho is a widespread, long lived line of severe storms that mostly occur during the warm, summer months and are most commonly seen in the Central Plains and Midwest. There had never been a derecho recorded in the U.S. in December before this event. It was a historic outbreak and numerous records were set. Record warmth, rapid snow melt, dense fog, tornadoes, and widespread strong, damaging winds occurred. During the span of the event, there were 94 reports of tornadoes, 564 damaging wind reports, and 22 reports for hail. Along with that, there were 64 reports of winds greater than 65 knots (75 mph), and 4 reports of hail greater than 2 inches. Over 600,000 people lost power and an estimated 1.8 billion dollars in damages occurred as a result of this storm.

The National Weather Service La Crosse local area saw over 18 reports of 70+ mph wind gusts and 8 reports of 80+ mph measured gusts. The Rochester, MN airport reported its second-highest wind gust on record at 77 mph. NWS La Crosse also surveyed 27 tornadoes as a result of this event. The greatest concentration of damage generally occurred north and west of La Crosse, WI. The strongest tornadoes (4 EF2s) occurred in north-central WI, where snow depths ranged from 4 to 6 inches the day prior. Record setting warm temperatures helped provide the fuel needed for this unprecedented storm to occur in December.

A strong 300 mb trough was present heading into the Rocky Mountains around 1200 UTC (6 am CST) on December 15th. Over time, this trough strengthened and became negatively tilted by 00 UTC (6 pm CST) on December 16th. Negatively tilted troughs are usually associated with more severe weather outbreaks because they increase instability - the fuel for thunderstorms. This is because warm air is brought from the south underneath colder air aloft, resulting in steeper lapse rates. Additionally, there is usually a large wind shift from the surface to upper levels in the atmosphere, called wind shear, which aids in the formation of severe weather.

As a significant band of 8 to 12 inches of snow was winding down on December 11, preparation for the December 15 storm began to increase. Well above-average moisture and warmth was first predicted on the afternoon of December 10. This was followed by an increase in forecast wind gusts for December 15th. By December 11, signals of winds over 55 mph were seen in some forecast models. Forecast confidence continued to increase in higher wind gust potential and a High Wind Watch was issued for December 15 at 4 am on December 13 (image on right). The potential for severe thunderstorms was still in question, with instability and the cooling effect of a deeper snowpack on temperatures expected to be limiting factors. The image to the right shows messaging about the expected severe weather from National Weather Service La Crosse issued on December 13.

Record Heat, Rapid Snow Melt, and Early-Morning Fog: All-time record high temperatures for the month of December were set at almost 50 different sites, with many topping out in the middle 60s to lower 70s. These record high temperatures were 35 to 40 degrees above the climatological normals for the day in the local area.