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Unprecedented Midwest wind storm caps 2021’s siege of extreme weather

Like many extreme weather events this year, its wrath was fueled by exceptionally warm temperatures

Utility poles lay in a field near Jefferson, Iowa, on Dec. 16 after a band of severe weather produced strong wind gusts and reports of tornadoes across much of the state the night before. (Bryon Houlgrave/Des Moines Register/AP)

It unleashed the most hurricane-force wind gusts of any storm since at least 2014. Tornadoes carved paths in places they never had before in a December, like western Iowa and Minnesota. Fires erupted. Walls of dust turned day to night.

Wednesday’s wind storm, which blasted areas from New Mexico to Michigan, was the second historic extreme weather event to wallop the Lower 48 states this week, among an onslaught this year.

2021′s various extreme weather events have featured record-crushing heat, wind, fire, snow, tornadoes and deluges. This one had it all, if you consider this storm had its roots in an atmospheric river that dumped flooding rain and massive amounts of snow in California a few days ago.

In particular, this storm stands out for the record-setting warmth that fueled it, the number of reports of destructive winds it generated and for spreading severe thunderstorms, including tornadoes, farther north than previously observed in December.

Cold, heat, fires, hurricanes and tornadoes: The year in weather disaster

Headlined by a violent complex of thunderstorms, known as a derecho, it unleashed damage in seven states along a 665 mile path. It charged ahead at breakneck speeds averaging over 60 mph; individual thunderstorms progressed as fast as 110 mph.

The National Weather Service received reports of 23 tornadoes and over 500 instances of severe winds, mostly generated by the derecho. But even before the derecho formed, ferocious winds over 90 mph blasted the Rockies and western Plains. Several gusts over 100 mph were clocked in Colorado. Gusts over 60 mph affected 16 states.

The winds sheared off roofs, overturned vehicles and toppled trees. More than 600,000 customers in the Central United States were without power on Thursday morning in the storm’s wake. The winds also triggered blinding dust storms in southeast Colorado and western Kansas and fanned fires in the panhandles of Texas and Oklahoma and central Kansas. Remarkably, the storm has been blamed for only one death so far.

Storms, extreme winds from Colorado to Michigan leave more than 600,000 without power

The wind storm came just five days after the devastating tornado outbreak in the Mid-South on Friday night, the deadliest on record during December. Between the two events, few areas of the Central United States have been left untouched amid 222 tornado warnings and 357 severe thunderstorm warnings issued by the National Weather Service.

“[T]his single past week is unprecedented in U.S. meteorological history for most of the central U.S.,” wrote Christopher Burt, an expert on weather extremes, in an email.

Iowa and Minnesota saw their first December tornadoes while high winds slammed Colorado, with some gusts reaching 107mph, on Dec. 15. (Video: The Washington Post)

Both of the week’s extreme weather events like many this year were fueled by record-setting warmth, intensified by human-caused climate change. And both events were also powered by a very strong winter jet stream diving into the central states, driving cold air south to clash with that warmth and incite the outburst of violent storms.

While Friday night’s tornado outbreak was far more deadly and destructive, concentrating its wrath mostly over five states (Arkansas, Tennessee, Kentucky, Illinois and Missouri), Wednesday’s wind storm was in some ways more exceptional meteorologically. It affected far more people — nearly 100 million Americans were under weather alerts. And its list of weather records and meteorological firsts is seemingly endless.

The record-breaking tornadoes that swept the United States, by the numbers

The exceptional nature of the wind storm, by the numbers

The National Weather Service Storm Prediction Center logged 61 reports of hurricane-force winds gusts (at least 75 mph), the most of any event since 2004 when it began tracking these numbers. The fact this occurred in December makes it even more remarkable; derechos are rare in wintertime.

Tornado watches and warnings were issued in parts of Iowa, South Dakota, Minnesota and Wisconsin where they are practically unheard of at this time of year.

Both southern Minnesota and western Wisconsin for the first time were under tornado watches in December.

More than 30 tornado warnings were issued in Iowa; before Wednesday, only two had ever previously been issued during the month. Minnesota and South Dakota also saw their first tornado warnings during a December.

In unprecedented circumstances, La Crosse, Wis., hovered at 70 degrees after 8 p.m. — in mid-December — with a tornado warning nearby. Eau Claire was under a severe thunderstorm warning with snow on the ground.

Several tornadoes touched down in central and western Iowa for the first time on record in December. The Weather Service confirmed a tornado touched in Minnesota for the first time in December, near Lewiston.

Wind gusts, in many instances, were off the charts. Here are some of the more impressive reports:

Abnormally high temperatures ahead of the derecho set scores of records, many not just for Dec. 15 but for the entire month. Both Wisconsin and Iowa saw their highest December temperatures on record. Iowa was as warm as 78 degrees (in Oskaloosa), breaking its previous mark by four degrees. Wisconsin was as warm as 72 degrees (in Boscobel), surpassing its previous record by two degrees.

Monthly record highs were beaten or tied in numerous cities, including: Columbia, Mo. (76 degrees), Des Moines (74), Kansas City, Mo. (74-tie), Cedar Rapids, Iowa (73), La Crosse, Wis. (74), Madison, Wis. (68), and Green Bay, Wis. (65).

Record highs for Dec. 15 were set in dozens of locations from New Mexico to Michigan. Temperatures in Iowa were up to 40 degrees above normal and some records were broken by more than 10 degrees. Dew points, an indicator of moisture and humidity, also rocketed to record levels in the state for the time of year.

How the wind storm unfolded

The wind storm that swept an enormous area across the North Central United States was composed of different phenomena, acting on different scales of atmospheric motion, that at times blended in such a way as to seem inseparable. This is best shown in the graphic below, a true-color satellite image that captures the counterclockwise swirl of a cyclone or low pressure system centered over Kansas and Nebraska in the late afternoon (red “L”).

Multiple air masses were being drawn into this storm from all directions: unseasonably warm, humid and unstable air from the Gulf of Mexico (the “warm sector”), cooler air from Canada, and a very dry air mass descending from high altitudes along the jet stream to the west. High temperature records; intense, widespread violent winds; and a derecho occupied the warm sector. The derecho was a fast moving arc of thunderstorms generating a narrow band of extreme wind gusts along the cold front (solid blue line).

The almost perfect arc of thunderstorms comprising the derecho is shown in the next image, as imaged by weather radar in the early evening. Note the lightning strikes recorded at the time of the radar scan. This arc remained continuous for several hours and hundreds of miles as it moved rapidly eastward within the cyclone’s warm sector. Some of the most extreme wind gusts of the day (many reports at or exceeding 75 mph) were generated by downdrafts within individual thunderstorm cells making up the arc.

The widespread winds were particularly intense just behind the storm’s cold front, in a broad swath that progressively extended from the Front Range of the Rockies, eastward into Illinois (image below).

There are multiple culprits behind such widespread winds. First, the storm was intensifying and generating a tightening pressure gradient, which is the change in pressure over distance which drives wind.

In the next image, the analyzed low pressure zone (red “L”) has dropped to 986 millibars at 5 p.m. Central time. Isobars, lines of constant pressure, are closely packed southwest of the low, over Kansas, where winds are particularly intense.

Second, this same analysis chart shows a series of red and blue contour lines superimposed on the pattern of isobars. The blue zone indicates the region where surface pressure is falling most rapidly (2 millibars per hour) and the large red zone indicates large pressure rise (5 millibars per hour). The total difference of 7 millibars per hour across eastern Kansas and Nebraska represents a powerful pressure surge across the cold front and derecho, accelerating the wind.

The derecho, pressure gradient and pressure surge all conspired in the region of several Midwestern states to generate a phenomenal, widespread wind storm — the extent and intensity of which has never been documented before at this time of year.

To be sure, this year’s La Niña event, which favors a strong jet stream diving into the central states to power such storms, has set the stage for both this event and Friday night’s tornado outbreak. But — in both cases — the record-setting warmth acted as the fuel, creating unprecedented storm environments for December. This warmth derives from Gulf of Mexico waters which have warmed by several degrees in the last century.

As climate change is intensifying these storm environments and increasing the likelihood that they’re record-breaking, we find it difficult to conclude that it is not playing some role in the extreme nature of these events.

A warming world could add more fuel to tornadoes, scientists say