Between December 15th and 17th of 2021, a squall line of severe thunderstorms produced over forty-five confirmed tornadoes across the Great Plains and Midwest. Most deaths related to this outbreak occurred in the state of Kansas, where residents in affected areas also received a powerful dust storm and damaging fires. Depicted below is the squall line over Kansas and part of southern Nebraska that later strengthened over the Midwest.
[Figure 1. A squall line of dry severe thunderstorms that later strengthened over the Midwest shown on GR2Analyst. (Source: AWS & GR2Analyst)]
This unprecedented and historic event was very unusual for this time of the year. Dust storms will typically brew when unsettled dust or dirt meets with an outflow boundary (cooler surface winds produced by the downdraft of a severe thunderstorm). These will usually occur in dry, arid climates because there are large amounts of dry soil and sand. In North America, dust storms may occur in the summer if the topography allows for it. In this case, not only was there loose dirt, but the unseasonable thunderstorms were also dry thunderstorms, producing only 0.19’’ of rain. The warm base at the bottom of these clouds may have been warm enough to evaporate the rain before it could reach the surface. In the Dust RGB image, the dust is depicted as magenta or violet while the mid-level clouds appear as a moss green. The dust storm caused power outages and near-zero visibility.
[Figure 2. The outflow boundary forms from the downdraft of a severe thunderstorm and brings a swath of cooler surface winds, which are strong enough to move dust and dirt at high speeds. (Source: University Corporation for Atmospheric Research)]
[Figure 3. This Dust RGB satellite image shows dust blowing across western Kansas. The dry air is to the left of the comma-shaped cloud while the moist air at the lower levels is to the right and appears blue. (Source: NWS)]
Combined with lightning, parts of Kansas, Colorado, and Nebraska experienced scattered fires after the dust storm. According to the National Weather Service, primary fire warning criteria requires that the relative humidity be less than 15% with 25 mph sustained wind speeds for at least three hours in a 12 hour period; however, the relative humidity requirement does not need to be met if the area of interest experienced hot and dry conditions preceding the thunderstorm. Lightning from dry thunderstorms was enough to ignite fires, and with the high wind speeds in Kansas, the fires were enhanced, resulting in damaged pastures and multiple injuries.
[Figure 4. Depicted is the mid-level jet stream responsible for the fast winds aloft. These will commonly cause wildfires if there is a dry air mass near the surfac for the jet to spread. This setup, along with dry thunderstorms that produced lightning, was more than enough to cause a wildfire outbreak in south central Kansas. (Source: NWS)]
[Figure 5. This shortwave infrared satellite image shows the fires in western and central Kansas within a three-hour period. The highest brightness temperatures (black) are associated with higher brightness temperatures, which means that the object is absorbing radiation instead of reflecting it (white). For this reason, these spots are much warmer than the surrounding area, which means that the black spots are indicative of fire outbreaks. (Source: NWS)]