The Dynamics and Predictability of a Mesoscale Windstorm and Wildfire Outbreak in the Central Great Plains on 15 December 2021

Extratropical cyclones can produce damaging windstorms that result in significant socioeconomic impacts. These windstorms have been documented extensively in Europe associated with landfalling oceanic extratropical cyclones, but they have also occurred along the west coast of North America and other regions across the globe. The strongest winds typically occur in a mesoscale region near the end of the bent-back warm front on the south side of the low center. These winds are associated with the cold conveyor belt airstream, or in some cases, a descending airstream termed the sting jet. Most sting jets occur with oceanic cyclones that follow the Shapiro-Keyser warm seclusion cyclone conceptual model. One such cyclone occurred in an unusual location – the central Great Plains – and produced simultaneous high-impact weather events on 15 December 2021. A derecho along the cold front produced widespread significant wind damage from central Kansas to Wisconsin. Concurrently, an outbreak of wildfires occurred in west-central Kansas along a mesoscale corridor of intense near-surface winds on the southern flank of the surface cyclone center. Pre-existing drought conditions throughout western and central Kansas, low 2-m dewpoint temperatures < 5°F, and near-surface wind gusts over 35 m s^-1, all contributed to rapid growth of several wildfires that were ignited by power lines toppled by the strong winds. These wildfires burned over 160,000 acres, killed hundreds of cattle, and resulted in 2 fatalities.

The aim of this presentation is to (1) diagnose the processes that resulted in the mesoscale area of strong winds in the wildfire region in Kansas and (2) determine whether these strong winds were associated with a cold conveyor belt airstream and/or a sting jet. Observations and a convection-allowing WRF-ARW simulation will be used to address these science goals. Detailed surface and radar analyses showed the extratropical cyclone rapidly deepened to 983 hPa as it moved from the Colorado high plains to central Nebraska between 1200–2100 UTC. By 2100 UTC, the frontal structure resembled a Shapiro-Keyser warm seclusion cyclone, with the bent-back warm front wrapping around the west side of the cyclone center and reaching the wildfire region in west-central Kansas. East of the cyclone center, the warm front extended northeastward into Iowa. The warm and cold fronts were oriented as a classic T-bone shape in the Shapiro-Keyser cyclone model, with the cold front extending from the warm front in eastern Nebraska southwestward to Texas. The derecho was located along the cold front from Nebraska into Kansas. An additional region of strong winds was located in the wildfire region in an area of light precipitation near the tip of the bent-back warm front. By 2200 UTC, this area of precipitation rapidly dissipated, consistent with a rapidly descending airstream characteristic of a sting jet. A Lagrangian analysis confirmed the presence of a rapidly descending airstream by revealing that the mesoscale windstorm was connected to the dry slot and sting jet airstreams that accelerated as they descended into the higher pressure gradient near the tip of the bent-back front. Finally, short-term prediction of the mesoscale windstorm using the convection-allowing NSSL Warn-on-Forecast System (WoFS) ensemble showed a high probability of damaging winds in western Kansas on the southwest side of the extratropical cyclone, but with significant uncertainty in the timing and areal coverage of the strongest winds. Initial results suggest that spatial and temporal variability in the consolidation of terrain-induced vorticity banners along the bent-back front may have modulated the occurrence of the mesoscale windstorm in the WoFS guidance.