Wildfire and Damaging Winds in Kansas, December 15 to 16, 2021

A historic onslaught of swiftly-moving severe thunderstorms followed by several hours of intense/damaging non-convective winds that stirred up clouds of blowing dust and sparked a few large wildfires unfolded within this six-county North Central Kansas area on this Wednesday afternoon-evening. The combination of blowing dust and wildfire smoke created near-apocalyptic scenes within parts of the area during the afternoon, firmly cementing this event in the history books. Above all else, the wildfires were the biggest story of the day locally, as three fairly large fires (and a few smaller ones) impacted primarily southern portions of Rooks/Osborne counties, cumulatively burning approximately 19,300 acres/30 square miles and sadly contributing to one fatality. The largest fire in southwestern Osborne County actually represented the northern fringes of the so-called Four Counties Fire that primarily impacted portions of Ellis/Russell counties just outside the local area (this fire as a whole consumed over 121,000 acres/190 square miles). Turning to the convection that preceded the big blazes, although North Central KS did not endure the blitz of quasi-linear convective system (QLCS) tornadoes that states to the north-northeast did, the local area nonetheless served as the initiation zone of what was ultimately classified a serial derecho given that a nearly continuous swath of damaging winds extended hundreds of miles northeastward into southeast MN/western WI...the first December derecho on record in the United States. Within this six-county area, peak thunderstorm wind gusts registered up to 80 MPH and the more impactful and longer lasting non-thunderstorm gusts were commonly 65-80 MPH, with unofficial mesonets clocking incredible 95+ MPH gusts in a few spots. Fortunately, there were no reported injuries from the fierce winds, despite countless instances of damage primarily to trees, outbuildings and power lines/poles. Aside from the wildfires, the primary (and more widespread) issue caused by the relentless non-convective winds was a transient wall of dense blowing dust (mainly emanating from the eastern CO/western KS area), with ground-truth reports from observers near Damar and also Phillipsburg confirming visibility down to one-quarter mile or less for up to a few hours.

Breaking down event evolution/timing for the local area, first of all, even before convection flared up it was already an unusual/noteworthy December weather day given unseasonable warmth/humidity being pumped into the area on strong south winds. By late morning-early afternoon, southerly gusts were commonly 45-55 MPH as temperatures soared to near-record/record levels in the low-70s F, accompanied by dewpoints surging at least briefly into the 50s F. Early in the afternoon a few local mesonet stations even recorded marginally-severe gusts around 60 MPH from the pre-convective southerlies. This set the stage for a quick round of severe wind-producing convection, as a small line of showers and initially-weak thunderstorms that first got going in northwest KS rapidly intensified and expanded in north-south extent as it entered the local area into Phillips/Rooks counties between 12:30-1 p.m. Over the next couple hours, this severe squall line raced across the remainder of the six-county area at roughly 60 MPH, already clearing eastern Jewell/Mitchell counties by 2:30 p.m. Along the way, area mesonets clocked several gusts into the 60-75 MPH range, with isolated 80 MPH gusts noted in southern Phillips and southeastern Rooks counties. Specific damage reports were limited but included: tree damage in Mankato and Downs; a large tree blown down and roof damage to outbuilding in far southeastern Osborne County. There were no local reports of severe-criteria hail.

Unfortunately, even with the convective line departing stage-right from the area, the worst weather of the day was yet to come as incredibly-strong (and generally westerly) non-convective winds surged across North Central KS behind a composite dryline/cold front, peaking in intensity primarily 2-7 p.m. During this time sustained speeds commonly averaged 35-50 MPH with frequent gusts of 65-80 MPH, with unofficial peak mesonet gusts highlighted by an incredible 97 MPH in far northern Smith County and 95 MPH in southeastern Rooks County. Although these remarkable winds undoubtedly caused considerable minor damage and/or exacerbated damage already done by earlier convection, specific reports were actually quite limited beyond the aforementioned significantly-reduced visibility from blowing dust. No matter, the majority of attention quickly turned to the overall-most-significant events of the day in North Central KS...the rapidly spreading wildfires in Rooks/Osborne counties. Although fire ignition was largely attributed to downed power lines, and rapid fire spread was clearly driven by the phenomenal winds, area vegetation was also riper-than-usual for extreme fire growth thanks to a multiple-week stretch of very dry conditions. In fact, between Nov 1-Dec 15, the majority of the six-county area measured less than 0.15 of precipitation, making grasses/pastures very fire prone. Although more details regarding these Osborne/Rooks County wildfires can be found in their respective event narratives, several aspects of this event...including that they burned roughly 19,300 acres (cumulatively) within the two counties, killed hundreds of cattle, destroyed two homes and contributed to the death of an elderly man who inadvertently drove into the path of one of the fires...clearly cemented this as one of the worst wildfire events in North Central KS history. Although parts of the area continued to burn well into the night while blazes were gradually contained, thankfully west-northwest winds underwent a steady decrease through the evening, with severe-criteria gusts abating by around 7 p.m. and easing down to under 30 MPH by 9-10 p.m.

Examining the meteorology behind this event, this would have been considered a powerhouse setup for severe convection/high winds during the more typical spring season usually associated with this type of system, let alone in mid-December. In the mid-upper levels, not only was the parent, negatively-tilted shortwave trough notably intense as it slammed across the central United States, but it was also notably fast. At daybreak, its vorticity maximum was just passing by the Four Corners, but by mid-day it had already reached the NE/CO/KS border area and by sunset had advanced into far northeast NE. At the 500-millibar level, a stout jet streak of 100+ knots lifted directly over North Central KS from the south-southwest during the afternoon, while lower in the column a very potent southerly low level jet (evident at 850 millibars) ramped up to 40-60 knots before turning more westerly and intensifying to 60-80 knots. At the surface, a vigorous low pressure center tracked slightly north of the area across NE during the afternoon, deepening to around 984 millibars along the way and driving a sharp dryline/cold frontal combo eastward through the region. As mentioned, surface dewpoints climbed into the 50s F across much of the area prior to convective initiation, fostering a corridor of mixed-layer CAPE up to around 500 J/kg. Although only modest instability by warmer-season standards, in this case it was more than enough to promote severe convection in the presence of extreme wind fields. In the wake of the squall line, the ensuing and more notable non-convective wind event corresponded to a zone of deep subsidence (likely associated with a sting jet) and a pronounced surface pressure gradient on the backside of the departing low, with a nearly 10-millibar gradient transecting North Central KS from northeast-to-southwest at 5 p.m. (from around 990 mb at Hebron NE to around 1000 mb at Hill City KS).

Closing with some comments regarding official NWS forecasts leading up to this historic event, there was excellent awareness several days in advance regarding the likelihood of powerful non-convective winds both ahead of (southerlies) and behind (westerlies) the intense surface low pressure system, along with an unusually-concerning fire weather threat. A High Wind Watch was issued for North Central KS early on the 13th (48-60 hours in advance) and was subsequently upgraded to a High Wind Warning early on the 14th. Meanwhile, a Fire Weather Watch was issued early on the 14th (36 hours in advance) and upgraded to a Red Flag Warning that afternoon. However, what was not as well anticipated (both at the local and national forecast level) was that the area would also face the threat of severe convection, as the fast-tracking squall line ended up forming slightly earlier and several counties farther west-southwest than most model data suggested (especially 12+ hours out). As a result, there was a late ramp-up of convective threat categories within North Central KS on Dec. 15th SPC Day 1 Outlooks, progressing from the initial 06Z outlook that kept any official severe risk altogether east of the local area, to the 1630Z outlook that at least shifted a sliver of Marginal (level 1) and Slight (level 2) categories westward into parts of (albeit not enough of) the area.