Significant damaging wind event strikes south central Nebraska during the late evening hours of July 9, 2021. This event began well west and northwest of the Hastings County Warning Area (CWA), within the Nebraska Panhandle, during the late afternoon and early evening hours as a small cluster of thunderstorms. These thunderstorms became severe at various points during their eastward track across the Nebraska Sandhills through the evening, but it wasn’t until they reached the North Platte and Broken Bow areas around 9PM CDT that they became significantly organized and severe. This activity quickly moved into W and NW portions of the CWA between 9-10PM CDT, with the strongest thunderstorms and associated wind gusts focused across Dawson County. Wind gusts of 65 MPH to nearly 80 MPH were common across the county, which resulted in widespread tree damage and numerous power outages. Activity quickly grew upscale in terms of coverage and intensity as it shifted SE, and by 10 PM CDT, there was a solid line of thunderstorms from Nance County to Buffalo County to northern Gosper County that would prove to become a prolific damaging wind producer over the next couple of hours, including for the three largest cities in the Hastings CWA - Kearney, Grand Island, and Hastings. Wind gusts of 60-80 MPH were either reported or estimated in all of these areas. Not surprisingly, impacts were fairly significant, especially in terms of the overall scope of tree damage and power outages, including snapped power poles and healthy trees that were snapped off near the ground. Some of the trees fell onto homes and vehicles. In Kearney, a roof was partially torn off a movie theater, which also led to extensive water damage due to the torrential rain. Conditions became treacherous along Interstate 80 as the strong wind gusts on the leading edge of the thunderstorms led to overturned semi-trucks, other trailers, and campers, as well as very limited visibility due to blowing dust. As the line of thunderstorms moved east of Highway 281, some localized corridors of particularly intense winds, observed or estimated to be as high as 80-95 MPH, developed across portions of eastern Hall County, southern Hamilton County, and northern Clay County between 10:30-11:30 PM CDT. The most intense wind occurred in a swath from around Central Nebraska Regional Airport, where an 84 MPH gust was measured, southeast towards areas near and south of Aurora, to near Giltner. Damage in these areas included: significant tree and power line damage, snapped power poles, flattened crops, overturned pivots, damaged or destroyed grain bins and outbuildings, and two blown over semi-trucks on Interstate 80. Some tree limbs fell on cars and buildings, and some minor roof and window damage occurred to a few businesses. Perhaps the most impressive damage was downed high-voltage transmission lines approximately 4 miles south of Aurora where they cross Interstate 80. This led to traffic issues in the area for a time. Similar tree, power line, crop, irrigation pivot, and grain bin/outbuilding damage was reported in portions of Clay County. The line of thunderstorms weakened slightly as they shifted E and SE towards the Highway 81 corridor between 11PM and midnight, though gusts of 65-75 MPH were still reported in York, Fillmore, and Thayer Counties. Most notable damage (other than tree and power line damage) were half dozen overturned pivots, flattened corn, and a tree falling on a car. Severe thunderstorms finally exited south central Nebraska by 1AM on July 10. When all was said and done, a nearly continuous swath of relatively concentrated wind damage 30 to 50 miles wide occurred from near North Platte, NE to areas just E/NE of Hebron, NE. Gov. Ricketts declared disaster declaration for 14 total counties in Nebraska, including Clay, Fillmore, Hall, Hamilton and York counties in our CWA, and it was ultimately approved by President Biden. Preliminary assessment by FEMA estimated over 30 million dollars in damages.
Meteorologically speaking, this complex of thunderstorms was not unexpected, though intensity forecasts were ramped up just hours before the onset. SPC outlooks increased from a Slight risk in the morning, to an Enhanced risk for the western half of the CWA in the 3PM CDT update. The Enhanced risk was expanded to include all of south central Nebraska with the evening update. Strong wind shear (effective shear 50-60kt) was a consistent feature of the forecast, with the primary uncertainty being low level moisture quality and how much instability there would be for storms to work with as they moved off of the High Plains. Ultimately, low level moisture, in particular surface dew points, were much higher than modeled. For example, surface dew points were well into the mid 70s across a large portion of central and south central Nebraska, vs. modeled dew points in the 60s. This discrepancy was likely due to strong evapotranspiration from irrigation and mature crops, as well as weak low level mixing. As such, instability was very strong with MUCAPE of 5000-7000 J/kg, and this continued well into the overnight hours. Downdraft CAPE was also high at around 1000-1300 J/kg. Not surprisingly, the Derecho Composite parameter peaked between 4-6, which was significant.