On August 1st, 2000 from Beulah, WY to Rapid City, SD an intense downburst spawned ferocious winds. Our office went on a storm damage survey on August 2nd, 2000 and found evidence of straight line winds estimated at 90 to 110 MPH. Tornado evidence was not found.
During the evening of 1 August, 2000, a powerful thunderstorm moved through northeast Wyoming and into western South Dakota. This system accounted for several reports of large hail and severe winds as it moved through extreme northeast Wyoming, and a swath of damaging winds as it moved toward the southeast through western South Dakota. A particularly devastating area of winds occurred in the Spearfish area. This overview addresses the severe nature of this event, and the factors which led the forecasters at the National Weather Service office in Rapid City to conclude that strong 'downburst' winds were accountable for most of the observed damage.
A loop of radar reflectivity data from the August 1 storm shows the event well. Around 9:00 PM MDT, the radar indicated a line of storms roughly parallel to the Wyoming/Montana border. This line quickly intensified as it moved toward the southeast, perhaps even attaining supercellular structure as it moved toward South Dakota. As the storm crossed the Wyoming/South Dakota border and approached the Spearfish area, a significant outflow boundary (gust front) became visible 5-10 miles ahead of the heaviest precipitation. When this feature moved through the Spearfish area around 10:30 PM MDT, winds estimated at 90-110+ blew through the area causing a considerable amount of damage and several injuries.
Soon after the storm passed Spearfish, the downbursts which caused the visible outflow boundary and severe winds in Spearfish evolved the storm into a bow echo - a storm with a curved, or bowing shape associated with severe winds. This bow echo moved across the Sturgis area with winds estimated at 65 - 80 mph, toppling and blowing away many merchandise tents which had been set up for the Sturgis Rally.
As the bow echo dissipated and moved toward the southeast, strong outflow from the storm continued to produce winds in excess of 70 mph in the Black Hawk, Piedmont, Rapid City, and Ellsworth AFB areas. As the storm continued to dissipate and move to the southeast of Rapid City, a broader, less intense swath of winds continued to the Nebraska border. Peak winds of 50 - 60 mph were observed in this area. A very interesting aspect of this storm was the very strong winds lasted for a very long time in most areas - over 30 minutes in some locations.
A map illustrating the breadth of the severe winds from this event is shown below.
The day after the storm, the NWS in Rapid City visited the area to perform a storm damage survey and assess the nature of the event which caused the damage. After spending almost 11 hours in the area, talking to numerous people, and taking a considerable number of measurements, it was concluded that most of the observed damage was caused by severe downburst (straight-line) winds. A map of the measured direction of the wind damage illustrates that most all the damage was produced by winds from the northwest. Areas of the most severe winds illustrated a divergent signature, that is the damage was blown away from some point. Tornadic damage illustrates very different characteristics. As opposed to a divergent signature, tornadic damage frequently exhibits convergent damage (like was observed with the Carter, SD tornado earlier this year).
If this storm exhibits anything, it is the fact that you don't need a tornado to cause tornadic-like damage which is capable of severe injuries, or even death. Downburst-producing wind storms are not uncommon in Western South Dakota and Northeast Wyoming - what made this event unusual was the fact that it struck a very populated area.