The following is a series of pictures taken from Wonderland Homes just southeast of Piedmont. They show the "shelf cloud" formation that oftentimes indicates that strong winds are occurring with the thunderstorm.
What is a downburst? A downburst is an outrush of air on or near the ground the originates from a parent cloud or thunderstorm. Wind from these events can be "tornado-like" in their damage. They produce damage paths that lay the debris end-to-end, all pointing in the general same direction. We sometimes call these winds straight-line winds. Downbursts occur when a pocket or pouch of air above the surface of the earth is too heavy to be held up in the thunderstorm. All of sudden the rain, cooler air, and cloud material violently descends toward the ground and can cause significant damage.
The first picture was taken at 7:53 PM, and the last picture taken at 8:01 PM just as the forecaster headed into the house to "seek shelter". Winds were estimated up to 70 mph at 8:05 PM as the gust front passed through the area, with numerous trees having limbs snapped off and a few trees completely blown down. Some hail up to 1" in diameter was reported 9 miles east of Piedmont, but otherwise the hail was 1/2" or less. The worst of the storm hit Ellsworth Air Force Base at 8:18 PM where they had a gust to 89 mph. Between that time and 8:30 PM, the wind never decreased below 50 mph at the base, and in fact the gusts got even higher. There are two wind sensors on the 2.5 mile long runway, one on the north end and one on the south end. The north end sensor measured a gust of 129 mph at 8:25 PM, and the south end sensor measured a gust to 165 mph! Surveys done by Air Force officials after the storm showed that the damage was minimal, luckily, due to its rural location. There were several trees around the base blown over, and some minor damage to the areas near the runway where vendors were setting up for the Annual Dakota Thunder Air Show which occurred the next morning.
Warnings were issued ahead of this event for Western and Southern Meade County. There was also a warning issued for North Central Pennington county, but the winds in Rapid City never went over 45 MPH. Further west of Piedmont, at Tilford, the winds were only gusting to around 50 to 55 mph, and therefore the storm increased in intensity as it moved eastward, delivering its most powerful punch over the Air Force Base. No strong winds or hail reports were received after the storm passed over the base.
Above picture: Taken at 753 pm looking south-southwest. The interstate is at the bottom of the valley.
Above picture: Taken at 756 pm. Same view to the south-southwest.
Above picture: Taken at 800 pm. View is to the south. Notice the "wedge shape" from the bottom of the cloud. This is not a funnel cloud. There was no rotation with this portion of the cloud.
Above picture: Taken at 800 pm also. Just a zoomed in view, showing the base of the cloud lowering towards the ground.
Above Picture: Taken at 800 pm. Shows the ominous dark clouds that preceded the strong gust of wind which blew over several trees and damaged more then one garden in the neighborhood.
Above picture: Taken at 801 pm looking to the south. The "finger" that showed clearly just a minute ago is dissipating and being pulled into the updraft of the thunderstorm. This same storm continued to progress to the east, eventually hitting Ellsworth AFB about 15 minutes later.