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Overview
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Severe thunderstorms marched across northeastern Wyoming and the Black Hills area the afternoon and evening of July 17, 2019. Tennis ball to baseball size hail was reported around Gillette and the northern Black Hills, and a tornado occurred near Four Corners, Wyoming. Higher concentrations of cloud-to-ground lightning flashes can be seen in these areas in the image to the right. |
.png) 24-hour cloud-to-ground lightning from 6 am July 17 to 6 am July 18, 2019 |
Tornado:
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Tornado - Four Corners
Track Map   |
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The Enhanced Fujita (EF) Scale classifies tornadoes into the following categories:
| EF0 Weak 65-85 mph |
EF1 Moderate 86-110 mph |
EF2 Significant 111-135 mph |
EF3 Severe 136-165 mph |
EF4 Extreme 166-200 mph |
EF5 Catastrophic 200+ mph |
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Satellite Loop
The Rapid City NWS radar was not working on Wednesday, July 17, 2019, because of a lightning strike the night before. Therefore, we relied heavily on GOES-17 1-min satellite and lightning data. Below is a visible animation (10-min interval to reduce file size) showing the developing storms in northeastern Wyoming that eventually moved across western South Dakota. A longer, infrared satellite animation is below that to illustrate the cold cloud-top temperatures associated with the strongest storms.
| Visible satellite loop from 2 pm to 7:45 pm MDT on July 17, 2019. Red plus signs represent positive cloud-to-ground lightning flashes (cyan is for negative flashes). |
| Infrared satellite loop from 2:15 pm to 10:30 pm MDT on July 17, 2019. Red plus signs represent positive cloud-to-ground lightning flashes (black is for negative flashes). |
Photos & Video
Below are some photos of large hail that fell in the Gillette area (first row), along with damage to a vehicle from the Gillette Police Department (second row). The last picture at the bottom is a screen capture from a video of the tornado west of Four Corners, Wyoming.
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| Hail estimated at golf ball size from Legacy Parkway & Stafford Bend in Gillette (photo courtesy Geno Palarazzi) |
Hail estimated at ping pong ball size from downtown Gillette (photo courtesy City of Gillette) |
Hail estimated at golf ball size from the north side of Gillette (photo courtesy Daren Wenzel) |
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| Vehicle damage to police car (photo courtesy Gillette PD) |
Vehicle damage to police car (photo courtesy Gillette PD) |
Vehicle damage to police car (photo courtesy Gillette PD) |
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| Screen capture from video of tornado west of Four Corners, Wyoming. (photo courtesy Ryan & Jess Lorenz) |
Storm Reports
Here are the preliminary severe thunderstorm storm reports for July 17, 2019 (link to reports).
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| Preliminary storm reports for July 17-18, 2019 (green H's represent hail; blue W's represent wind gusts; and red T's represent tornado reports). |
The storm reports listed below are in descending order and may not necessarily be the final storm reports. Hail reports listed by size (inches)
SIZE LOCATION ST COUNTY TIME ------ ----------------------- -- -------------- ------- 3.00 9 W DWTN SPEARFISH SD LAWRENCE 0615 PM 2.50 TILFORD SD MEADE 0710 PM 2.50 2 S BEULAH WY CROOK 0559 PM 2.25 3 SSE DWTN GILLETTE WY CAMPBELL 0510 PM 2.00 DWTN GILLETTE WY CAMPBELL 0515 PM 2.00 11 NNW DWTN GILLETTE WY CAMPBELL 0434 PM 1.75 6 SE DWTN GILLETTE WY CAMPBELL 0528 PM 1.75 1 ESE DWTN GILLETTE WY CAMPBELL 0515 PM 1.75 1 N DWTN GILLETTE WY CAMPBELL 0456 PM 1.50 1 S DWTN GILLETTE WY CAMPBELL 0502 PM 1.25 9 W DWTN SPEARFISH SD LAWRENCE 0600 PM 1.00 4 NNW TILFORD SD MEADE 0702 PM 1.00 2 N FOUR CORNERS WY WESTON 0653 PM 1.00 3 ESE DWTN SPEARFISH SD LAWRENCE 0630 PM 1.00 11 SSE WESTON WY CAMPBELL 0456 PM 0.75 MOORCROFT WY CROOK 0545 PM 0.70 4 SE PINE HAVEN WY CROOK 0613 PM
Tstm wind reports listed by speed (mph) SPEED LOCATION ST COUNTY TIME ------ ----------------------- -- -------------- ------- 65.00 2 ENE WHITE OWL SD MEADE 0810 PM 60.00 1 SSE STURGIS SD MEADE 0700 PM 57.00 2 SW BRIDGER SD ZIEBACH 0836 PM 55.00 MOORCROFT WY CROOK 0545 PM 52.00 4 E DWTN SPEARFISH SD LAWRENCE 0635 PM
DAMAGE 3 ENE DWTN GILLETTE WY CAMPBELL 0500 PM
POWER POLE SNAPPED AT THE BASE FROM STRONG WINDS. ALSO
HAD SEVERAL REPORTS OF TENNIS BALL SIZED HAIL THROUGHOUT
NORTHEASTERN GILLETTE THAT CAUSED EXTENSIVE DAMAGE.
Environment
The atmosphere was very unstable over northeastern Wyoming and western South Dakota on Wednesday, July 17th, 2019, per the information obtained from the weather balloon released in East Rapid City at 6 pm MDT (left image). The vertical wind shear (right image) was rather strong, which favored severe storms with especially large hail, but also damaging winds.
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| July 17, 2019, 6 pm upper-air sounding for Rapid City (00z on the 18th) | July 17, 2019, 6 pm upper-air hodograph for Rapid City (00z on the 18th) |
A moist and unstable air mass was in place with southeast winds bringing humid air into the region, per the images below. Strong winds well above the ground created more than sufficient vertical wind shear that is needed to sustain severe thunderstorms.
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| SPC surface map valid 4 pm MDT 17 July 2019 | SPC mixed-layer CAPE (a measure of buoyancy) valid 4 pm MDT 17 July 2019 | SPC vertical wind shear from the surface to 6 km above ground (about 18,000 ft) valid 4 pm MDT 17 July 2019 |
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