Tornado:

Tornado - Four Corners Weston County Date July 17, 2019 Time (Local) 6:45 pm to 6:58 pm MDT EF Rating EF-2 Est. Peak Winds 115 mph Path Length 1.4 miles Max Width 100 yards Injuries/Deaths 0 Summary: The survey conducted by the NWS found tornado damage from 3 miles west of Four Corners, WY to 2 miles west-southwest of Four Corners, WY.  It snapped and uprooted numerous large pine trees, but caused no structural damage. The track will be adjusted as additional damage information is received.  Track Map

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

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.

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.

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)

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)

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).

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.

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.

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