July 21, 2018 Severe Thunderstorms and Flash Flooding over northeast Wyoming and western South Dakota

Wind Damage

Downed trees, east Rapid City (courtesy Rapid City Municipal Government)

Downed tree branch, Elks Golf Course (courtesy Teresa Komandt)

Flooding

Flash flooding at 3rd and St. Francis in Rapid City. Video courtesy Stacey Morris Roberts.

Flooding and High Water Photos

Rapid Creek, near downtown Rapid City (courtesy Rapid City Municipal Government)		Rapid City, near Indiana and 5th (courtesy Kim Stackenwalt)	Rapid City, near Meade St. (courtesy Heather Turnbaugh)

Rapid City, on St. Patrick (courtesy Jiggs Cressy)	North of Ellsworth AFB (courtesy Krista Fox)	North of I-90 on Dyess Ave (courtesy Lois Peterson)

Rapid Creek, near downtown Rapid City (courtesy Rapid City Municipal Government)		Hill City (courtesy Sarah Johnson)	Rapid Creek, near the airport (courtesy Kristen Kruse)

Green Valley (courtesy Leanne Olson)	The Elks Golf Course (courtesy Teresa Komandt)	North of I-90 on Dyess Ave (courtesy Lois Peterson)

Rapid Creek, near 5th and Omaha in Rapid City (courtesy Bob Julius)

Hydrographs

Rapid Creek above the water reclamation facility east of Rapid City	Grizzly Bear Creek near Keystone

Radar

Radar reflectivity (precipitation intensity) for 3 PM through 9 PM MDT on July 21, 2018.

Storm Reports

Hail reports listed by size (inches)

  SIZE   LOCATION                 ST  COUNTY           TIME
 ------  -----------------------  --  --------------   -------
  1.50   3 SW BROWNSVILLE         SD  LAWRENCE         0515 PM
  1.25   6 SSE DOWNTOWN GILLETTE  WY  CAMPBELL         0458 PM
  1.00   7 SW RAPID CITY AIRPORT  SD  PENNINGTON       0722 PM
  1.00   3 N SHERIDAN LAKE        SD  PENNINGTON       0640 PM
  1.00   6 SSE BROWNSVILLE        SD  LAWRENCE         0533 PM
  1.00   8 SE DOWNTOWN GILLETTE   WY  CAMPBELL         0514 PM
  1.00   6 SE DOWNTOWN GILLETTE   WY  CAMPBELL         0502 PM
  1.00   7 NNW WESTON             WY  CAMPBELL         0235 PM
  0.75   2 SW DOWNTOWN RAPID CIT  SD  PENNINGTON       0709 PM
  0.50   HISEGA                   SD  PENNINGTON       0642 PM
  0.50   17 ESE RENO JUNCTION     WY  CAMPBELL         0625 PM
  0.25   LEAD                     SD  LAWRENCE         0418 PM

Thunderstorm wind reports listed by speed (mph)
 

 SPEED   LOCATION                 ST  COUNTY           TIME
 ------  -----------------------  --  --------------   -------
 70.00   7 SW RAPID CITY AIRPORT  SD  PENNINGTON       0722 PM
 67.00   RAPID CITY AIRPORT       SD  PENNINGTON       0725 PM
 66.00   6 WNW DOWNTOWN NEWCASTL  WY  WESTON           0755 PM
 63.00   4 S DOWNTOWN RAPID CITY  SD  PENNINGTON       0657 PM
 60.00   3 NNE DOWNTOWN NEWCASTL  WY  WESTON           0755 PM
 55.00   1 E DOWNTOWN RAPID CITY  SD  PENNINGTON       0721 PM

Rain Reports

Radar estimated rainfall during the afternoon and evening of July 21, 2018.

 Rainfall amounts (inches)

 AMOUNT  LOCATION                 ST  COUNTY           TIME
 ------  -----------------------  --  --------------   -------
  2.53   5 NE ROCKERVILLE         SD  PENNINGTON       0700 AM
  2.37   HILL CITY                SD  PENNINGTON       0821 AM
  2.35   10 SSW DOWNTOWN RAPID C  SD  PENNINGTON       0739 PM
  2.29   4 NE ROCKERVILLE         SD  PENNINGTON       0742 AM
  2.28   5 W ELLSWORTH AFB        SD  MEADE            0700 AM
  2.13   4 SSW DOWNTOWN RAPID CI  SD  PENNINGTON       0735 PM
  2.07   1 ESE PACTOLA RESERVOIR  SD  PENNINGTON       0700 AM
  2.00   2 SSE DOWNTOWN RAPID CI  SD  PENNINGTON       0733 PM
  1.90   2 SW DOWNTOWN RAPID CIT  SD  PENNINGTON       0818 PM
  1.85   5 NW SWETT               SD  BENNETT          0700 AM
  1.81   3 NE HISEGA              SD  PENNINGTON       0700 AM
  1.80   11 S FOLSOM              SD  CUSTER           0804 AM
  1.65   5 ENE DOWNTOWN CUSTER    SD  CUSTER           0900 PM
  1.64   1 W HISEGA               SD  PENNINGTON       0807 AM
  1.60   4 W DOWNTOWN RAPID CITY  SD  PENNINGTON       0700 AM
  1.57   5 NW DOWNTOWN RAPID CIT  SD  PENNINGTON       0806 AM
  1.57   7 WSW FOLSOM             SD  CUSTER           0700 AM
  1.45   3 E HISEGA               SD  PENNINGTON       0808 AM
  1.43   2 SW DOWNTOWN RAPID CIT  SD  PENNINGTON       0737 PM
  1.37   1 NNW DOWNTOWN RAPID CI  SD  PENNINGTON       0810 AM
  1.26   1 E DOWNTOWN RAPID CITY  SD  PENNINGTON       0747 AM
  1.25   1 SSW HERMOSA            SD  CUSTER           0805 AM
  1.24   3 W RAPID VALLEY         SD  PENNINGTON       0808 AM
  1.24   2 NNW MOUNT RUSHMORE     SD  PENNINGTON       0805 AM
  1.17   1 NE DOWNTOWN RAPID CIT  SD  PENNINGTON       0700 AM
  1.16   6 W HERMOSA              SD  CUSTER           0700 AM
  1.12   4 WSW DOWNTOWN RAPID CI  SD  PENNINGTON       0811 AM

Environment

A sounding, or weather balloon with attached instrumentation, launched from the Rapid City National Weather Service Forecast Office (below) showed that precipitable water values (a measure of atmospheric moisture content) on the evening of July 21st were nearly 170% of normal for this time of year. Additionally, the sounding revealed extreme instability and strong vertical wind shear, key ingredients for severe weather.

Figure 1: Sounding launched from Rapid City at 5 PM on July 21, 2018.	Figure 2: Wind hodograph from the Rapid City sounding shown at left.

Accumulated precipitation this year shows that Rapid City has experienced a wetter than average spring and summer. This anomalously high precipitation led to saturated soils across the Black Hills and Rapid City, providing antecedent conditions favorable for flash flooding.

Figure 3: 2018 Precipitation to Date for Rapid City.