Flash Flooding and Flooding in South Dakota, July 21 to 22, 2018

Name: Flash Flooding and Flooding in South Dakota, July 21 to 22, 2018
Start: 2018-07-21T16:15
End: 2018-07-22T13:00
Location: South Dakota

July 21 to 22, 2018 · NWS UNR · 14 events in 2 counties and zones

A severe thunderstorm developed over the central Black Hills and tracked eastward through the Rapid City area before weakening over the plains. The storm initially contained large hail over the Black Hills, then also produced strong winds as it moved through the Rapid City area.

Where it happened

Every event with a place to put it. Tap one for its entry below.

Tornadoes
Tropical storms and surge
Floods
Wind
Hail
Winter weather
Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
Other

Open the live radar over this ground

The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 14

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS UNR wrote it up.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Rapid City, SD wrote its last area forecast discussion before the first event at Jul 21, 4:15 PM MST, 0 minutes earlier.

Area forecast discussion, Jul 21, 4:15 PM MST, 0 minutes before

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
515 PM MDT Sat Jul 21 2018

.DISCUSSION...(This Evening Through Saturday)
Issued at 125 PM MDT Sat Jul 21 2018

19z surface analysis had low over eastern MT with cold front into
northwest WY and boundary into eastern WY. Ahead of low,
sustained southeasterly winds have pushed dew points to 65-70F -
IA humidity levels continue. 0.5km 1-minute visible imagery shows
moist tongue well with well-defined cumulus field. Agitated
CU/TCU over far northwest SD and northeast WY. MLCAPE at 2-3KJ/kg+,
but MLCIN 50-150J/kg particularly on the SD plains. Hard to pick
out a distinct wave on the water vapour matinee, but it shows
copious amounts of monsoonal moisture over the CWA wrapping around
upper ridge centered over CO/NM. 0-6km bulk shear marginal for
organized storms now, but it slowly increases by evening to
15-20m/s as cap erodes. Expect at least isolated T+ coverage later
this afternoon with large hail possible for supercells and severe
wind gusts in other convection. Precipitable waters peak at
150-200% of normal with supercell motions over the Flash Flood
Watch areas as low as 5-10kts by evening, so torrential rain
possibility remains. Activity will shift east/northeast tonight
with lows near guidance.

Sunday, upper trough currently over southwest Canada zips across
south central Canada pushing cold front through the CWA as it
flattens upper level ridge. 850mb winds behind front 20-30kts with
modest cold air advection - windy spots will be seen. Much drier
air behind the front will suppress thunderstorms chances behind it
Sunday afternoon, but moderate instability/shear forecast ahead
of the front with marginal threat for large hail/damaging winds
east of the Black Hills. Temperatures will be near guidance. Enjoy
a nice respite Sunday night.

Next week, upper ridge remains flat as it slowly moves west. West-
northwest flow aloft will become northwest as a series of
disturbances moves through. Looks seasonal first part of the week
with cooler than normal temperatures for the second half.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 512 PM MDT Sat Jul 21 2018

Scattered thunderstorms will move across the are this evening,
with IFR conditions. Coverage of storms will diminish after 06Z.
Gusty northwest winds expected across far western South Dakota
plains behind a cold front. Thunderstorms will develop across
south central South Dakota Sunday afternoon.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Flash Flood Watch through late tonight for SDZ024>030-041-042-
     072-074.

WY...Flash Flood Watch through late tonight for WYZ057.

&&

DISCUSSION...Helgeson
AVIATION...7

Hazardous weather outlook, Jul 21, 5:56 AM MST, 10 hours 19 minutes before

Hazardous Weather Outlook
National Weather Service Rapid City SD
656 AM MDT Sat Jul 21 2018

SDZ024>030-041-042-072-074-WYZ057-221300-
Northern Black Hills-Northern Foot Hills-Rapid City-
Southern Foot Hills-Central Black Hills-Southern Black Hills-
Custer Co Plains-Fall River-Oglala Lakota-
Sturgis/Piedmont Foot Hills-Hermosa Foot Hills-
Wyoming Black Hills-
656 AM MDT Sat Jul 21 2018

This Hazardous Weather Outlook is for portions of northwestern
South Dakota...southwestern South Dakota...the Black Hills of
South Dakota...and the Black Hills of Wyoming.

.DAY ONE...Today and Tonight

Isolated thunderstorms are possible today, with scattered storms
expected this evening. Some thunderstorms could become severe.
Additionally, heavy rain on top of already saturated soils could
cause flash flooding.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

An active weather pattern will bring near daily chances for
showers and thunderstorms. Strong to severe storms will be
possible at times.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today.

&&

Stay tuned to NOAA weather radio for further updates...or check
our web site at weather.gov/rapidcity

SDZ001-002-012-013-031-043-044-073-WYZ054>056-058-071-221300-
Harding-Perkins-Butte-Northern Meade Co Plains-
Pennington Co Plains-Jackson-Bennett-Southern Meade Co Plains-
Northern Campbell-Southern Campbell-Western Crook-Weston-
Northeastern Crook-
656 AM MDT Sat Jul 21 2018

This Hazardous Weather Outlook is for portions of northwestern
South Dakota...southwestern South Dakota...and northeast Wyoming.

.DAY ONE...Today and Tonight

Isolated thunderstorms are possible today, with scattered storms
expected this evening. Some thunderstorms could become severe.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

An active weather pattern will bring near daily chances for
showers and thunderstorms. Strong to severe storms will be
possible at times.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today.

&&

Stay tuned to NOAA weather radio for further updates...or check
our web site at weather.gov/rapidcity

SDZ014-032-046-047-049-221300-
Ziebach-Haakon-Mellette-Todd-Tripp-
656 AM MDT Sat Jul 21 2018 /756 AM CDT Sat Jul 21 2018/

This Hazardous Weather Outlook is for portions of western South
Dakota.

.DAY ONE...Today and Tonight

Isolated thunderstorms are possible today, with scattered storms
expected this evening. Severe weather is not expected at this
time.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

An active weather pattern will bring near daily chances for
showers and thunderstorms. Strong to severe storms will be
possible at times.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today.

&&

Stay tuned to NOAA weather radio for further updates...or check
our web site at weather.gov/rapidcity

In the hours before

Special weather statement, Jul 21, 3:08 PM MST, 1 hour 7 minutes before

Special Weather Statement
National Weather Service Rapid City SD
408 PM MDT SAT JUL 21 2018

SDZ024-025-072-212245-
Northern Black Hills SD-Northern Foot Hills SD-
Sturgis/Piedmont Foot Hills SD-
408 PM MDT SAT JUL 21 2018

...SIGNIFICANT WEATHER ADVISORY FOR WESTERN MEADE AND SOUTHEASTERN
LAWRENCE COUNTIES UNTIL 445 PM MDT...

At 408 PM MDT, Doppler radar was tracking a strong thunderstorm near
Brownsville, or 7 miles southeast of Lead, moving southeast at 15
mph.

Pea to half inch hail and winds in excess of 40 mph will be possible
with this storm.

Locations impacted include...
Southern Sturgis, Lead, Deadwood, Galena, Brownsville, Apple Springs
and Roubaix Lake.

This Includes Interstate 90 in South Dakota between Mile Markers 34
and 38.

LAT...LON 4430 10382 4443 10372 4437 10339 4414 10361
      4414 10363
TIME...MOT...LOC 2208Z 301DEG 14KT 4429 10366

Eagan

Special weather statement, Jul 21, 3:12 PM MST, 1 hour 3 minutes before

Special Weather Statement
National Weather Service Rapid City SD
412 PM MDT SAT JUL 21 2018

WYZ054-055-212245-
Northern Campbell WY-Southern Campbell WY-
412 PM MDT SAT JUL 21 2018

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL CAMPBELL COUNTY UNTIL 445
PM MDT...

At 411 PM MDT, Doppler radar was tracking a strong thunderstorm 10
miles west of Gillette, moving east at 20 mph.

Up to half inch hail and winds in excess of 40 mph will be possible
with this storm.

Locations impacted include...
Northwestern Gillette.

This Includes Interstate 90 in Wyoming between Mile Markers 109 and
125.

LAT...LON 4416 10582 4430 10580 4431 10550 4408 10553
TIME...MOT...LOC 2211Z 275DEG 16KT 4422 10569

Eagan

Local storm report, Jul 21, 3:21 PM MST, 54 minutes before

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE RAPID CITY SD
421 PM MDT SAT JUL 21 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0418 PM     HAIL             LEAD                    44.35N 103.76W
07/21/2018  E0.25 INCH       LAWRENCE           SD   TRAINED SPOTTER

            A LOT OF PEA SIZED HAIL

&&

KDS

Special weather statement, Jul 21, 3:43 PM MST, 32 minutes before

Special Weather Statement
National Weather Service Rapid City SD
443 PM MDT SAT JUL 21 2018

WYZ054-055-212330-
Northern Campbell WY-Southern Campbell WY-
443 PM MDT SAT JUL 21 2018

...SIGNIFICANT WEATHER ADVISORY FOR EAST CENTRAL CAMPBELL COUNTY
UNTIL 530 PM MDT...

At 442 PM MDT, Doppler radar was tracking a strong thunderstorm 5
miles southeast of Gillette, moving east at 25 mph.

Dime size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Gillette, Rozet and Camplex Event Facility.

This Includes Interstate 90 in Wyoming between Mile Markers 122 and
147.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

A severe thunderstorm watch remains in effect until 1100 PM MDT for
northeastern Wyoming.

LAT...LON 4430 10558 4434 10509 4425 10508 4396 10519
      4413 10564
TIME...MOT...LOC 2242Z 281DEG 20KT 4420 10544

Eagan

Special weather statement, Jul 21, 3:52 PM MST, 23 minutes before

Special Weather Statement
National Weather Service Rapid City SD
452 PM MDT SAT JUL 21 2018

SDZ024-025-072-212330-
Northern Black Hills SD-Northern Foot Hills SD-
Sturgis/Piedmont Foot Hills SD-
452 PM MDT SAT JUL 21 2018

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHWESTERN MEADE AND
SOUTHEASTERN LAWRENCE COUNTIES UNTIL 530 PM MDT...

At 452 PM MDT, Doppler radar was tracking a strong thunderstorm over
Brownsville, or 6 miles southeast of Lead, moving southeast at 10
mph.

Pea to half inch hail and winds in excess of 40 mph will be possible
with this storm.

Locations impacted include...
Brownsville, Galena and Roubaix Lake.

A severe thunderstorm watch remains in effect until 1100 PM MDT for
west central South Dakota.

LAT...LON 4428 10381 4438 10363 4431 10343 4414 10367
TIME...MOT...LOC 2252Z 307DEG 9KT 4429 10367

Eagan

Local storm report, Jul 21, 4:07 PM MST, 8 minutes before

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The radar

The Rapid City radar (KUDX), 73 km away, was scanning. The volume nearest the first event began at 23:17 UTC.

Open the radar archive at this moment

Event by event

5 hail, 5 thunderstorm wind, 2 flash flood, 2 flood. Local standard time, as filed.

Hail, Lawrence

When: July 21, 4:15 PM MST
Where: Lawrence County, 7.65 mi SW of Galena
Measured: 1.5 in hail
Reported by: Trained Spotter
Coordinates: 44.240, -103.730

Hail, Lawrence

When: July 21, 4:33 PM MST
Where: Lawrence County, 7.46 mi W of Nemo
Measured: 1 in hail
Reported by: Trained Spotter
Coordinates: 44.200, -103.650

Hail, Pennington

When: July 21, 5:40 PM MST
Where: Pennington County, 2.76 mi N of Sheridan Lake
Measured: 1 in hail
Reported by: NWS Employee
Coordinates: 44.020, -103.480

Thunderstorm Wind, Pennington

When: July 21, 5:45 PM MST
Where: Pennington County, 3.74 mi WSW of Rapid City
Measured: 61 kt (70 mph) estimated gust
Reported by: Broadcast Media
Coordinates: 44.060, -103.300

A large tree was downed by the winds.

Thunderstorm Wind, Pennington

When: July 21, 5:47 PM MST
Where: Pennington County, 2 mi SW of Rapid City
Measured: 61 kt (70 mph) estimated gust
Reported by: Public
Coordinates: 44.059, -103.258

The strong winds blew down a tree and damaged a garage door.

Thunderstorm Wind, Pennington

When: July 21, 5:57 PM MST
Where: Pennington County, 4 mi S of Rapid City
Measured: 55 kt (63 mph) measured gust
Reported by: NWS Employee
Coordinates: 44.022, -103.230

Hail, Pennington

When: July 21, 6:09 PM MST
Where: Pennington County, 2 mi SW of Rapid City
Measured: 0.75 in hail
Reported by: Public
Coordinates: 44.059, -103.258

Thunderstorm Wind, Pennington

When: July 21, 6:10 PM MST to 6:25 PM MST
Where: Pennington County, (Rap)rapid City Arpt
Measured: 58 kt (67 mph) measured gust
Reported by: ASOS
Coordinates: 44.050, -103.050

Hail, Pennington

When: July 21, 6:22 PM MST
Where: Pennington County, 7.52 mi SSE of Rapid City
Measured: 1 in hail
Reported by: NWS Employee
Coordinates: 43.980, -103.170

Thunderstorm Wind, Pennington

When: July 21, 6:22 PM MST
Where: Pennington County, 7.52 mi SSE of Rapid City
Measured: 61 kt (70 mph) estimated gust
Reported by: NWS Employee
Coordinates: 43.980, -103.170

Wind gusts were estimated at 70 mph.

Flash Flood, Pennington

When: July 21, 6:45 PM MST to 9:00 PM MST
Where: Pennington County, 6.09 mi NW of Rapid City to 1.64 mi ESE of Rockerville
Cause: Heavy Rain
Reported by: Public
Coordinates: 44.140, -103.320

Street flooding occurred throughout Rapid City. Water was reported over Third Street, St. Francis Street, Meade Street, and West Main Street. Rapid Creek quickly rose and inundated lower portions of the bike path through Rapid City.

Flash Flood, Pennington

When: July 21, 8:00 PM MST to 10:00 PM MST
Where: Pennington County, 3.41 mi NW of Mt Rushmore to 1.7 mi SW of Mt Rushmore
Cause: Heavy Rain
Reported by: Public
Coordinates: 43.920, -103.490

Runoff from heavy rain caused flash flooding in and around Keystone. This flash flooding was exacerbated by the already saturated ground from previous rainfall and flooding four days earlier. Water covered Grizzly Gulch Road, parts of Cemetery Road, and parking lots of businesses along Iron Mountain Road. Levels on Grizzly Bear Creek and Battle Creek quickly rose several feet.

Flood, Pennington

When: July 21, 9:00 PM MST to July 22, 1:00 PM MST
Where: Pennington County, 0.61 mi SW of Keystone to 1.05 mi SE of Mt Rushmore
Cause: Heavy Rain
Reported by: Public
Coordinates: 43.895, -103.430

Runoff from heavy rain continued to cause flooding in and around Keystone. Water covered Grizzly Gulch Road, parts of Cemetery Road, and parking lots of businesses along Iron Mountain Road. Levels on Grizzly Bear Creek and Battle Creek quickly rose several feet.

Flood, Pennington

When: July 21, 9:00 PM MST to July 22, 8:00 AM MST
Where: Pennington County, 4.84 mi SE of Rapid City to 3.97 mi SSW of (Rap)rapid City Arpt
Cause: Heavy Rain
Reported by: Public
Coordinates: 44.040, -103.150

Flooding continued along Rapid Creek, especially in Rapid Valley near Green Valley Road and Reservoir Road. The river gauge near the Water Reclamation facility crested slightly above flood stage with a peak flow around 1430 cfs.

What these are

Hail: Hail three-quarters of an inch or larger, or smaller hail that caused damage, filed with the largest stone measured.
Thunderstorm Wind: Winds from a thunderstorm of 50 knots (58 mph) or more, or lower winds that caused damage or casualties. Measured gusts are marked MG, estimated EG.
Flash Flood: Flooding that begins within six hours of the rain that causes it, or of a dam or levee failure or ice jam release, often with deadly speed.
Flood: Flooding that develops more slowly than a flash flood: rivers leaving their banks, or water standing over land for an extended time.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in South Dakota in 2018 Every state in 2018 This day in weather history Warnings in South Dakota, July 2018

Sources

NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-10-01. Each event’s entry at NCEI: 765828, 765831, 765833, 765834, 765835, 765836, 765837, 765841, 765838, 765840, 769614, 769591, 769596, 769697.