Damaging Wind in South Dakota, May 20, 2021

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
522 PM MDT Thu May 20 2021

.DISCUSSION...(This Evening Through Thursday)
Issued at 223 PM MDT Thu May 20 2021

Upper analysis this aftn shows southwesterly flow aloft over the
forecast area ahead of an upper low over the western CONUS. Latest
surface analysis shows low pressure centered over SW WY/NE UT
with a trough/front extending northeastward into SE MT, resulting
in breezy to windy southeasterly low level flow across the
forecast area. Current dewpoints are mainly in the 40s across
northeastern Wyoming and in the 50s over western SD.

MLCAPE is currently around 1000J/KG with 0-6KM bulk shear ranging
from 30-45kt to the southwest of our northeastern Wyoming zones.
An upper shortwave, combined with diurnal heating and steepening
lapse rates, has helped to develop scattered thunderstorms over
the higher terrain of eastern Wyoming. Storms will continue to
move northeastward into the forecast area this afternoon and
evening. A few strong to severe storms are possible, with large
hail and damaging wind gusts.

The frontal boundary will move southeastward as a cold front
tonight. Behind the front, low stratus and possibly some drizzle,
will develop over northeastern Wyoming and parts of far western
South Dakota with the stratus expected to linger at least through
the morning.

On Friday, the cold front will stall somewhere east of the Black
Hills. Southwesterly flow aloft will continue to strengthen with
embedded waves. 1000-2000J/KG MLCAPE will develop along and to the
east of the cold front by afternoon. Relatively strong MLCIN will
be an issue, but suspect forcing along cold front will be
sufficient for surface based convection, some of which could be
strong/severe. However, notable uncertainty exists given forecast
soundings and potential for an extended period/area of stratus.

On Saturday and Sunday, deep southwesterly flow will continue as
moisture deepens. The upper low will eject into MT late Sunday.
This will promote rather high pops for the weekend with diurnally
driven convection. Temperatures will be near/slightly cooler than
average.

Next week looks drier, but storm track along US/Canadian border will
support periodic frontal passages and chances for showers and
thunderstorms.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 519 PM MDT Thu May 20 2021

Showers and thunderstorms will continue across northeastern WY
and far western South Dakota, shifting north and east into
northwestern South Dakota. Thunderstorm activity will continue
across northwest South Dakota through the evening hours. Isolated
to scattered severe storms could produce hail, brief strong wind
gusts, and localized MVFR/IFR conditions.

Following the thunderstorm activity, a cold front will cross the
area this evening. Behind the front, conditions will deteriorate
toward IFR as low stratus develops and persists through Friday
morning, across northeastern WY and far western SD. LIFR
conditions will be found across higher elevations late tonight
into Friday morning.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

DISCUSSION...10
AVIATION...15

Hazardous Weather Outlook
National Weather Service Rapid City SD
525 AM MDT Thu May 20 2021

SDZ001-002-012>014-024>032-041>044-046-047-049-072>074-WYZ054>058-
071-211300-
Harding-Perkins-Butte-Northern Meade Co Plains-Ziebach-
Northern Black Hills-Northern Foot Hills-Rapid City-
Southern Foot Hills-Central Black Hills-Southern Black Hills-
Custer Co Plains-Pennington Co Plains-Haakon-Fall River-
Oglala Lakota-Jackson-Bennett-Mellette-Todd-Tripp-
Sturgis/Piedmont Foot Hills-Southern Meade Co Plains-
Hermosa Foot Hills-Northern Campbell-Southern Campbell-
Western Crook-Wyoming Black Hills-Weston-Northeastern Crook-
525 AM MDT Thu May 20 2021 /625 AM CDT Thu May 20 2021/

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

.DAY ONE...Today and Tonight

Isolated to scattered thunderstorms are expected this afternoon
and evening, potentially lingering into tonight. Some of these
storms could be severe, with damaging winds and large hail the
primary threats. Highest coverage of thunderstorms and potential
for severe storms will be over northeastern Wyoming and far
western South Dakota, including the Black Hills.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Shower and thunderstorm chances will continue across the area
through the weekend. Isolated strong to severe storms will also be
possible, particularly on Friday and Sunday.

.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

Special Weather Statement
National Weather Service Rapid City SD
623 PM MDT Thu May 20 2021

SDZ028-029-WYZ057-058-210115-
Central Black Hills SD-Southern Black Hills SD-Weston WY-
Wyoming Black Hills WY-
623 PM MDT Thu May 20 2021

...STRONG THUNDERSTORM OVER NORTHWESTERN CUSTER...WEST CENTRAL
PENNINGTON AND EAST CENTRAL WESTON COUNTIES...

At 622 PM MDT, Doppler radar was tracking a strong thunderstorm near
Elk Mountain Lookout, or 10 miles southeast of Newcastle, moving
north at 30 mph.

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

Locations impacted include...
Newcastle, Deerfield, Moon, Deerfield Reservoir, Redbank Spring
Campground, and Elk Mountain Lookout.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4354 10407 4367 10429 4409 10412 4401 10371
TIME...MOT...LOC 0022Z 197DEG 28KT 4373 10409

Eagan

Special Weather Statement
National Weather Service Rapid City SD
643 PM MDT Thu May 20 2021

SDZ012-WYZ056-057-071-210130-
Butte SD-Northeastern Crook WY-Wyoming Black Hills WY-
Western Crook WY-
643 PM MDT Thu May 20 2021

...STRONG THUNDERSTORM OVER WEST CENTRAL BUTTE AND NORTHEASTERN CROOK
COUNTIES...

At 643 PM MDT, Doppler radar was tracking a strong thunderstorm 8
miles northeast of Aladdin, or 13 miles northwest of Belle Fourche,
moving north at 30 mph.

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

Locations impacted include...
Colony.

LAT...LON 4502 10383 4463 10407 4499 10450 4500 10448
      4501 10404 4510 10403
TIME...MOT...LOC 0043Z 197DEG 28KT 4474 10410

Eagan

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE RAPID CITY SD
650 PM MDT THU MAY 20 2021

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

0615 PM     TSTM WND GST     2 SE HULETT             44.66N 104.57W
05/20/2021  M52 MPH          CROOK              WY   AWOS

&&

MLS

Special Weather Statement
National Weather Service Rapid City SD
707 PM MDT Thu May 20 2021

SDZ026-028-029-074-210145-
Hermosa Foot Hills SD-Central Black Hills SD-Southern Black Hills SD-
Rapid City SD-
707 PM MDT Thu May 20 2021

...STRONG THUNDERSTORM OVER CENTRAL CUSTER AND SOUTHWESTERN
PENNINGTON COUNTIES...

At 706 PM MDT, Doppler radar was tracking a strong thunderstorm
cluster over southwestern Custer State Park, or 7 miles southeast of
Custer, moving northeast at 30 mph.

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

Locations impacted include...
Custer, Hill City, Hermosa, Keystone, Pringle, Hayward, Rockerville,
Wind Cave National Park, Mount Rushmore National Memorial, Stockade
Lake, Legion Lake, Crazy Horse Memorial, State Game Lodge, Sheridan
Lake, Custer State Park, Horse Thief Lake, and Palmer Gulch Resort.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4348 10343 4373 10375 4405 10353 4388 10308
      4383 10309
TIME...MOT...LOC 0106Z 208DEG 26KT 4369 10350

Eagan

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE RAPID CITY SD
721 PM MDT THU MAY 20 2021

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

0715 PM     HAIL             1 ENE CUSTER            43.77N 103.58W
05/20/2021  M0.38 INCH       CUSTER             SD   COCORAHS

&&

MLS

Special Weather Statement
National Weather Service Rapid City SD
743 PM MDT Thu May 20 2021

SDZ026-028-029-031-072>074-210230-
Hermosa Foot Hills SD-Pennington Co Plains SD-Central Black Hills SD-
Southern Meade Co Plains SD-Southern Black Hills SD-
Sturgis/Piedmont Foot Hills SD-Rapid City SD-
743 PM MDT Thu May 20 2021

...STRONG THUNDERSTORM OVER SOUTHERN MEADE...NORTH CENTRAL CUSTER AND
SOUTH CENTRAL PENNINGTON COUNTIES...

At 742 PM MDT, Doppler radar was tracking a strong thunderstorm 3
miles northeast of Hayward, or 11 miles south of Rapid City, moving
northeast at 30 mph.

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

Locations impacted include...
Rapid City, Rapid Valley, Ellsworth Air Force Base, Box Elder, Black
Hawk, Summerset, Hermosa, Caputa, Hayward, Rockerville, Rapid City
Airport, Bear Country, Hart Ranch, and Reptile Gardens.

This includes Interstate 90 in South Dakota between Mile Markers 50
and 76.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4377 10321 4385 10343 4423 10333 4410 10286
TIME...MOT...LOC 0142Z 204DEG 24KT 4391 10329

Eagan