Dangerously hot conditions are occurring today in southeast Oklahoma, the ArkLaTex, lower Mississippi Valley, and Puerto Rico. Hot temperatures surround this area throughout the southern U.S. Excessive Heat Warnings and Heat Advisories in effect. Excessive rainfall may lead to flash flooding today over parts of northern Alabama into eastern Tennessee and the Cascades. Read More >
Overview
On May 23, 2021, a spring storm system brought numerous strong to severe thunderstorms to much of western and central South Dakota. These storms spawned at least four tornadoes and produced widespread wind gusts of 60 to 70 mph. Morning low clouds and fog across the western South Dakota plains and Black Hills gave way to an initial round of thunderstorms producing heavy rain and small hail by early afternoon. A second round of storms over the Black Hills produced two tornadoes in the Custer area, damaging straight-line winds, and hail up to half dollar size. As these storms moved eastward over the western South Dakota plains, the primary threat transitioned to straight-line winds. Several wind gusts of 60 to 70 mph were measured across the plains during the afternoon and evening hours. This line of storms continued to produce damaging winds near and east of the Missouri River into the night. East of this line of storms, supercells developed across the western South Dakota plains during the early and mid-afternoon hours. One supercell spawned two tornadoes in Perkins County. No damage was reported with either tornado as they traveled over mostly open land. |
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Tree Damage from Custer County caused by Tornado 4 (photo from NWS Storm Survey) |
Perkins County Tornado 1 (photo by Janelle Uyeda) |
Tornadoes:
Tornado 1
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Tornado 2
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Tornado 3
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Tornado 4
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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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Photos & Video
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Perkins County Tornado 1 (photos by Janelle Uyeda) |
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3 miles east of Custer (photos by Matt Selena Spring) |
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4 miles south of Custer along Sidney Rd (photos by Brandon Zapp) |
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Southeast of Custer (photos by Custer County Sheriff) |
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Line of storms approaching Box Elder (photo by Amanda Breann) |
Radar
Radar imagery from noon MDT through 10 PM MDT on May 23, 2021 |
Storm Reports
Preliminary local storm reports from May 23, 2021 into early May 24, 2021. Red "T"s, blue "W"s, and green "H"s represent tornado, severe wind (58 mph or greater), and severe hail (1" or larger) reports. Italic blue "W"s show significant severe wind (75 mph or greater) reports. (Image courtesy of the Storm Prediction Center) |
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 ------ ----------------------- -- -------------- ------- 1.25 1 SSW CUSTER SD CUSTER 0402 PM 1.00 1 E DWTN RAPID CITY SD PENNINGTON 0458 PM 1.00 4 S DWTN RAPID CITY SD PENNINGTON 0447 PM 0.75 1 ENE CUSTER SD CUSTER 0408 PM Thunderstorm wind reports listed by speed (mph) SPEED LOCATION ST COUNTY TIME ------ ----------------------- -- -------------- ------- 70.00 WASTA SD PENNINGTON 0540 PM 68.00 2 SSW CUSTER SD CUSTER 0405 PM 66.00 1 NNE FAITH SD MEADE 0656 PM 66.00 1 NNE FAITH SD MEADE 0640 PM 62.00 4 SSE WITTEN SD TRIPP 0845 PM 62.00 CACTUS FLAT SD JACKSON 0559 PM 60.00 WINNER SD TRIPP 0900 PM POWER IS OUT IN TOWN 60.00 10 WSW RED ELM SD ZIEBACH 0650 PM 60.00 3 NNE CREIGHTON SD PENNINGTON 0605 PM PEA TO DIME SIZED HAIL 60.00 12 WSW HEREFORD SD MEADE 0510 PM 60.00 PRINGLE SD CUSTER 0400 PM 60.00 8 WNW USTA SD PERKINS 0335 PM 59.00 HOT SPRINGS SD FALL RIVER 0341 PM 58.00 RAPID CITY ARPT SD PENNINGTON 0455 PM 58.00 ELLSWORTH AFB SD MEADE 0453 PM 50.00 3 N JOHNSON SIDING SD PENNINGTON 0445 PM PEA SIZED HAIL AND OVER AN INCH OF RAIN SO FAR. Rain amounts (inches) AMOUNT LOCATION ST COUNTY TIME ------ ----------------------- -- -------------- ------- 2.89 9 W BISON SD PERKINS 0700 AM 24-HOUR RAINFALL. 1.91 14 S REVA SD HARDING 0625 AM 4 DAY TOTAL... 0.25 INCHES ON THURSDAY, 0.46 INCHES ON FRIDAY, AND 1.20 INCHES OVER THE WEEKEND 1.90 MOUNT RUSHMORE SD PENNINGTON 1200 AM STORM TOTAL 1.87 7 W RAPID CITY SD PENNINGTON 0700 AM 24-HOUR RAINFALL. 1.65 1 NNW PIEDMONT SD MEADE 1200 AM STORM TOTAL 1.64 10 WNW RAPID CITY SD PENNINGTON 0637 AM 24-HOUR RAINFALL. 1.56 14 NW EDGEMONT SD FALL RIVER 0800 AM 24-HOUR RAINFALL. 1.48 5 E PIEDMONT SD MEADE 0600 AM 24-HOUR RAINFALL. 1.36 6 SSW LEAD SD LAWRENCE 0700 AM 24-HOUR RAINFALL. 1.35 RAPID CITY ARPT SD PENNINGTON 1200 AM 24 HOUR TOTAL 1.33 6 NNE RAPID CITY SD MEADE 0700 AM 24-HOUR RAINFALL. 1.32 8 NNE BOX ELDER SD MEADE 0700 AM 24-HOUR RAINFALL. 1.28 1 ESE PACTOLA RES SD PENNINGTON 0800 AM STORM TOTAL 1.27 2 NE BOX ELDER SD PENNINGTON 0800 AM 24-HOUR RAINFALL. 1.27 12 WSW HEREFORD SD MEADE 1000 PM STORM TOTAL RAINFALL 1.23 3 WNW RAPID CITY SD PENNINGTON 0700 AM 24-HOUR RAINFALL. 1.20 5 ENE CUSTER SD CUSTER 0600 AM 24-HOUR RAINFALL. 1.16 TILFORD SD MEADE 0700 AM 24-HOUR RAINFALL. 1.15 3 WNW RAPID CITY SD PENNINGTON 0700 AM 24-HOUR RAINFALL. 1.15 4 S RAPID CITY SD PENNINGTON 0700 AM 24-HOUR RAINFALL. 1.12 11 NW UNION CENTER SD MEADE 0605 AM 24-HOUR RAINFALL. 1.11 4 W HOT SPRINGS SD FALL RIVER 0700 AM 24-HOUR RAINFALL. 1.11 1 E DWTN RAPID CITY SD PENNINGTON 1200 AM 24 HOUR STORM TOTAL 1.10 4 NE RAPID CITY SD PENNINGTON 0600 AM 24-HOUR RAINFALL. 1.04 LEAD SD LAWRENCE 0900 AM STORM TOTAL 1.04 1 W NEWCASTLE WY WESTON 0519 AM 24-HOUR RAINFALL. 1.03 4 NNW WHITEWOOD SD LAWRENCE 0530 AM 24-HOUR RAINFALL. 1.00 HOT SPRINGS SD FALL RIVER 0700 AM 24-HOUR RAINFALL. 0.90 10 WSW RED ELM SD ZIEBACH 0950 PM 6-HOUR RAINFALL. 0.50 11 ENE WANBLEE SD JACKSON 1004 PM 12-HOUR RAINFALL.
Environment
From the morning of May 23, 2021 into the afternoon, a strong mid-to-upper-level trough and associated upper-level jet streak approached the area from the west. These features supported the strengthening of a surface cyclone over the central Rockies, allowing a warm front to lift northward across western South Dakota and leading to a moderately unstable, strongly sheared environment favorable for severe weather.
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Figure 1: 12z (6 AM MDT) 5/23/21 300 mb (upper-level) observations showing wind speeds, streamlines, and divergence. Note the trough axis west of the Four Corners region and jet streak extending from AZ northward into WY. (Image courtesy Storm Prediction Center) |
Figure 2: 12z (6 AM MDT) 5/23/21 500 mb (midlevel) observations showing lines of constant height, wind barbs, and temperatures. A midlevel low center is evident near the ID/NV/WA border. (Image courtesy Storm Prediction Center) |
Figure 3: 12z (6 AM MDT) 5/23/21 700 mb (low-to-midlevel) observations showing lines of constant height, wind barbs, temperatures, and dew points. (Image courtesy Storm Prediction Center) |
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Figure 4: 12z (6 AM MDT) 5/23/21 surface analysis courtesy of the Weather Prediction Center. Note warm front south of the NE/SD border and low pressure center over south central WY. | Figure 5: 15z (9 AM MDT) 5/23/21 surface analysis courtesy of the Weather Prediction Center. Note that the warm front has lifted northward as the surface low has moved eastward. | Figure 6: 21z (3 PM MDT) 5/23/21 surface analysis courtesy of the Weather Prediction Center. The warm front has now lifted north of the Black Hills as the surface low has moved near the NE/SD/WY border. |
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Figure 7. Mixed-layer convective available potential energy (MLCAPE) and convective inhibition (CIN) from the SPC mesoanalysis page valid at 22z (4 PM MDT) 5/23/21. This plot reveals a moderately unstable environment across much of the region, with MLCAPE values of 1,000-2,000 J/kg. | Figure 8: Effective-layer (based upon the expected storm depth) vertical shear from the SPC mesoanalysis page, valid at 22z (4 PM MDT) 5/23/21. Deep-layer shear magnitudes at this time, which was approximately the time when tornadoes occurred in Perkins and Custer counties, were strong (40-50 kt). | Figure 9: Combination plot from the SPC mesoanalysis showing 0-3 km CAPE and 0-3 km vertical wind shear vectors, valid at 22z (4 PM MDT) 5/23/21. These two parameters are useful in highlighting areas favorable for quasi-linear convective system (QLCS; aka squall line) tornadoes, such as those observed near Custer. Values of 0-3 km CAPE (100-150 J/kg) and 0-3 km shear (30-40 kt) are both considered supportive for the development of QLCS tornadoes. |
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