Union County, South Dakota

What hazards hit Union County most?

The most frequent records are winter storm records: 166 since 1996. 48% of its winter storm records since 1996 fell in December and January. The rank column places the county among South Dakota's counties by days with a record per 1,000 square miles of land over each hazard's complete period.

Source: NOAA NCEI Storm Events Database (county and forecast-zone records attributed to the county; a zone record shows its own totals on each county it covers). Days: dates with at least one record of the hazard in the county. Ranks: days with a record per 1,000 square miles of land (Census Bureau 2024 Gazetteer), since 1950 for tornadoes, 1955 for hail and thunderstorm wind, 1996 for the rest.

How many tornadoes has Union County had?

NOAA records 24 tornadoes in Union County from 1954 to 2014, the strongest rated F3, with 0 deaths and 1 injury recorded in the county. That is 9th of 66 South Dakota counties by days with tornadoes per 1,000 square miles since 1950. A tornado that crossed a county line is counted in each county it touched, with the length of its path inside the county.

Counted as whole tornadoes rather than county records, the Storm Prediction Center's database holds 24 tornadoes whose track touched Union County since 1950 (NOAA's county records number 24, the same count); along their whole tracks they killed 0 people.

Map of the strongest tornado tracks in South Dakota.

Source: NOAA NCEI Storm Events Database, tornado records by county (path length in the county, maximum width); ratings as published: F scale before February 1, 2007, EF scale after. Whole-track figures: NOAA Storm Prediction Center tornado database (1950-2025). SPC severe weather database.

When was the last damaging storm in Union County?

The most recent record with damage, injuries or deaths is the June 21, 2024 flood ($51.3 million in damage in 2025 dollars).

Rainfall amounts across Union County from June 20-22 ranged from around a third of an inch across southern areas to 1 to 2 inches across northern areas. Farther north and upstream, 6 to 10 inches of rain fell across portions of southeast South Dakota and northwest Iowa, with locally higher amounts in excess of 12 inches. This rainfall resulted in record flooding along the Big Sioux River and extensive overland flooding along the adjoining flood plain. County-wide, 44 residences were destroyed and another 182 received minor to major flood damage. Estimated losses exceed 52 million dollars. At the peak of the flooding, there were an estimated 20 to 30 water rescues. Residents along Northshore and Penrose Drives on the north side of McCook Lake were evacuated on the evening of June 23 as flood waters rose rapidly. …
National Weather Service event narrative, NOAA Storm Events Database (search the full record, event 1193159)

Source: NOAA NCEI Storm Events Database; every record of 2025-2026 is listed, earlier ones when they caused deaths, injuries or $250,000 of damage (2025 dollars), and every tornado.

The deadliest and costliest records

Source: NOAA NCEI Storm Events Database: deaths and injuries direct and indirect; damage is property plus crops as estimated by the National Weather Service, converted to 2025 dollars.

Is it getting more frequent?

Records per decade for Union County's most frequent hazards. Counts rose everywhere after 1996, when NOAA began recording every event type, so compare decades within a hazard's complete period only (the shaded cells come before it).

Drought in the 2020-26: 45 records against 12 in the decade before, 45 of them forecast-zone records. High wind and dust storms in the 2020-26: 29 records against 8 in the decade before, 29 of them forecast-zone records. Cells marked z rose mostly through zone records. The local National Weather Service office files these once per forecast zone, so a change in how it draws its zones or splits a storm into records moves this count as much as the weather does; read it beside the Union County days with a record above.

Source: NOAA NCEI Storm Events Database, records per calendar decade; zone records from the CZ_TYPE field (Z).

What time of year?

48% of its winter storm records since 1996 fell in December and January. The two busiest months are in violet.

Source: NOAA NCEI Storm Events Database, records by the month they began.

Federal disaster declarations for Union County

FEMA's register lists 19 declarations that include Union County: 16 major disasters, 3 emergencies and 0 fire management assistance declarations. Most were for flood (9) and severe storm (6).

Source: FEMA OpenFEMA Disaster Declarations Summaries (v2), declarations through October 2, 2026; one declaration is counted once however many areas it names. OpenFEMA dataset page.

How many floods has Union County had?

NOAA's Storm Events Database holds 160 flood or flash flood records for Union County from 1997 to 2025, on 107 days, with 0 deaths. By flood insurance claims since 1978 (OpenFEMA, as of September 8, 2026), Union County ranks 560th of 2,942 U.S. counties.

The National Flood Insurance Program has paid $7.6 million on 443 claims filed in Union County; the most in one year was 366 in 2011. Claims count insured losses only: uninsured flood damage is not in this file.

Source: FEMA OpenFEMA NFIP Redacted Claims, as of September 8, 2026; claims by year of loss, payments in the dollars of the year paid. OpenFEMA dataset page.

FEMA's modeled risk for Union County

FEMA's National Risk Index rates Union County Very Low overall, with an expected annual loss of $16.3 million. These are FEMA's model estimates of future loss from 18 hazards, not a count of what happened; they can differ from the record above.

Source: FEMA National Risk Index, county table, December 2025 release. FEMA's National Risk Index map.

Counties that share a border with Union County

1. Woodbury County, IA 1,213
2. Sioux County, IA 1,046
3. Plymouth County, IA 1,017
4. Lincoln County 959
5. Union County 834
6. Clay County 708
7. Dixon County, NE 657
8. Dakota County, NE 511

Source: U.S. Census Bureau county adjacency file; NOAA NCEI Storm Events Database, records since 1950. The Census Bureau's adjacency file.

Next

• Tornadoes in South DakotaEvery South Dakota tornado since 1950 and the counties with the most per square mile
• Winter storms in South DakotaThe state's winter storm records, by county, year and month
• The South Dakota recordEvery hazard in the state, its deadliest events and its federal declarations
• FEMA's risk rating for each hazard in Union CountyThe hazard risk calculator: 18 hazards beside the county's record
• Download Union County's FEMA declarations (CSV)19 rows: every declaration that named the county, for your own analysis