239 flood insurance claims·$2.1M paid
Share of the whole county outline, rivers and lakes included; parts FEMA hasn’t mapped count as zero.
Shaded areas are FEMA’s mapped flood hazard zones in and around Williamson County, Illinois. Zoomed out they are simplified; at street level the map switches to FEMA’s exact layer. Tap anywhere to check that spot, or search an address above.
Flash flooding from summer thunderstorms dominates the flood profile of Williamson County, accounting for 40 events compared to 28 general floods over the last 30 years. Recent severe weather events illustrate this pattern, including a slow-moving MCS that brought torrential rainfall rates on July 19, 2025, and a surface low-pressure system that produced minor street flooding in Marion on May 14, 2024. Additional major storm outbreaks occurred on May 8 and May 26 of 2024.
National Flood Insurance Program records show claims across multiple zones, with the highest concentration in high-risk areas. Zone A accounts for 142 claims with an average water depth of 3 inches and an average payout of $11,618, while properties categorized as Zone UNKNOWN recorded 59 claims with an average water depth of 2 inches and a $4,670 average payout. Properties in Zone X recorded 15 claims with an average water depth of 6 inches and an average payout of $22,231. Zone X_UNSHADED recorded 15 claims averaging 2 inches of water depth and $17,712, and Zone X_SHADED recorded 8 claims averaging 4 inches of water depth and $4,902. Residents in Zone A and Zone X should pay the most attention to flood risk given the documented claim history and water depths.
Summary written from NOAA storm reports and NFIP claims for this county. Not a prediction.
14% of Williamson County’s flood insurance claims were on properties rated outside FEMA’s high-risk zones.
165 NFIP claims with a recorded rated flood zone, 1978 to date. The zone is the one FEMA used to rate that policy, which may not match today’s map. Source: OpenFEMA NFIP claims.
In flood insurance claims here that reported a flood depth, the average was 3 inches.
Drawn to scale against an 80-inch door. Even a few inches soaks flooring, the bottom of the drywall and anything plugged in near the floor.
Average of the depths reported on claims (zero and implausible values left out); FEMA’s record doesn’t say exactly where each depth was measured. Source: OpenFEMA NFIP claims.
The worst year was 1996, with 38 claims and $467,821 paid, the year of Severe Storms and Flooding.
Bar height is scaled by square root so ordinary years stay visible next to the worst one. NFIP claims by year of loss. Source: OpenFEMA.
32 NOAA storm reports from Williamson County describe what happened, in the words of the forecasters who covered it. Read the Illinois flood stories →
9 federal disaster declarations since 1982.
| Disaster | Began |
|---|---|
| Severe Storms And Flooding | Apr 19, 2011 |
| Severe Storms, Flooding, And Tornadoes | May 8, 2009 |
| Hurricane Katrina Evacuation | Aug 29, 2005 |
| Record/near Record Snow | Dec 21, 2004 |
| Severe Storms, Tornadoes And Flooding | Apr 21, 2002 |
| Severe Storms And Flooding | Apr 28, 1996 |
| Severe Storms & Tornadoes | Jun 5, 1982 |
Weather and water disasters only; most recent first. Source: OpenFEMA Disaster Declarations.
68 since 1996: 40 flash floods and 28 river or area floods.
| Event | Date | Damage |
|---|---|---|
| Flash Flood | Jul 19, 2025 | — |
| Flood | May 26, 2024 | — |
| Flood | May 14, 2024 | — |
| Flash Flood | May 8, 2024 | — |
| Flood | Mar 24, 2023 | — |
| Flash Flood | Jul 19, 2023 | $25,000 |
| Flood | Mar 3, 2023 | — |
| Flood | Mar 30, 2022 | — |
— = no damage estimate in the record. Source: NOAA Storm Events Database.
Flash Flood · Jul 19, 2025. As high pressure slowly crept northward towards the Quad State region, ridging remained over the middle Mississippi River Valley. A slow-moving MCS moved through the northeastern half of the Quad State midday to near sunset. CAPE rose to 1000-2000J/kg in northern portions of Southern Illinois with effective bulk shear around 30kts. Extremely high moisture content with precipitable water values of 2.25in resulted i…
Flood · May 26, 2024. The second major severe weather outbreak for the month occurred on the 26th for the Quad State region. On the synoptic scale, a shortwave trough centered in the middle of the country with a 60 kt mid-level jet moved across northern Arkansas. A weak surface low was moving into SE Missouri during the morning with a secondary low located further northwest. A warm frontal boundary was draped across our area. A bowing…
Flood · May 14, 2024. Surface low pressure was drifting across the Quad State on the 14th. Showers and thunderstorms popped up in the afternoon to evening hours with the strongest storm in Southern Illinois moving through Marion, producing damage to vinyl fencing and minor street flooding. CAPE values around 1000 J/kg dropped off after sunset. Lapse rates were meager, around 6 C/km, and shear was also limited to around 25 kts.
Flash Flood · May 8, 2024. A major outbreak of severe weather occurred on the 8th for the Quad State region. On the synoptic scale, a longwave trough was centered across the Rockies with deep-layer southwesterly flow from the Southern Plains to the Ohio Valley. A 110 kt upper jet extended from northern KS to IA while a 60 kt mid-level jet was positioned across northern MZ into IA. Surface low pressure near the MZ/KS border lifted to St. Lou…
National Weather Service narratives, lightly trimmed. Source: NOAA Storm Events Database.
Every colour on the map above is a place on this drawing: where the ground sits relative to the flood FEMA plans for.
If a building in Williamson County is in a high-risk zone (A or V) and its mortgage comes from a federally regulated or insured lender, or is federally backed, flood insurance is required.
Outside those zones it is optional, and often worth having: from 2014 to 2024 nearly one-third of NFIP claims came from outside the high-risk Special Flood Hazard Area. Since Risk Rating 2.0, NFIP premiums are set for each building, so the only reliable price is a quote: FloodSmart.gov or a local agent.