Nearly 13 million high-risk properties sit outside FEMA’s mapped flood zones, where just 2% carried federal flood insurance as of April.
Homeowners across the country could enter a potentially historic El Niño winter believing their home sits safely outside a high-risk flood zone—because according to FEMA's maps, they do.
But nearly 13 million properties outside the Federal Emergency Management Agency’s Special Flood Hazard Areas are considered high risk by climate-risk firm First Street, according to a Government Accountability Office report released last week.
Flood risk caused by heavy rain accounts for much of the mismatch. Among the high-risk properties First Street identified outside FEMA’s zones, 62% had rainfall alone as their predominant flood threat.
Those properties also sit on the side of FEMA’s maps where federal flood insurance is far less common. About 43% of properties inside Special Flood Hazard Areas had National Flood Insurance Program coverage as of April, compared with just 2% outside them.
The findings come as the National Oceanic and Atmospheric Administration (NOAA) says the current El Niño has a greater than 90% chance of becoming “very strong” this fall and winter, with wetter-than-normal conditions favored across much of the southern United States.
The coming winter could expose the consequences of a system in which FEMA’s flood-zone boundary can more strongly influence whether homeowners buy insurance than the underlying flood risk itself.
Why millions of high-risk homes fall outside FEMA’s flood zones
Heavy-rain flooding—also called pluvial flooding—occurs when rain falls faster than the ground and drainage systems can absorb or carry it away.
As the water accumulates, it can flood streets and spill into homes well outside the river and coastal areas typically associated with flooding risk.
FEMA’s maps have a built-in blind spot for that kind of flooding, too.
A FEMA flood map for Austin, TX, shows a property sitting just outside the agency’s Zone AE high-risk floodplain. (FEMA) The federal mapping system grew out of technology better able to model river and coastal flooding, and Special Flood Hazard Areas still primarily reflect those threats.
Among the nearly 12.8 million high-risk properties First Street identified outside FEMA’s zones, 62% had rainfall alone as their predominant flood threat. Inside the zones, just 19% did.
FEMA has since broadened the risks it considers when pricing flood insurance, including through Risk Rating 2.0. But those additional hazards don't necessarily determine whether a property falls inside the zone that can require its owner to buy coverage.
But that mandate largely shape who gets insured.
GAO found homeowners were more likely to carry NFIP coverage on a low-risk property inside a FEMA flood zone than on one of the highest-risk properties outside one.
El Niño could stress-test the rainfall blind spot
The coming winter could put that gap under new pressure.
NOAA’s outlook favors wetter-than-normal conditions across much of the southern United States, with the strongest signal around southern Georgia and northern Florida. Along parts of coastal California, the probability of above-normal precipitation rises to 50% to 60% later in the winter.
“With an event of this magnitude, the chances of experiencing impacts consistent with El Niño are larger, though not guaranteed,” NOAA says.
Southern California’s flood history shows what can happen when those wetter odds translate into damaging storms.
Scripps Institution of Oceanography researchers Tom Corringham and Daniel Cayan analyzed NFIP claims from 1978 through 2017 and found El Niño was associated with more frequent flooding and larger insured losses in coastal Southern California and the Southwest.
“We found that damages in Southern California were 10 times greater in El Niño winters than in La Niña winters over the 40-year period from 1978–2017,” Corringham says.
Those losses can be driven by remarkably few storms. Across the Western states studied, just 1% of extreme flood events accounted for more than 66% of insured losses.
That means a costly El Niño winter may not even require the months of relentless rain that the forecast promises.
Instead, just a handful of extreme storms could produce an outsized share of the damage—while many properties vulnerable to rainfall flooding remain outside the FEMA zones most likely to prompt their owners to buy flood insurance.