How much flood risk does Jefferson County actually carry?
Jefferson County lands at the 91.4th percentile of FEMA's National Risk Index for flooding, "Relatively Moderate" territory — a middling flood hazard by national standards — with the hazard driven by inland rivers and flash flooding rather than the coast — FEMA's model puts a flood event here at about 1.8 times a year. That's a county-wide figure, not a per-address one — FEMA's parcel maps are a live lookup, not a bulk file.
Mandatory flood insurance compliance in Jefferson County
Homes with a federally backed mortgage inside Jefferson County's mapped high-risk flood zone (the Special Flood Hazard Area, or SFHA) are legally required to carry an NFIP flood policy. In practice, 19.5% of them do — 466 active policies against 2,395 structures FEMA counts in that zone, leaving a real compliance gap. Across all residential structures in Jefferson County, not just the ones FEMA mandates, 0.7% carry an NFIP policy — most flood damage nationally happens outside the mapped high-risk zone, so that broader number matters too.
How Jefferson County compares nationally
Jefferson County sits at rank 287 of 3144 counties on FEMA's flood hazard scale (1 is worst) and rank 311 of 3144 by population (1 is biggest). Jefferson County, Missouri covers 656.042 square miles and counts roughly 231,888 residents.
Your street may differ from Jefferson County's average
Across all of Jefferson County's 87,272 residential structures, 591 carry an NFIP policy today — a county-wide count that still can't tell you one address's own mapped zone.
More on how this number was built
The methodology guide shows exactly which FEMA and Census tables feed Jefferson County's numbers. The guide to what the Special Flood Hazard Area actually means explains why the mandatory-insurance rule exists and who it applies to.