St. Pete Property-Risk Analysis

The data

What the data showed

I wanted to go deeper on flood exposure than "is this house in Flood Zone X?" While digging, I decided to use this as an opportunity to learn how to build a catastrophe model around it. The TLDR: the market hasn't finished pricing flood risk, and elevation is the hedge.

The market's flood discount is about 3× too small

Buyers haircut a flood-zone home by ~9.3% — about $27k off the median, a one-time, permanent discount. But a single 2024 storm season (Helene + Milton) paid out roughly $85k per insured home in flood-heavy block groups: ~3× the entire lifetime discount, in one season. Federally subsidized flood insurance has absorbed that cost so it never had to show up in the price — and FEMA's Risk Rating 2.0 is now unwinding the subsidy onto premiums, which do capitalize into price. Flood-zone homes carry repricing downside the current discount doesn't reflect.

It's an elevation story, and two independent methods agree

Read straight off the federal claims data, with no model in the loop: a block group's low-lying ground predicts its 2024 NFIP claims better than its average does (rank correlation ρ ≈ −0.77), and the gradient survives normalizing for who bought insurance — a ~7× swing in claim frequency from coast to ridge among homes that all carried a policy. A from-scratch flood catastrophe model, built from elevation and depth-damage physics, then reproduces the same result (the dry 33713 ridge contributes ≈ $0 of modeled loss). Two roads, same destination.

FEMA's "safe" label is trustworthy for surge: Zone X is real high ground

The skeptic's worry is that the flood maps are stale and the cheap, low-insurance "Zone X" homes are hiding risk. In St. Pete they aren't: sampling ground elevation for all 59,230 Zone-X parcels puts the median at 29 ft, with only 0.1% below the surge-plausible 8 ft line. The label corresponds to genuine elevation — so a dry, high-ground Zone-X home isn't just safer, it's on the right side of the repricing flood-zone homes haven't taken yet. (Honest caveat: this holds in St. Pete, not county-wide — a 99%-Zone-X pocket in inland Largo still took ~$26M in rain-driven flooding.) Zone X is trustworthy about surge — but it says nothing about rainfall, drainage, or the sewer network, each its own layer (next).

The hazard the flood map and my own surge model both miss: rainfall

Surge isn't the only way water reaches a house. I ingested St. Pete's actual storm-drain network (~22k inlets, ~4k outfalls) and scored every parcel on pluvial risk — low ground that's poorly served by drainage, where heavy rain ponds with nowhere to go. It flags 1,130 homes that FEMA calls Zone X and my surge model leaves bone-dry, yet sit low and under-drained — the inland-Largo caveat above, now mapped citywide. It's the local-infrastructure layer Zillow and even First Street structurally can't see. Honest caveat: a relative screen, not a validated probability — there's no public record of where it actually floods, so this points at suspects, not verdicts.

A third water hazard the map can't see: the sewer itself backing up

Rain ponding on the surface is one failure mode; the sanitary sewer overflowing in wet weather is another. Using the city's sewer-network condition data, I scored all 151 sewersheds for inflow & infiltration (I&I) risk: unlined clay pipe, inflow-prone manholes, and deep manholes, the plumbing drivers of wet-weather overflows, independent of topography. It's a genuinely distinct layer, not a restatement of the rain-drainage score: the correlation between the two is essentially zero (ρ ≈ −0.03). The payoff is the overlap. 12 of the 151 sewersheds fall in the worst quartile of both, the "double-jeopardy" areas where heavy rain ponds on the surface and the sewer is most prone to back up. Honest caveat: a relative percentile screen, not an overflow probability. The condition data has gaps (pipe-lining status is known for only ~22% of the network) and there's no public overflow-event record to validate against (the national EPA dataset carries zero Florida records), so it ranks suspects, not confirmed failures.