Why a Hurricane's Category Number Ignores Your Flood Risk

Hurricane Katrina made landfall in Louisiana on Aug. 29, 2005, rated a Category 3 on wind speed alone. Two decades later, the number that defined it in every headline still can't explain why the storm remains the costliest hurricane in U.S. history.

That's because the Saffir-Simpson Hurricane Wind Scale, the 1-to-5 rating system almost everyone reaches for during hurricane season, measures exactly one thing: a storm's maximum sustained wind speed. The National Hurricane Center says so directly: This scale does not take into account other potentially deadly hazards such as storm surge, rainfall flooding, and tornadoes. The category tells you how hard the wind will blow. It says nothing about whether your street will flood.

What the number actually measures

The scale's five tiers run on wind speed and nothing else. A Category 1 starts at 74 mph. Category 2 begins at 96 mph. Storms become "major hurricanes" at Category 3, when winds reach 111 mph, through Category 5, at 157 mph and up. Each step describes worse wind damage: shingles and gutters at the low end, total roof failure and uninhabitable neighborhoods at the top.

Wind is genuinely dangerous. But it is not what has been killing the most people. Water is. According to figures cited by the storm-forecasting firm StormGeo, water causes 90% of the loss of life related to powerful storms, per NOAA. A hurricane can weaken to a lower category and still drown a region.

Why does the scale ignore rain?

It didn't always. The Saffir-Simpson scale once included storm surge ranges alongside wind categories. The National Hurricane Center dropped that piece because tying two very different hazards to one number confused the public more than it informed them — a storm surge outcome depends heavily on a coastline's shape and a storm's exact track, not just its wind category. So NHC split the hazards apart: wind gets the category number, and storm surge gets its own dedicated watches and warnings, a system NHC began issuing separately in 2017.

Rain never got an equivalent public number at all. That gap is exactly what a team of researchers, including Jim Kossin at NOAA's National Centers for Environmental Information and colleagues at the University of Wisconsin-Madison, set out to close. Writing in the March 2020 issue of the Bulletin of the American Meteorological Society, they proposed the Extreme Rain Multiplier, a metric that compares an incoming storm's expected rainfall to the heaviest rain a given region typically sees once every two years. Rather than telling people a flood is "500-year" or "1,000-year" — phrasing NCEI's research summary notes has repeatedly failed to convey real risk — the ERM frames the threat against a baseline people already recognize.

Three storms the wind number badly undersold

The researchers tested the metric against historical hurricanes and found it flagged exactly the storms that wind categories missed.

Hurricane Harvey made landfall in Texas in 2017 at Category 4, then weakened to Category 1 within a day — and then did something the wind scale has no way to score: it stalled. Parked over southeast Texas, Harvey dropped 1,500 mm of rain, about 60 inches, on Nederland, the highest tropical-cyclone rainfall total ever recorded in the continental United States. Hurricane Florence broke North Carolina's tropical-cyclone rainfall record the following year for the same reason: it slowed to a crawl after landfall.

Two older storms make the same point with less wind behind them. Hurricane Georges came ashore near Biloxi, Mississippi, on Sept. 28, 1998, as a Category 2. Within 24 hours it had weakened to a tropical depression and gone nearly stationary, dumping 561 mm — 22 inches — of rain in two days. Hurricane Floyd hit North Carolina on Sept. 19, 1999, also as a Category 2, and left 415 mm, or 16 inches, of rain in a single day near Bald Head Island.

None of those four storms reached Category 5. All four are remembered, locally, for water.

A separate fix aimed at the same blind spot

Wind speed has another blind spot: it says nothing about a storm's physical size, which drives how long an area gets battered and how much storm surge and rain it can generate. StormGeo, a private weather-intelligence firm, built the Hurricane Severity Index to close that gap, scoring a storm 0 to 25 on intensity and 0 to 25 on size for a 50-point total. Run through Katrina, the index would have ranked the storm's damage potential in line with a Category 5 rather than the Category 3 the wind scale assigned — a 13-out-of-25 on intensity paired with a 23-out-of-25 on size.

"Size is very important because it matters for the aerial extent of the impact — how much time a hurricane is going to hit you. Things like storm surge, wave heights, rain amounts are greater."

Bob Weinzapfel, Hurricane Severity Index co-creator

Weinzapfel put the public-perception problem even more bluntly, describing how the country processed Katrina at the time.

"With Hurricane Katrina in 2005, we didn't have in the collective like public mindset, you know, the difference between a really large, powerful storm and a compact powerful storm. The pictures and the memories of Katrina aren't really about the winds, the wind damage. It's mostly about the storm surge."

Bob Weinzapfel, Hurricane Severity Index co-creator

Neither the ERM nor the Hurricane Severity Index has replaced the Saffir-Simpson scale in official NHC advisories, and there's a reason: a single wind number is simple, and simplicity is what makes a warning spread fast during an evacuation. But that simplicity is a trade, not a free good. Some hurricanes stall over land for reasons meteorologists are still working to fully explain — a pattern that turns a modest wind category into a catastrophic flood event, as Daybreak Wire has covered separately. And a quieter-than-usual season, the kind some forecasters have tied to a strengthening El Niño heading into 2026, doesn't mean a lower-category storm can't still park itself over one unlucky county and rewrite its rainfall record, as this year's outlook discussion has noted.

The category number will keep leading every forecast graphic, because changing five decades of habit is its own kind of storm. Reading past it — asking not just how hard the wind will blow but how long the storm plans to sit still — is the part still left to the resident, not the rating.