Tropical Cyclones

Hurricane chances on Peril Report come from NOAA IBTrACS (International Best Track Archive for Climate Stewardship). Storms are counted from 1950 onward when the U.S. wind speed reaches hurricane force (64 knots, Category 1) or major-hurricane force (96 knots, Category 3).

A storm counts if its center passed within 50 miles of the Census county that contains the address. Unique storm IDs are used so one hurricane is not counted twice. The annual chance is a Poisson estimate from that history: 1 − exp(−count / years). The rating in that section is FEMA’s National Risk Index tropical-cyclone expected annual loss rating for the county, which is separate from the IBTrACS strike chances. The percentile bar is the tropical-cyclone expected annual loss rate compared with other U.S. counties. The text also prorates historic and modeled building-loss ratios over 100 years.

For Gulf and East Coast counties that Colorado State University covers, Peril Report also shows that year’s Tropical Cyclone Impact forecast: the probability of a hurricane and of a major hurricane tracking within 50 miles. Those seasonal odds are adjusted from 1880–2020 climatology using CSU’s latest Atlantic forecast. They are omitted when CSU does not publish a forecast for the county.

Tropical cyclone is the generic name for a warm-core storm with a closed circulation. A tropical depression has winds below 39 mph (34 kt), with rain and flooding the main threats. At 39–73 mph (34–63 kt) it is a tropical storm and is given a name. At 74 mph (64 kt) or higher the same storm is called a hurricane in the Atlantic and Northeast Pacific, a typhoon in the Northwest Pacific, and a cyclone (or severe tropical cyclone) in the South Pacific and Indian Ocean. Super typhoon is a regional term for the strongest Northwest Pacific storms. A subtropical cyclone is a hybrid of tropical and extratropical storms; it can still bring damaging wind, rain, and surge, and may later become fully tropical.