How NCEI records storm events

The Storm Events Database is compiled by NOAA's National Centers for Environmental Information from the monthly Storm Data reports that each National Weather Service forecast office files for its area. Every county page on this site draws on it.

Who reports an event

NCEI describes Storm Data as compiled from the National Weather Service, other federal, state and local agencies, the media, trained spotters, emergency managers and the public. An event enters the record only if someone observed and reported it and the forecast office accepted it. Dense reporting networks produce more records than sparse ones, so counts partly measure population and spotter coverage.

County or forecast zone

Events that happen at a point or along a path (hail, tornadoes, thunderstorm wind, flash floods) are recorded against a county. Events that cover a wide area (winter storms, high wind, heat, cold, drought) are recorded against an NWS public forecast zone, which may be a county, part of a county or several counties. This site attaches a zone-recorded event to a county only when the zone carries that county's name, and marks it "Forecast zone" in tables. Where zones are split ("Northern Litchfield") or grouped, the event counts toward the state only.

Event types over time

From 1950 to 1954 only tornadoes were recorded; from 1955, tornadoes, hail and thunderstorm wind; from 1996, all 48 event types NCEI uses today. Decade totals on this site rise sharply in the 1990s for that reason.

Magnitude

Hail is recorded by stone diameter in inches, often with an NWS descriptor such as "golf ball" (1.75 in) or "baseball" (2.75 in). Thunderstorm and high wind carry a speed in knots, flagged as measured or estimated. Tornadoes carry a Fujita rating before February 2007 and an Enhanced Fujita rating after; the two scales use different wind estimates for the same number.

Damage, deaths and injuries

Property and crop damage are estimates made at the time of the report and are not revised for inflation or later insurance totals. Deaths and injuries are counted as direct (caused by the event) or indirect; this site adds the two. NCEI publishes a short narrative for each event; this site does not republish narratives.

Timing

Forecast offices file Storm Data about 60 days after the month ends and NCEI publishes the compiled files about 120 days after. The "last 12 months" figures here therefore end at the newest month in the file, not at today's date.