Rainfall accumulation is the total depth of rain that falls at a place over a stated period, usually reported in inches in the United States. A total means little without its time window, so a useful report always says whether it covers 1 hour, 24 hours, several days or an entire storm.
How totals are measured
Gauges provide the ground truth. Automated Surface Observing System (ASOS) stations use heated tipping-bucket gauges or all-weather accumulation gauges, and volunteers read manual gauges by hand.
One volunteer network is the Community Collaborative Rain, Hail and Snow Network (CoCoRaHS), whose observers use 4-inch-diameter gauges, according to a National Weather Service (NWS) training slideshow. It began at the Colorado Climate Center at Colorado State University in 1998, partly because of the Fort Collins flood the year before, and the NWS is among the many users of its data.
Radar fills the gaps between gauges with estimates. The NWS retired its older radar Storm Total Precipitation product in 2023 in favor of dual-polarization rainfall estimates, and some River Forecast Centers publish totals that blend gauge reports with radar estimates.
24-hour totals versus storm totals
A 24-hour total depends on when the clock starts. CoCoRaHS training prefers a 7 AM daily reading, and the Middle Atlantic River Forecast Center's daily maps cover periods ending at 7 AM EST, or 8 AM EDT (1200 UTC).
Official climate sites instead use a midnight-to-midnight local standard time day, which runs 1 AM to 1 AM during daylight saving time.
A storm total spans the whole event, which may cross two or three observation days. An overnight storm can be split between two daily totals, so neither daily number shows how much fell in the event.
Reading totals with care
Totals can vary sharply over short distances, and radar estimates carry errors that gauges help correct. Before comparing reports, check the period each covers and whether it came from a gauge or a radar estimate.
Accumulation says how much fell, not how fast it fell. The rainfall rate and the condition of the ground decide much of whether a given total floods.