Rainfall rate is the amount of liquid precipitation falling per unit time at a given location, most often expressed in inches per hour or millimeters per hour.
How it is measured and estimated
At the surface, a tipping-bucket or weighing gauge measures accumulation over short intervals, and dividing by time gives a rate. Above the surface, radar estimates rainfall by relating measured reflectivity, differential reflectivity, and specific differential phase to expected drop size distributions. Dual-polarization radar and multi-radar multi-sensor (MRMS) products merge these approaches into gridded quantitative precipitation estimates (QPE).
Different sensors report at different cadences. A radar-derived Surface Precipitation Rate might update every couple of minutes, while a gauge reports at longer intervals. Comparing them requires matching the time window and the effective sampling area.
How forecasters use it
For flash flooding, rainfall rate is one of the primary ingredients. The ingredients-based method used by NWS forecasters combines rate with storm motion and duration to estimate accumulated rainfall over a specific area. A slow-moving storm producing several inches per hour over an urban basin is a very different threat from a fast-moving storm producing the same rate over open country.
Forecasters also compare current rates against local precipitation return periods. A rate that produces a one-hour total near the local 100-year value is a strong flash flood signal, particularly if the same cells are training over the same watershed. Rate maps and short-term accumulation grids from MRMS are common tools for that comparison.
Important limits
Radar-estimated rates carry error. Beam blockage, hail contamination, evaporation of rain below the beam, and drop size assumptions can all bias the estimate high or low. Forecasters use gauge and dual-pol adjustments to reduce this error but do not rely on any single sensor.
A high peak rate at one point also does not describe the storm. Storm-total rainfall is driven by how long a location remains under high rates, which depends on cell motion, boundary interactions, and whether new cells train over the same ground. Rate is one input to a storm-scale picture, not the whole picture.