A Quantitative Precipitation Estimate (QPE) is an estimate of how much precipitation has already fallen, built from radar, rain gauges, satellite data or a blend of them. Where a quantitative precipitation forecast looks ahead, a QPE looks back at what actually fell.
From radar echoes to rainfall
Radar does not measure rain directly. It measures returned power, expressed as a reflectivity factor, and converts that to a rain rate through an empirical relationship between reflectivity and rain rate.
Adding those rain rates up over time gives an accumulation. Rain gauges are then used to adjust the radar estimate, combining the gauges' point accuracy with the radar's wide coverage.
Three kinds in MRMS
The Multi-Radar Multi-Sensor (MRMS) system offers radar-only QPE, which sums radar rain rates. Its 1-hour total updates every 2 minutes, and longer totals update hourly.
Radar QPE with gauge bias correction compares the radar estimate with quality-checked gauge reports and spreads each difference to nearby grid points. Multi-Sensor QPE fills gaps in poor radar coverage with gauges and model rainfall, with a first pass after about 20 minutes and a second, gauge-richer pass after about 60 minutes.
All three come in 1, 3, 6, 12, 24, 48 and 72 hour totals on a 1 km grid.
Known weak spots
In the melting layer, or bright band, reflectivity is enhanced, and National Weather Service (NWS) guidance on the legacy WSR-88D rainfall algorithm notes that rain rates estimated there can be up to five times too high. Hail also returns strong echoes, so that legacy algorithm ignored reflectivity above a set threshold on the assumption that it came from hail.
Far from the radar, Earth's curvature lifts the beam above the ground and the beam widens, so it can overshoot shallow rain and smooth out small features. Mountains block the beam at many Western radars, and sparse gauges there limit the correction.
Using it during an event
QPE answers how much has fallen, which forecasters weigh against flash flood guidance, the rain a basin can take before flooding. MRMS even offers a ratio of QPE to flash flood guidance for that comparison.
Check gauge reports where they exist, and expect the later multi-sensor passes to differ from early radar-only totals.