How NOAA's MRMS Rainfall Estimates Work
What the Multi-Radar Multi-Sensor system is, how it turns radar echoes and gauge reports into hourly rainfall on a 1 km grid, and what its passes and flags mean.
Published September 29, 2026 · Raindex
What MRMS is
MRMS (Multi-Radar Multi-Sensor) is a NOAA system, developed at the National Severe Storms Laboratory, that merges data from many radars, surface and upper-air observations, satellites and forecast models into national weather products (NSSL).
For rainfall, its output is a quantitative precipitation estimate: how much water fell in each grid cell over an hour and longer windows. It mosaics about 180 operational radars in the US and Canada at 1 km and 2-minute resolution (Zhang et al., 2016).
From radar echo to inches of rain
A radar does not see rain directly. It measures how much energy comes back from drops, and an algorithm converts that into a rain rate. The right conversion depends on the kind of precipitation, so MRMS classifies each cell (for example stratiform rain, convective rain, snow or hail) and applies a relationship suited to it.
Rates are then accumulated into hourly totals and longer windows. Where radar coverage is poor, such as behind mountains, other inputs help fill the gap.
Radar-only, then gauge-corrected
MRMS publishes more than one estimate for the same hour. A radar-only product comes out first. Later passes incorporate rain-gauge reports, correcting the radar estimate toward what gauges in the area measured and weighting each gauge by how close and how dense the network is (Zhang et al., 2016).
Raindex reads the gauge-corrected pass for its totals. That pass runs roughly an hour and a half behind real time, which is why the most recent hour on a reading can be marked provisional and then revised.
- Earlier pass: quicker, built from the gauge reports that have arrived so far; it will change.
- Later (gauge-corrected) pass: slower, uses more gauge reports, the one to trust for totals.
- Gauge influence: MRMS also publishes how strongly gauges shaped each cell's value, which Raindex uses to separate gauge-adjusted from radar-only readings.
What MRMS cannot tell you
It is an estimate for a cell, not a measurement at your door. Frozen precipitation is harder for radar than rain, which is why Raindex reports snow only as liquid equivalent and labels it. And coverage ends at the edge of the radar network: Raindex covers the lower 48 United States only.
The data is public: NOAA publishes MRMS openly. NOAA does not endorse Raindex or this site.
See the last 24 hours for any ZIP code with the free rainfall lookup, or keep a labelled, day-by-day record for the places you care about in Raindex.
Common questions
What resolution is MRMS rainfall?
About 1 km, with the radar mosaic updated every 2 minutes and rainfall accumulated into hourly and longer totals.
How long after rain falls is MRMS data available?
The radar-only estimate comes quickly. The gauge-corrected pass Raindex relies on arrives roughly an hour and a half after the hour it describes.
Does MRMS cover Alaska or Hawaii?
Raindex uses the CONUS (lower 48) MRMS grid only, so readings are available for the contiguous United States.
Rain totals by city
More guides
- Why Your Rain Gauge and a Rainfall App DisagreeRadar rainfall and a backyard gauge measure different things in different places. Here is where the gap comes from, and how to compare them fairly.
- How Much Water Is an Inch of Rain?An inch of rain on an acre is about 27,154 gallons. The same inch on a roof, a lawn or a square mile, with the arithmetic shown.
- Gauge-Adjusted, Radar Only, Provisional: What Rainfall Labels MeanEvery Raindex reading carries a label saying how the number was made. What gauge-adjusted, radar only, provisional and no reading mean, and why there is no accuracy percentage.