Hail Methodology
How hail values shown in address, hail explorer, and PDF weather reports are derived.
The rule in one sentence
The estimate is the MRMS MESH radar value in the radar grid cell that contains the property; a ground hail report (NWS Local Storm Report) within 3 miles can set a higher value, capped at the largest radar value within 3 miles.
How SAHE-2 works
The formula: estimate = greater of (MESH in the radar cell that contains the property) and (smaller of: largest ground report within 3 miles, largest radar value within 3 miles). Only hail reports on the same storm date count; when several are within 3 miles, the largest is used; the cap is the largest radar value within 3 miles on that date, so with no radar hail within 3 miles a ground report cannot set a number.
MESH (Maximum Estimated Size of Hail) is a radar-derived estimate of the maximum hail size reaching the ground, calibrated against ground hail reports, on a roughly 1-km grid from NOAA's Multi-Radar Multi-Sensor system (Witt et al. 1998). Only the radar cell that contains the address sets a radar-based number; nearby cells are shown as context with their distance, never counted at the address. The MESH archive stores cells of 0.50 in. and larger, so a value below 0.50 in. at the property means the radar estimate in that exact cell was below half an inch.
Reported hail sizes are printed unrounded. A radar-set value such as 0.59 in. is the MESH algorithm's output for the property's cell, not a value rounded toward a size bin; a report-set value is the size in that ground report, within the 3-mile radar cap. Treat the size class (sub-severe, severe, significant) as the robust signal and the decimal as the point value.
Radar coverage is not uniform: in parts of the intermountain West the nearest NEXRAD beam overshoots low storms, and at long range everywhere the beam samples higher in the storm — both reduce hail-detection accuracy, so an absence of radar-estimated hail in radar-sparse areas is weaker evidence than the same absence near a radar site.
NWS severe thunderstorm warnings document forecaster expectations in real time, and Local Storm Reports capture what people measured or observed on the ground; warnings, and reports farther than 3 miles, are shown as context and do not change the estimate.
What hail values mean — and don't
MESH is a radar-derived estimate of the maximum hail size at the ground; melting, wind, and storm motion can make individual stones larger or smaller. A large MESH value near an address raises the likelihood of hail at the address; it does not show impact, and a lack of reports does not rule out hail. StormAuditor never converts a radar estimate into a damage conclusion.
Where hail data appears
The statewide Hail Explorer maps swaths by date, Hail Report by Address and Hail History cover single properties, and the free Hail Size Guide visualizes sizes from pea to softball. Sources are documented on Data Sources; caveats on Limitations.
Primary source documentation
Every dataset StormAuditor uses is public and officially documented — verify any layer at the source:
- NWS MRMS documentation — the Multi-Radar Multi-Sensor system behind MESH radar-estimated hail size, with the algorithm lineage in Witt et al. (1998), Weather and Forecasting 13(2), doi:10.1175/1520-0434(1998)013<0286:AEHDAF>2.0.CO;2.
- NOAA NCEP RTMA/URMA products — the real-time and un-restricted mesoscale analyses that serve as the NWS analysis of record for surface wind.
- NOAA HRRR model — the High-Resolution Rapid Refresh fields that supplement recent dates.
- NWS ASOS program — the automated airport-grade surface observing network supplying measured gusts (with AWOS operated under FAA standards).
- SPC severe weather reports and NWS Local Storm Reports — spotter, public, and emergency-management reports of hail and wind.
- NCEI Storm Events Database — the official post-storm federal record.
- FEMA disaster declarations (Data.gov) — federal recognition of major events.