HailMapr field guide
How to read HailMapr's data—and its limits
A useful storm map should make clear what was observed, what was estimated and what still needs checking on the ground. HailMapr brings these sources together for storm research and field planning, without treating them as interchangeable evidence.
Radar-derived hail swaths
HailMapr uses radar-derived hail data, including NOAA's Multi-Radar/Multi-Sensor (MRMS) products, to build mapped storm footprints and estimated hail-size bands. MRMS combines information from multiple radar sources; it is not a person measuring hail at every location.
A mapped swath summarizes a storm's estimated footprint over its processing window. Grid resolution, radar coverage, beam height, storm structure and processing choices can affect the estimate. A boundary on the map is not a surveyed property boundary or an exact line between damaged and undamaged roofs.
Source background: NOAA National Severe Storms Laboratory — MRMS.
Ground Impact versus the broader hail footprint
Ground Impact is HailMapr's additional radar-derived estimate of likely ground-level hail exposure. Its size bands and persistent versus isolated signal display help teams prioritize areas for inspection. It is not a replacement for on-site findings.
The underlying hail signature and conditions between the radar sampling height and the ground can differ. Do not interpret a displayed maximum as a measurement at a specific roof. Use the feature guide and map legend to understand each available layer before comparing values.
Observed reports and warnings
Available National Weather Service and Storm Prediction Center reports add ground-observation context. Hail size may be measured or estimated, and report locations and times have their own precision limits. Reports are unevenly distributed: populated or well-observed areas may have more reports than rural areas.
A lack of reports does not prove a lack of hail. Likewise, a report within a city does not mean every property in that city experienced the same hail. Warning polygons describe a risk area, not a confirmed damage footprint.
Source background: SPC storm reports and National Weather Service thunderstorm safety.
Historical radar replay and timing
The radar replay overlay shows archived reflectivity through the Iowa Environmental Mesonet. Reflectivity is not hail size. Its strongest colors can be offset from a hail-estimate layer, particularly when the source timestamps, resolutions or products differ.
Read the timestamp before making comparisons. Replay displays Central Time; records from other sources may use UTC or another stated time zone. Daily map processing windows can extend beyond midnight. Archived data may have gaps, and active maps may change as additional data arrives.
Property research and responsible use
Use storm history reports to identify dates worth investigating, then document the actual inspection separately. Weather data cannot determine the age or cause of every mark on a roof, policy coverage or whether an insurance claim will be approved.
Start with a location you know. Compare the map, source reports and field findings. If something looks inconsistent, send HailMapr the storm date, location and layer so it can be reviewed. Never rely on historical replay for immediate safety decisions; follow current NWS warnings and local emergency instructions.