Data Sources
The public weather data StormAuditor uses, what each source is best for, and the limitations to keep in mind when reading it.
NCEI Storm Events Database
Reported · NOAA National Centers for Environmental Information
- Used for: The official archive of significant storms (wind, hail, tornado, flooding, tropical) behind the storm history on every state and city page, back to 2010.
- Resolution: County or forecast zone, with narratives and point coordinates where available.
- Updates: Monthly releases.
- Limitations: Published about 75–90 days after the end of each month (per NCEI). An event is recorded when reports reach the local NWS office, so coverage varies by county.
National Weather Service Alerts
Warning (alert) · NWS / api.weather.gov
- Used for: Warnings, watches, and advisories (severe thunderstorm, tornado, flood, hurricane, wind) shown over a property and on the maps.
- Resolution: Polygon (storm-based) or UGC county/zone.
- Updates: Real time.
- Limitations: A warning alerts an area to an expected hazard; it is not a report that the hazard reached every address inside the polygon.
Storm Prediction Center Reports
Reported · NOAA / NWS SPC
- Used for: Daily preliminary wind, hail, and tornado reports from trained spotters and NWS offices.
- Resolution: Point reports with city, county, and magnitude.
- Updates: Daily.
- Limitations: Preliminary and unfiltered; may contain duplicates, estimates, or later corrections.
Iowa Environmental Mesonet Local Storm Reports
Reported · Iowa State University / NWS Local Storm Reports
- Used for: Spotter, public, and emergency-manager reports relayed by NWS offices, with magnitudes (hail size in inches, wind in mph) — measured reports feed the wind and hail estimates.
- Resolution: Point reports with latitude and longitude.
- Updates: Real time to near real time.
- Limitations: Reports depend on spotter coverage and may be delayed, missing, or corrected.
ASOS/AWOS Station Observations
Observed · FAA / NWS via Iowa Environmental Mesonet
- Used for: Measured sustained wind, gusts, and daily peak gusts at automated airport weather stations — the measurements that correct the wind estimates.
- Resolution: One point per station; the nearest station may be many miles from a searched address.
- Updates: Sub-hourly to hourly.
- Limitations: Stations are sparse in rural areas and away from airports. Observations describe the station, not the address.
NOAA MRMS Hail Products
Estimated (modeled) · NOAA / NSSL Multi-Radar Multi-Sensor
- Used for: Radar-derived Maximum Estimated Size of Hail (MESH) — the basis of every hail estimate and hail swath.
- Resolution: About 1 km grid, updated at radar cadence.
- Updates: Every few minutes when active; archived thereafter.
- Limitations: A modeled estimate from radar signatures, not a measurement of stones on the ground; values can over- or under-estimate actual hail.
NOAA URMA / HRRR (Wind Fields)
Analyzed (gridded) · NOAA
- Used for: URMA, the NWS analysis of record, is the wind background at an address (its grid cell over the property); HRRR, a 3 km weather model, is used where no analysis exists and to classify the storm type.
- Resolution: URMA 2.5 km analysis; HRRR 3 km model.
- Updates: URMA hourly, run about 6 hours after the hour to take in late observations; HRRR hourly.
- Limitations: Values are an analyzed or modeled field over a 2.5 km (URMA) or 3 km (HRRR) cell, not an on-site measurement.
NEXRAD National Radar Composite
Observed · NWS radar network via Iowa Environmental Mesonet
- Used for: The live animated national radar mosaic and full-day storm replay (any date back to 2012) on the Live Radar & Storm Replay page, plus the SPC Day-1 categorical outlook layer shown alongside it.
- Resolution: National composite imagery in 5-minute frames.
- Updates: Every 5 minutes live; archived frames back to 2012.
- Limitations: A radar mosaic shows reflectivity aloft — precipitation type and what reaches the ground can differ, and beam coverage varies with distance from each radar site.
NWS Gridded Point Forecast & GOES-19 Imagery
Modeled · NWS / api.weather.gov · NOAA NESDIS
- Used for: The free 7-Day Forecast page (hourly and day/night point forecasts with active alerts for any U.S. location) and the GOES-19 satellite deck on the radar page.
- Resolution: About 2.5 km forecast grid; GOES CONUS imagery refreshed every 5 minutes.
- Updates: Forecasts as NWS offices publish them; satellite every 5 minutes.
- Limitations: A forecast is a prediction issued by NWS meteorologists, not an observation; satellite imagery shows cloud tops, not surface conditions.
NOAA Climate Data Online (GHCND)
Observed · NOAA NCEI
- Used for: Daily peak wind, precipitation, and hail-occurrence flags from surface stations.
- Resolution: Daily summaries at station locations.
- Updates: Daily or near-daily, by station.
- Limitations: Station coverage varies. Daily summaries keep only the reported daily metrics, not every intra-day peak.
OpenFEMA Disaster Declarations
Reported · Federal Emergency Management Agency
- Used for: Federal disaster declaration context (hurricanes, severe storms, flooding) on state storm dashboards and in date-of-loss ranking.
- Resolution: County level.
- Updates: As FEMA publishes.
- Limitations: Declarations record federal designation decisions, not individual property impact.
How the sources are combined
Address-level wind estimates start from the URMA analysis at the property and are corrected with measured station gusts and measured storm reports under the SAWE-2 wind methodology. Hail estimates start from MRMS MESH over the property, with ground reports within 3 miles under the SAHE-2 hail methodology.
State and city pages combine NWS warnings, storm reports, station gusts, and NCEI records into deduplicated storm days for all 50 states — browse U.S. storm data by state. Every source has resolution and coverage limits; see Limitations, Data Disclaimers, and the data license for reuse.
Related: Wind methodology · Hail methodology · Limitations · U.S. storm data by state