How NOAA SPC Storm Reports Work (And Why They Say “Preliminary”)
If you’ve visited FindTheTornado.com or explored the NOAA Storm Prediction Center website, you’ve noticed that all storm reports carry the label “Preliminary.” This isn’t a caveat buried in fine print — it’s a fundamental part of how storm reporting works in the United States.
What Is the SPC?
The Storm Prediction Center (SPC) is a branch of NOAA’s National Weather Service located in Norman, Oklahoma. It operates 24 hours a day, 7 days a week, and is responsible for:
- Issuing convective outlooks (predicting severe weather probability 1–8 days out).
- Issuing tornado watches and severe thunderstorm watches.
- Collecting and publishing Local Storm Reports (LSRs) — observations of actual severe weather events.
The SPC’s daily storm report pages (like `spc.noaa.gov/climo/reports/today.html`) are one of the most valuable and widely-referenced public datasets in U.S. meteorology.
What Goes Into a Storm Report
A storm report (or LSR) is filed when someone observes or records evidence of severe weather. Sources include:
- Storm spotters — trained volunteers with SKYWARN and other programs who report conditions to local NWS offices.
- Emergency managers — county and city officials who observe and report to NWS.
- Law enforcement and fire departments — often the first to observe and report damage on the ground.
- NWS meteorologists — who may observe storm features via radar or receive reports from field personnel.
- Automated surface observing systems (ASOS) — airport and weather station instruments.
Reports are transmitted to the local NWS Weather Forecast Office (WFO), which relays them to the SPC. The SPC aggregates these into the publicly accessible CSV files that FindTheTornado.com reads every 15 minutes.
The Convective Day
A crucial detail: SPC storm reports run on a “convective day,” not a calendar day. The SPC day starts at 12:00 UTC (approximately 7 AM CDT or 8 AM EDT) and ends at 12:00 UTC the following day.
This is why FindTheTornado.com shows an empty feed early in the morning — the `today_torn.csv` and `today_hail.csv` files are legitimately empty until reports start coming in around or after noon UTC.
Why “Preliminary”?
Storm reports are called preliminary because they reflect initial observer reports that haven’t yet been fully verified. Several aspects may change after publication:
EF ratings are almost always unconfirmed initially. An EF scale rating for a tornado requires a physical damage survey by NWS meteorologists — walking or flying the tornado path, examining damage indicators, and applying the EF scale damage indicators. This can take anywhere from hours to weeks depending on the scale and accessibility of the tornado path.
Hail sizes may be approximate. Spotters may estimate hail size by comparison (e.g., “golf ball size”) rather than measuring with calipers. While this is generally accurate, sizes can vary.
Locations may be approximate. The lat/lon coordinates in SPC reports are estimates based on spotter location and damage reports, not always the exact point of impact.
What This Means for You
If you experienced a tornado or severe hail event and the SPC data shows it as preliminary, that’s normal. For insurance and legal purposes, the finalized NWS Local Storm Report is the authoritative record. You can request it from your local NWS forecast office.
For general awareness and early damage assessment, the preliminary SPC data is valuable and timely.
Sources: SPC About Page, NWS LSR Documentation
*FindTheTornado.com is an educational resource. We do not perform inspections, repairs, or insurance work.*