Weather alerts in Yellowstone County, Montana, 2011

By month

4 of 7 tornado warnings covering Yellowstone County in 2011 had a tornado report inside them; 5 of 5 tornado reports fell inside a warning, on average 24 minutes after it went out.

Yellowstone County, 2011

Each office's whole area, 2011

An office's figures cover every county it serves, including across state lines.

Verified by the Iowa Environmental Mesonet's Cow, which matches each storm-based warning with the local storm reports inside its polygon while it was in force, within 15 km of its edge: a tornado warning counts as verified on a tornado report, a severe thunderstorm warning on hail of 1 inch or more, a gust of 58 mph or more, or wind damage, a flash flood warning on a flash flood report. The false alarm ratio is the share of warnings with no such report; "inside a warning" is the probability of detection, the share of reports that fell inside one; lead time runs from the warning's issue to each report inside it, counting zero for a report that came first. A warning with no report is not proof nothing happened: reports come from spotters, the public and damage surveys, and are thin where few people live.

1. June 7, 201110alerts
2. July 8, 20119alerts
3. July 14, 20118alerts
4. August 29, 20118alerts
5. July 4, 20116alerts
6. August 5, 20115alerts
7. July 3, 20114alerts
8. July 15, 20114alerts
9. July 16, 20114alerts
10. May 24, 20113alerts

Alerts in force nowTornadoes on recordForecastThe radar archive20102012

National Weather Service watches, warnings, advisories and statements from the Iowa Environmental Mesonet's VTEC archive, which reconstructs tornado, severe thunderstorm, flash flood and marine warnings back to 1986 from the NWS text archive. Storm Prediction Center watches and mesoscale discussions from SPC's archive through the Mesonet. County centers from the US Census Bureau. Warning verification from the Mesonet's Cow. Rebuilt daily.