False Alarm Rate for Tornado Warnings

The National Weather Service issues approximately 1,500-2,000 tornado warnings per year in the United States, of which about 75% are false alarms (no tornado occurs or is confirmed in the warned area). This means the probability of detection (POD) for tornadoes is about 75-80% (most tornadoes are warned), but the false alarm ratio (FAR) remains high at about 70-75%.

The high false alarm rate has been a persistent challenge. While it is better to warn and have no tornado than to miss a warning, excessive false alarms lead to "cry wolf" complacency: research shows that people who have experienced multiple false alarms are less likely to take shelter during subsequent warnings. This is a significant safety concern.

Forecast improvements have been substantial: in the 1980s, the POD was only about 50% and average lead time was about 5 minutes. Today, POD is about 75-80% and average lead time is 13-15 minutes. Dual-polarization radar (deployed across the US by 2013) has improved the ability to distinguish between tornadoes and other phenomena. The NWS is working toward "Warn-on-Forecast" capabilities using ensemble modeling, which could further improve both accuracy and lead time. Impact-based warnings (indicating expected severity) have been introduced to help people assess the level of threat.