Five Thirty Eight recently ran a post about the false alarm rate of tornado warnings. Tornado warnings fail to verify (i.e. have no tornado) approximately 75% of the time, a number that has held steady for years. This comes as no surprise to meteorologists, and probably not to the general public. What’s disappointing about the article is that it doesn’t address the reason that the false alarm rate hasn’t improved: because it’s not a priority.

The ideal case, of course, is a false alarm rate of zero. While the article quotes the reasoning (“you would rather have a warning out there and have it miss than have an event and not have one out there”), it doesn’t explain why that reasoning leads to a high false alarm rate.

The first reason is that an emphasis on maximizing detection means that in questionable scenarios, forecasters will lean toward issuing a warning instead of not. I’ve been in an office when an unwarned tornado has been reported. The forecasters are not happy about that. They take the National Weather Service mission of protecting life and property seriously. The impact of a false alarm (inconvenience and lost productivity) outweighs the potential loss of life from a missed event.

After inadvertently posting this when I meant to save the draft, a friend commented that the “ideal FAR is actually non-zero if you want lead time.” This leads to the second reason an emphasis on detection increases the false alarm rate. Issuing a tornado warning seconds before the tornado hits is of limited utility. People in the warned area need time to move to safety. The article does point out that lead time has increased steadily for the past few decades. But the more lead time you have, the more likely it is that a warned storm won’t produce a tornado. Tornadoes are exceptional events.

There’s a balance between detection rate and lead time on one side and false alarm rate on the other. Like a seesaw, lowering one side raises the other (if you play with the signs on the numbers, that is). Prudent policy focuses first on detection and then on lead time, so the false alarm rate has to suffer. Improvements in technology and science will hopefully move the fulcrum such that we can lower the false alarm rate without reducing the lead time or probability of detection too much.