海角大神

On April 27, 2011, TV weatherman James Spann warned viewers that a 鈥渓arge, multiple-vortex tornado鈥� was bearing down on Tuscaloosa, Ala.

鈥淵ou should have been in your safe place 20 minutes ago,鈥� he said as a camera tracked the funnel clouds, 鈥渂ut if by chance you鈥檙e hearing me at the last minute on the radio, get into a safe place right now!鈥�

Despite these warnings, 252 Alabamians died that day, victims of the fourth-deadliest tornado season in US history. Mr. Spann, who still serves as chief meteorologist for Birmingham鈥檚 WBMA-TV, drew a clear lesson from the tragedy.

鈥淲hat we learned that day is that physical science could not have been better,鈥� he remembers in a phone interview with 海角大神, 鈥渂ut what maybe we don't understand is the social science part of it.鈥�

The nature of twisters makes their exact timing and location difficult to predict. But since 2011, sociological research and storms like Alabama鈥檚 have spurred a shift in forecasting practices. Meteorologists now aim to reduce false alarms 鈥� even at the risk of missed storms and delayed warnings 鈥� in the hopes that residents will heed the warnings they do issue.

鈥淭here is a recognition that the false alarm rate is something that we need to take into consideration,鈥� explains economist Kevin Simmons, who researches the statistics of natural disasters.

A high-stakes trade-off

Tornado forecasters鈥� performance gets measured by three key numbers:

• False-alarm rate (FAR), the percentage of tornado warnings issued for storms that never touch down
• Probability-of-detection (POD), the percentage of warnings that accurately predict tornadoes
• 鈥淟ead-time,鈥� the minutes between when a warning is issued and when the storm touches down

鈥淪ince around 2011, both tornado lead-time and detection have gotten worse. But the false alarm rate has improved (decreased),鈥澛燭he Washington Post reported聽on April 20.

Harold Brooks, a senior scientist at the National Severe Storms Laboratory, explains that he and other forecasters face a trade-off: Longer lead-times and higher POD鈥檚 versus a lower false alarm rate.聽As forecasters wait for more evidence 鈥� from radar, spotter teams, and other sources 鈥� before sounding the alarm, they issue fewer false ones. But they鈥檙e also more likely to miss some, and give residents less time to take cover.

鈥淥ur skill hasn鈥檛 changed鈥� in recent years, he tells the Monitor over the phone, but they have employed 鈥渁 higher threshold for warning.鈥�

Despite the obvious risks of missing a storm or cutting down on warning time, Dr. Simmons, a professor of economics at Austin College, in Sherman, Texas,聽says there鈥檚 merit in reducing false alarms. 鈥淚f a tornado strikes an area with a higher-than-average false alarm rate, it's more likely that that tornado would generate fatalities [than in an area] with lower-than-average false alarm rates,鈥� he tells the Monitor.

In 2009, he and economist Dan Sutter, currently at Alabama's Troy University,聽assessed the trade-off between false-alarm rates and POD, finding 鈥渟trong [statistical] evidence that a higher local, recent FAR significantly increases tornado fatalities and injuries.鈥� As residents hear one false alarm after another, the 鈥渃ry-wolf effect鈥� takes hold, and they鈥檙e less disposed to heed the one warning that could save their lives.

Two years after they published these findings, the devastating 2011 season bore them out. In Birmingham, where聽almost 80 percent of warnings聽were false, Spann insists, 鈥渢here's no doubt in my mind a high false alarm ratio in 2011 killed people."

The National Weather Service (NWS) reached a similar conclusion in Joplin, Mo., which suffered one of that year鈥檚 worst twisters. It found that 鈥渢he perceived frequency of siren activation (false alarms) led a large number of [residents] to become desensitized or complacent to this method of warning.鈥� Dr. Brooks cites this survey as a key reason behind the new focus on false-alarm rates. It's since edged down,聽from 73 percent in 2011 to 70 percent in 2015.

Sure enough, the same period saw forecasters detecting fewer storms, with POD going from聽75 percent to 58 percent. But Eric Waage, director of emergency management for Hennepin County, Minn., says not all of the missed storms give cause for concern.

鈥淭he most problematic thing we have are these small ... EF-0 tornadoes,鈥� he tells the Monitor via phone. These are the weakest storms on the Enhanced Fujita scale that measures tornado strength in the United States and Canada based on the damage they cause. Spann explains that, because of their short duration, 鈥渢rying to warn for those suckers ... is like playing Whack-a-Mole."

These days, forecasters may find it harder to get enough evidence to warn against these storms. But they also have more room for error. EF-0 and EF-1 storms occasionally make the NWS鈥檚 killer tornado list, but most deadly storms are EF-2 and up. Mr. Waage says that 鈥渋n most places in our state, those small ones aren't really a huge problem. They're hitting cornfields and forests.鈥�

The third key tornado metric, lead-time, has also dropped amid the NWS's focus on false alarms, from 15 minutes in 2011 to just eight in 2015. But that national trend obscures local variations. Spann remembers that April 27, 2011 saw lead-times as high as 40 minutes. Since that tragic day, the local NWS office's average lead-times have dropped slightly 鈥� and false alarms have plunged. 聽

鈥淲e've got to get the FAR down,鈥� Spann says, 鈥渁nd if we lose a little lead-time, I don't have any problem with that.鈥�

'We are better than that'

A 2011-caliber season hasn鈥檛 yet tested this new mind-set, and Brooks, speaking with the Post鈥檚 Jason Samenow, cautioned that social science research on this topic is 鈥渄ifficult to conduct and often inconclusive.鈥�

But it could provide lifesaving guidance for the Midwest鈥檚 鈥淭ornado Alley鈥� and the Southeast鈥檚 鈥淒ixie Alley,鈥� at least until the next major leap in meteorology.

Last week, President Trump signed a bill aiming to extend tornado prediction time beyond one hour. But Brooks cautions that, to lengthen lead-times without raising too many more false alarms, 鈥渨e need to move a long way from that [current] skill line, and that's hard.鈥�

He says it鈥檚 happened before, thanks to Doppler radar and other innovations in the 1990s. One government program, Warn on Forecast, aims to repeat the feat with numerical models, but practical applications remain 鈥測ears away.鈥�

For the moment, forecasters are fine-tuning the balance of lead-time, POD, and false-alarm rate, and authorities are trying to get more people to respond to warnings. 鈥淭he missing link still is that public awareness,鈥� says Hennepin County鈥檚 Waage. To minimize confusion when the skies darken, for instance, he explains that Minnesota has recently standardized its siren procedures.

Birmingham, Ala., has also seen a push for greater tornado awareness since 2011, and Spann thinks it鈥檚 heading in the right direction.

鈥淭here have been too many funerals on my watch in 38 years,鈥� as a meteorologist, he reflects. 鈥淎nd we are better than that. And now, working with the social science people, I think that's going to make a huge difference.鈥�

[Editor's note:聽This article has been updated to correctly state the university where Dan Sutter currently works]