How Accurate Are Tornado Warnings? Here’s What the Last Five Years of Data Says

Tornado warnings provide valuable, in some cases life-saving, lead time to take action, particularly in the case of strong or violent tornadoes, according to the latest data from the National Weather Service (NWS).

From 2016 through 2020, the NWS issued an average of 2,389 tornado warnings per year, according to Greg Schoor, severe weather program manager at the NWS in Norman, Oklahoma.

The number of warnings varied widely over that five-year stretch, from a low of 1,974 in 2016, a more inactive year for tornadoes, to a peak of 3,052 in 2019, one of the most active years for tornadoes on record.

But there haven't been nearly that many tornadoes. From 2000 through 2019, the U.S. has averaged about 1,233 tornadoes per year, according to data from NOAA's Storm Prediction Center.

How to Measure Performance

Before we dig into the data, let's define a few metrics meteorologists use to assess the effectiveness of warnings.

The probability of detection (POD) is a percentage chance a tornado will be accompanied by a tornado warning, either before or after the tornado first happens, based on actual statistics. The higher the POD, the better the performance.

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Lead time (LT) is the amount of time between when a tornado warning is issued and when the tornado first touches down. In general, the more LT, the more advanced warning ahead of a tornado.

The false alarm ratio (FAR) is the percentage of tornado warnings in which a tornado doesn't happen. The lower the FAR, the better the performance.

It turns out POD and FAR are related.

The NWS could "detect every tornado" by blanketing every single thunderstorm with a tornado warning. But since only a small fraction of thunderstorms spawn tornadoes, that would result in a rash of false alarms.

Conversely, you can't have a false alarm if no tornado warnings are issued. But clearly, going back to the days before life-saving tornado warnings isn't feasible, either.

So the NWS faces the reality of striking an acceptable balance between detection and false alarms.

Statistics for All Tornadoes

Tornado warning performance has been consistent over the past five years. Here are the latest statistics for all tornadoes in the U.S. since 2016, according to the NWS.

A tornado warning accompanied an actual tornado roughly 60% of the time in that five-year period, with an average lead time between 8 and 10 minutes.

Nevertheless, over the past five years, roughly seven out of 10 tornado warnings have been false alarms.

"There will always be false alarms, given the majority of warnings are issued based on radar rotation," said Schoor in an email to weather.com.

One main reason for this is relatively few thunderstorms – even supercell thunderstorms – with rotation detected thousands of feet above the ground actually produce tornadoes.

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We documented a number of difficult cases NWS forecasters run into when assessing a storm's tornado potential in an article earlier this spring.

Another reason for a false alarm is the storm may be far from the nearest Doppler radar. Since a radar beam bends slightly upward, the farther a storm is from the radar, the more of the storm's lowest levels near the ground the radar will miss.

So the path of least regret is to err on the side of caution and issue a warning when rotation is detected, rather than risk missing a tornado.

"Tornadoes are erratic and evolve too quickly for NWS forecasters to take chances on what they may or may not eventually do, if they touch down," Schoor said.

Schoor added that the NWS goal is to issue tornado warnings before the tornado develops, which provides lead time in case one touches down and immediately begins to cause damage and is a threat to life.

While a tornado may not necessarily occur, a warning means a trained, experienced meteorologist using the latest technology and forecast techniques has determined a given storm has at least a chance of spawning a tornado, or has already done so.

The bottom line is you should take shelter immediately when a tornado warning is issued, instead of wasting time looking outside to verify you're in danger, or, worse yet, ignoring the warning.

Weaker Tornadoes Are More Challenging

But simply lumping all tornadoes together doesn't tell the entire story. Warning performance depends on a tornado's intensity.

From 2016 through 2020, about 84% of all tornadoes in the U.S. were weaker tornadoes, rated EF0 or EF1 on the Enhanced-Fujita Scale.

Despite the challenge, warning performance for these weaker tornadoes appears to have ticked up in the past two years.

A warning accompanied an actual weak tornado 59% to 62% of the time from 2019 through 2020, compared to only 55% to 57% from 2016 through 2018.

The average lead time for warnings accompanying weak tornadoes was roughly 8 to 10 minutes over the past five years.

Weaker, shallower radar signatures are often the reason these weak tornadoes are tougher to pick up ahead of time.

"They are commonly difficult to detect unless very close to the radar (EF0s mainly), since most only show evidential gate-to-gate features in the lowest few thousand feet," said Schoor.

Furthermore, weaker tornadoes often develop and fizzle out quickly and can occur in environments that are otherwise more marginal for tornadic thunderstorms.

Fortunately, they're the least damaging and are rarely deadly. Only about 5% of all tornado fatalities in the U.S. from 2000 to 2019 were from weak tornadoes.

So this large number of weaker, less damaging and less deadly tornadoes can weigh down the entire tornado warning performance database.

Stronger Tornadoes, Better Warning Performance

Tornado warning performance improves for stronger tornadoes that are more likely to inflict more serious damage and claim lives. That performance boost sets in beginning with EF2 tornadoes.

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Warnings accompanied these EF2+ tornadoes much more often than weaker tornadoes, with lead times from 12 to 15 minutes from 2016 through 2020.

"The data speaks for itself, especially considering that most tornado warnings are only valid for between 30 and 45 minutes," Schoor told weather.com.

EF2 tornadoes are capable of more considerable damage – they can tear roofs completely off homes, destroy mobile homes, uproot trees and lift vehicles off the ground. NOAA's Storm Prediction Center refers to tornadoes of at least this intensity as "significant."

Schoor said these EF2 and stronger tornadoes only made up 10% to 12% of all tornadoes the past five years, but account for much more and worse damage than weaker ones.

And when considering only the three strongest tornado categories – EF3, EF4 and EF5 – tornado warning performance ramped up another notch.

While these most destructive tornadoes were a much smaller sampler size, average lead times vaulted to 18 minutes in 2020.

A 2019 study of 16,000 tornadoes over 15 years found 87% of deadly tornadoes were warned ahead of time and almost 95% of tornado deaths happened in an area under a tornado warning at the time.

So the most destructive and deadly tornadoes are usually detected and warned ahead of time.

These most intense tornadoes typically have very distinct features in Doppler radar easily recognizable by NWS meteorologists, including intense rotation, hook echoes, debris balls or indications of debris lofted thousands of feet in the air, known as tornado debris signatures.

Tornadoes this intense are capable of destroying homes (even wiping some home foundations clean), stripping bark off trees, and tossing and crumpling vehicles like aluminum foil wrap.

The Work Continues

"The NWS has been making added efforts within our warnings to alert people in the path in different ways so that we are saying what we know, when we know it," Schoor told weather.com.

That includes specifying whether a warning is issued based on radar rotation, radar confirmation of an actual tornado (a tornado debris signature), or an actual sighting of a tornado.

For situations of a confirmed, damaging tornado, the NWS adds "considerable" or "catastrophic" damage threat tags at the bottom of those tornado warnings. For particularly rare, dire situations of a severe threat to human life and catastrophic damage, the NWS will issue a tornado emergency.

(MORE: All Tornado Warnings Are Serious, But They Aren't All the Same)

And as we wrote about previously, scientists at the National Severe Storms Laboratory and NWS are working on a new paradigm known as FACETs for issuing more precise, rapidly updating warnings that could replace the current polygon-shaped warnings.

The bottom line is to take every warning seriously and act immediately.

And prior to severe weather, make sure you know where to take shelter and have multiple ways of receiving watches and warnings, including smartphones (and apps like The Weather Channel), NOAA weather radio, TV or radio.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.