Why Some Tornadoes May Not Get Warned Even as Technology Improves

The National Weather Service issues hundreds, if not thousands, of warnings ahead of likely or already occurring tornadoes, but occasionally, some tornadoes slip under the radar.

In fact, the NWS missed more than 25 percent of tornadoes from October 2007 to September 2016, according to the National Centers for Environmental Information and Steven Nelson at the NWS office in Peachtree City, Georgia. That number climbed to 39 percent or higher in 2013.

Successful detection of tornado events, or the probability of detection, has been in a slump since 2013, falling as low as 57 percent.

Lead time – the time between warning issuance and when a tornado arrives in a given location in the warning – has also decreased during this period. 

Changes in both methodology of warning tornadoes and technology have affected how tornadoes can be missed in recent years.

Changes in Methodology

It's important to note that several changes in the NWS occurred in 2013 which turned several trends.

First, the probability of detection definition was altered to allow for minute-by-minute updates in observation.

Tornadoes are now broken down into one-minute segments for verification purposes. Each of these segments is given a "hit" or "miss" label. Any time tornadoes spend outside a tornado warning or any tornadoes that form or last outside of tornado warning times will be counted as misses, which may lower the rate at which tornadoes are successfully detected using this metric. 

Secondly, the NWS adopted a goal of lowering the "cry wolf" perception in the early 2010s. This is the public perception that if a tornado didn't hit one time during a previous tornado warning, it definitely won't happen next time. The NWS calls this metric the "False Alarm Ratio".

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Lessening false alarms is difficult because forecasters wait until higher confidence exists before issuing warnings in order to decrease the chance they could be wrong.

NWS forecasters now wait until a tornado has begun before issuing a warning more often than prior to this new policy, according to Harold Brooks in a presentation given at the 28th Conference on Severe Local Storms in 2016.

In the same presentation, Brooks said NWS offices issue warnings that are shorter-fused and cover less area than before 2012.

In addition, Brooks showed lead times – the time between when a warning is issued and when a tornado reaches a location – are at record lows dating back to 1986 for the weakest and most common types of tornadoes. This means you aren't getting as much time to get into your shelter or safe place as you did a few years ago.

Researchers are currently developing a way to monitor and forecast severe weather, including tornadoes, using satellites. The technique, called ProbSevere and developed in part at the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin, may become another method that helps the warning process and could serve as a backup if radars fail.

This technology will add to confidence levels for forecasters when it is perfected since ProbSevere gives a readout of the probability severe weather is occurring and the ingredients with which a particular storm has to work.

One other trend that could be to blame for the increase in the number of "missed" tornadoes is weaker twisters are reported afterward more than ever before.

Storm chasers and citizens on social media are taking more photos and video of short-lived tornadoes that would've otherwise been missed in the past. Emergency managers are also able to help with tornado surveys of weaker and more rural tornadoes that the NWS may not have rated until recently.

Changes in Technology

Advances in radar technology can be a double-edged sword, in some aspects.

The entire fleet of weather radars received a major upgrade, adding a capability known as dual-polarization, in 2013, which allows users to better discriminate between precipitation types, see lofted debris and better analyze weather systems.

With better radar technology, more subtle, generally weaker tornadoes are seen than ever before. These newly visible low-end tornado signatures on radar introduce lower levels of confidence for forecasters trying to pick out which features on radar are tornadoes, and which are non-threatening.

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These marginal tornado cases often do not get warned due to lower confidence. These subtle tornadoes are usually surveyed afterward to see if they were a tornado, straight-line winds or nothing at all. This change in technology may have added to the slump in successful detection in warnings, which began as the last few radars were upgraded.

Sometimes, the radar simply cannot see them. The Earth's curvature causes radar beams to slope upward after they travel a lengthy distance. This may cause the radar beam to cut through a storm near the top of the cloud – low enough that we know that a storm is there, but too high to discern if the storm contains a circulation.

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Radar coverage across the United States is some of the best in the world, but like most technology, our 122 radars break occasionally. Most highly-populated areas have Terminal Doppler Weather Radars (TDWR) that also run simultaneously with our national network of radars and can act as a backup in case the network radar fails.

"Since October, four radars have had bull gear failures, more than the average of one a year," said Jessica Schultz, NWS focal point at the Radar Operations Center. The bull gear is the component in a radar that spins the radar so it can scan the sky in all directions; if broken, it can take a week or two to repair.

"This is not unexpected," added Schultz. "We anticipated failures as the radars age, which is why we began planning the Service Life Extension Program five years ago. The Service Life Extension Program began in 2015 and is a $150 million, seven-year effort to refurbish and replace major system components."

Although it may seem like radars have gone down more often this year, they "have been available more than 96 percent of the time", according to Schultz.

The current set of radars were implemented in the early to mid-1990s and have a life expectancy of 20 years.

Tornadoes can occur in conditions that normally would not be supportive of severe weather. An example of this occurred in Valdosta, Georgia, in 2014. This tornado was spotted by the NWS, but it caught a lot of people off guard at the time.

The thunderstorm only grew to about 8,000 feet in height, substantially shorter than an average tornadic thunderstorm, which often climbs to well over 30,000 feet.

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Fortunately for Valdosta, the city is very close to the KVAX radar at Moody Air Force Base. The closeness of the radar allowed the beam to see the shallow thunderstorm at around 900 feet.

This storm would have gone completely undetected if it had occurred 85 miles away, and the tornado probably would not have been detected if the storm occurred more than 40 miles away. If the tornado was weaker and farther from a populated location, debris may have been nonexistent and clues of a tornado in progress would have been more limited.

Atmospheric conditions, in general, the day of that tornado were not greatly conducive to severe weather.

Another case of this overshooting radar problem occurred recently in northern Georgia, and the NWS did not catch it.

The number of ways tornadoes can go undetected is quickly shrinking as technology advances and the number of storm chasers grows.

Tornadoes are not the only hazard that should be taken wholeheartedly. Severe thunderstorms that contain high winds and hail can also cause harm to you and your property. Severe thunderstorm and flash flood warnings should not be taken lightly. 

Please heed all warning issued by the National Weather Service and be weather ready.

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