How National Weather Service Damage Surveys Estimate A Tornado's Strength

After a tornado strikes, National Weather Service damage surveys do much more than just determine the strength and path. They're a vital task to enhance our understanding of nature's most violent storms.

W​hat is a survey? When a tornado has occurred or is suspected, a team of meteorologists from the local National Weather Service office examines the damage.

The survey team is tasked with verifying whether the damage was caused by a tornado or thunderstorm winds, when a tornado first touched down and lifted, how wide it was and its intensity.

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Why surveys are needed: While we have a national network of Doppler radars, a tornado's wind speed at the ground while it's in progress is rarely measured.

That's because tornadoes often occur miles from the nearest radar. A radar beam is sent out at an angle slightly above horizontal, to avoid bouncing off nearby trees and buildings. Coupled with Earth's curved surface, that means the radar beam arrives at a thunderstorm at some elevation above the ground.

S​o, the Doppler-estimated winds from the radar aren't necessarily representative of actual wind speeds at the ground.

T​he damage scale: Instead, tornado wind speeds are estimated through damage inspections after the storm.

Theodore "Ted" Fujita was a pioneer in tornado research and post-storm damage surveys, beginning in the 1950s. Based on this work, Fujita and meteorologist Allen Pearson created the initial Fujita-Pearson scale in 1971.

The scale was updated in 2007 to the Enhanced Fujita scale to account for the quality of building construction and other types of of damage, from trees to small barns to large shopping centers.

A tornado is rated based on the most severe damage found along its path, from EF0 (the lowest) to EF5 (the most intense). For example, an EF3 tornado with a 30-mile long path may have produced EF3 damage to only one house or building, but EF0, 1 or 2 damage along the rest of its path.

This isn't easy: This sounds straightforward, but damage surveys can be both challenging and time consuming.

The survey team has to find the worst damage, sometimes among dozens, if not hundreds, of damaged buildings for miles. In higher-end damage cases - EF3 or stronger - other experts like engineers are consulted to help assess the damage.

The damage path may be hours away from the nearest NWS office. It may also require the team to crisscross the damage path multiple times to help establish the tornado's start and end points.

Drones have been one advancement in helping assessment teams determine a damage path. Below is an example by the NWS-Louisville, Kentucky, office from the Dec. 10-11, 2021 outbreak.

W​hat if there are many tornadoes? Sometimes, multiple tornadoes tear through a NWS forecast area. Some of these may be long-track supercells that spawned tornadoes over dozens of miles.

These require multiple damage survey teams, and it can take days to examine the damage of every tornado. Final tornado counts for larger outbreaks over multiple states can take a week or longer.

Challenges you may not think about: Rick Smith, warning coordination meteorologist at NWS-Norman, Oklahoma, shared other damage survey challenges in a thread after the Dec. 2021 outbreak.

They include the wait to access some areas, and weather delays. Sometimes, severe weather hits an area within 24 hours after it was first hit, not allowing meteorologists a clear window to access the damaged areas until the following day.

Smith told weather.com he has even had to dodge tornado warnings while surveying damage.

The human element: One important part of a storm survey is talking with those affected.

"It's a sobering thing," NWS-Omaha warning coordination meteorologist Brian Smith said in a 2016 NOAA article. "A lot of times when you tell them who you are, they open up and start talking... and that's part of the grieving process. You feel more welcome when they do that."

NWS meteorologists often work long shifts. They could issue warnings during an outbreak well into the night and then have to survey damage the next morning.

Why the surveys are important: First of all, they help educate the public. Seeing what a tornado's winds are capable of may, and should, prompt you to take cover immediately when a warning is issued.

Secondly, they help forecasters and researchers by adding to the historical database of where, when and how strong tornadoes occurred.

This can lead to improvements in the detection of tornadoes and the forecasting of the conditions most conducive for tornadoes by computer models.

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Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. He completed a Bachelor's degree in physics at the University of Wisconsin-Madison, then a Master's degree working with dual-polarization radar and lightning data at Colorado State University. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads, Facebook and Bluesky.