How meteorologists determine if a tornado is to blame for storm damage

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Assessing damage of the deadly tornadoes in Bowling Green, KY

FOX Weather Correspondent Nicole Valdes spoke with John Gordon, a meterologist with NWS Louisville, on how they are going about assessing the deadly tornado outbreak.

Strong winds from a thunderstorm swept through your neighborhood, causing damage to homes, businesses and trees. People claim it was a tornado that caused the damage, but how can you be sure? That’s where the National Weather Service steps in.

Whenever a tornado is believed to have occurred, a survey team from the NWS is dispatched to determine if that was indeed the case. They do everything from an assessment of the type of damage that happened to a reconstruction of wind patterns, before making a final determination.

Here’s a closer look at the damage survey process.

Starting the survey

According to the NWS, each survey team is equipped with a technology kit before they head to the site of the storm damage. The kit includes things such as a GPS, a computer with damage survey software, an atlas, a digital camera, a compass and a notebook.

"Most commonly, a survey team will conduct a full ground survey in order to assess tornado damage, but occasionally, a team may also conduct an aerial survey if the spatial extent of the damage is large enough," the NWS office in Amarillo, Texas, wrote in a 2011 post about damage surveys.

According to the NWS, most teams will start with determining the starting and ending points of the suspected tornado. They will then move on to determining the width of the damage path, followed by locating the worst damage along that path.

Conducting the survey

The survey team will assign one of 28 indicators to each type of storm damage they see. The indicators include all types of structures and even tree damage.

"Each one of the damage indicators has a description of the typical construction for that category of indicator," the NWS wrote. "For example, typical construction for one- and two-family residences includes asphalt shingles, tile, slate or metal roofing, attached single car garage, and brick veneer, wood panels, stucco, vinyl or metal siding."

Once all of the indicators are assigned, the team moves on to assign a degree of damage to each of them. Each of the damage categories has an upper-end and lower-end of wind speeds. This is what ultimately determines a wind speed range for the storm.

"This is where the job becomes difficult for the survey team because the team must know some basics about construction," the NWS wrote. "If the quality of construction meets strict building code, the survey team will likely assign an expected wind speed to the damage. If the construction fails to meet code, a lower bound wind speed may be assigned, but if the construction exceeds code and/or is well-engineered, it may be assigned an upper bound wind speed."

In addition, surveyors also look at the way trees were damaged by the storm. According to a post written by the NWS office in Binghamton, New York, members of the survey team use a compass aligned with the trunk of a felled tree to determine the direction of the wind. They also look at the way the trees landed on the ground.

"Microburst damage often looks laid or flattened out. Larger uprooted trees point in the same direction, or a fan-shaped divergent pattern," the NWS wrote.

"Tornado damage often has a chaotic appearance, with larger uprooted trees often crossing each other," the NWS wrote.

Making a determination

All of the information collected by the survey team is coupled with other data, such as radar images and eyewitness accounts to make the final determination about what caused the damage.

If meteorologists believe a tornado caused the destruction, they will assign the twister a rating on the Enhanced Fujita Scale.

The NWS will also issue a survey report that lays out everything from the estimated wind speed of the storm to the length of the damage path to the type of damage that was seen by the team.

Usually, the entire damage survey process takes a few days to complete.