Beyond The Forecast - How tornadoes are rated on the EF scale

There are a lot of steps the National Weather Service has to take before estimating how strong a tornado was.

We go beyond the forecast to explain how tornadoes are rated by the National Weather Service.

ROANOKE, Va. – Happy Monday, and welcome to another edition of Beyond The Forecast.

This spring has been very active in regards to tornadoes in the Plains and Midwest.

According to the Storm Prediction Center, there have 928 reported tornadoes as of May 25.

Once a tornado has done its damage, the first thing many people want to know is, “How strong was it?” A team of storm surveyors attempt to answer that question in the hours, days, weeks, etc. after a storm.

When determining storm damage, you first have to see if what was damaged is diverging outward (downburst/straight line winds) or if it is going in a multitude of directions.

A difference in damage swaths between downbursts and tornadoes.

Since 2007, the National Weather Service has used the Enhanced Fujita (EF) scale to rate a tornado’s strength.

This scale assigns an estimated wind speed to the damage observed by surveyors on the ground.

The Enhanced Fujita attempts to estimate wind speed based on damage.

How are they able to estimate the wind speed, then?

There are 28 damage indicators, ranging from small barns to large shopping malls and motels to trees.

Surveyors will then look at eight different degrees of damage (DODs).

For example, if a small barn or farm outbuilding is damaged, these are the degrees of damage surveyors would look for.

Degrees of damage for barns and outbuildings according to NOAA's Storm Prediction Center. (Copyright 2024 by WSLS 10 - All rights reserved.)

In addition to ground surveys, meteorologists with the National Weather Service will deploy drones to assess the damage path in more detail.

There are oftentimes in which different radar products can be used to assist in storm damage surveys.

Velocity can estimate the wind at a few different levels of the storm.

The height at which debris is lofted can also become a potential determining factor in how strong the twister was. This is estimated by using Correlation Coefficient.

You can learn more about these radar products here.

You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, the Lynchburg area, the New River Valley, Southside or the Highlands anytime at Know your zone!

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About the Author

Meteorologist Chris Michaels is an American Meteorological Society (AMS) Certified Broadcaster, forecasting weather conditions in southwest Virginia on WSLS 10 News from 5 a.m. to 7 a.m. weekdays on Virginia Today.

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