(WKBN)- With the recent waves of severe storms, quite a lot has happened–from damaging wind gusts all the way to a tornado touching down. The most recent tornado that moved through the Valley was rated an EF-0. What does that actually mean, and how do National Weather Service meteorologists determine that?
The first known scale to be invented was the Fujita (F) scale. It was created by Dr. Ted Fujita to estimate the wind speeds of a tornado by the damage it caused. The scale has been used since 1971 to assess and rate tornadoes.
There was only one issue. It had many limitations. It didn’t have any sort of damage indicators. There was no account for the quality of construction. The overall correlation (relation) between the damage seen and wind speeds was also very poor. This would result in wind speeds being overestimated a majority of the time.
So how did they fix it?
The Enhanced Fujita (EF) scale was created and began use in 2007.
This scale helped to relate damage to estimated wind speeds much better. It accounted for some of the factors the Fujita (F) scale lacked. How does it work?
When a tornado is actively being surveyed, a couple of steps are taken. It will be compared to a list of damage indicators and degrees of damage. This helps estimate the range of wind speeds much better. Then an EF rating will be assigned to the tornado.
The new EF scale is based on wind estimates, so it is not a measurement. It is not the same as a wind speed directly measured at the surface from a weather station. It then uses the wind gust estimations at the point of damage using 28 indicators and 8 degrees of damage.
The goal is to find the highest wind speed along the path of damage from the tornado. This is how it is done.
For example, say a tornado has just occurred. The next day, a survey team will head out to begin the assessment. In this case, we notice an area where a small barn was. It has been completely destroyed. We would then pick damage indicator number 1.
Images above courtesy NWS Norman, OK
In this section, we start with the typical description of how the structure is built. Now we know what pieces and parts to consider.
We now move to the degree of damage. We said, in this case, the entire structure was destroyed. This has us now picking degree of damage number 8. You will notice the abbreviations “EXP, LB, and UB”. This stands for expected, upper bound, and lower bound. It will be based on how well the structure was built. In this case, we’ll go with it being a brand new barn with the latest construction and integrity involved.
Based on the structure, degree of damage, and the quality of it, the wind speed will be estimated around 130 miles per hour.
The National Weather Service will repeat this process as many times as needed or up to what is provided in the damage path. They will then determine the maximum wind speed in the path. That will then determine the EF rating of the tornado.
If the total destruction of that barn was the highest wind speed found after further evaluation of other structures, we would rate this tornado a high-end EF-2.
Below are a few segments on how this was applied to the most recent EF-0 tornado from Chief Meteorologist Paul Wetzl and Meteorologist Craig Ziobert.