Why wasn’t the Greenfield tornado rated an EF-5?

GREENFIELD, Iowa — The road to recovery is still just beginning in Greenfield, but some discussions about the rating of the tornado that struck the town on May 21 are already in progress.

The tornado was rated an EF-4 by the National Weather Service in Des Moines with winds estimated to be between 175 and 185 mph. With such serious damage in the city, a “quick response team” was called in to help with the damage survey.

But findings by weather researchers over the last few months have caused some to question that survey. A crew with a “doppler on wheels,” a mobile radar unit, measured wind speeds over 300 mph. Those wind speeds are well into the territory of an EF-5 tornado, which has wind speeds over 200 mph.

But the survey process isn’t that simple. The Enhanced Fujita Scale, the system used to rate tornadoes, is a damage scale. In other words, the strength of tornadoes is measured by the damage they cause, not by the winds measured in the twister.

Damage surveys not only consider the extent of damage but also how strong the structures hit by the tornado were. So if a strong structure wasn’t hit by a tornado, only so high of a rating can be assigned.

Bill Gallus, a meteorology professor at Iowa State University puts it this way:

“The piece of information that everyone wants to know is what is the wind speed of a tornado. But tornadoes are very small. Our normal weather stations are about 50 miles apart. So it is extremely rare that a tornado would actually hit an instrument. The winds in a tornado are so strong that they’re full of debris and they would probably destroy most instruments when they hit. So the tornadoes do a very good job of keeping a secret of what their winds are. Therefore, it forces us to use a method that has its own flaws. And that method is to rate tornadoes based on the damage that they do.”

~Bill Gallus, Iowa State University Meteorology Professor

Gallus brings up an excellent point — it is very rare to gather wind speed measurements in tornadoes. For the sake of consistency, they can’t be included in the rating process.

So when some insisted the Greenfield tornado wasn’t properly rated, National Weather Service Warning Coordination Meteorologist Chad Hahn replies:

“The scale is a wind estimate and we’re talking about a difference of ten, 15 miles per hour and the different levels of ratings. And so what I would say is, it’s a possibility, but the damage didn’t support it that we saw on our ground based survey.”

~ Chad Hahn, National Weather Service Des Moines Meteorologist

Meteorologists and others in the weather world may bicker about how tornadoes should be rated or nit-pick the process. However, the human impact is still the same.

“No matter what wind speed, all that other stuff. It was a vicious tornado that came through here and a lot of people lost everything. You know, I mean, a few lost their lives, but most of them lost everything.”

~Mayor Jimmie Schultz, Greenfield

At the end of the day, all tornadoes are dangerous. In fact, all tornadoes EF-2 and above are considered “significant tornadoes” due to the damage they cause.

It’s a good reminder to take all tornado warnings seriously and to remember the human toll they cause.

For now, there are no plans to significantly change the tornado rating process. However, tornadoes with significant wind speed recordings, like the Greenfield tornado, will have a footnote added to their record along with the damage rating.