Effect of storm motion on EF scale

[Jeff Hawkins]

Effect of tornado speed in determining the EF scale

Hi everyone, a general question here. I searched for a thread on this and didn't find one specifically, so I'm gonna ask...and this isn't one of those earth-shattering issues of life, but it is something I'd probably be scratching my head about until I had a satisfactory answer....

Hypothetically, if a tornado passes over a point, with a given wind speed of, say, 100kts, and the speed the tornado passes over that point is at 10kts, will it do less destruction to that point, than if the storm motion were at 5kts or 2kts?
If so, then my question is: is that taken into account in the calculation for establishing the EF scale of a tornado? I realize it's probably moot, since the EF scale is based on the level of damage aftwerwards.

 

[Wes Carter]

Jim, the EF rating is a damage rating, nothing more. However, the storm motion can affect the damage done by the tornado and therefore can affect the EF rating. If a tornado has a forward motion of 50kts, and a max wind speed on 150 kts then on the lower portion of the vortex with the wind going the same direction as the mean storm motion you can have an effective wind speed of 200 kts while on the upper portion of the vortex with the wind flowing away from the mean storm motion the effective wind speed would be 100 kts.

Please keep in mind that my explanation is the highly simplified version that assumes a single vortice tornado with a steady storm motion and wind speed.

 

[Andrew Pritchard]

Indirectly, the inverse is also true - a slower storm motion can also increase the damage a tornado inflicts on a structure by allowing it to churn away for a greater amount of time. This issue has been covered in other discussions, however.

 

[rdale]

I realize it's probably moot, since the EF scale is based on the level of damage aftwerwards.

That's exactly correct. It doesn't matter how the damage got there - the EF scale still only measures damage. The wind speeds are simply inferred and not to be used as verified (or verifiable) numbers.

 

[Jeff Hawkins]

OK, thanks for answering that question! I appreciate your willingness to share your knowledge and technical insights.