slow moving tornadoes

[STurner]

I know that when a tornado is nearly stationary it may stay over the same structure for several minutes making it look like the damage was higher than it actually is. I was wondering in spite of stationary rotation if the tornado already destroyed the structure as it approached it or if it may have stayed there hitting the structure over and over again. For instance the Harper, Kansas tornado on May 12, 2004 did not receive an F5 rating because it was nearly stationary over the same area for several minutes. The guy who did this survey from the NWS in Wichita now wishes he would have rated this tornado an F5 but at the time thought since the tornado stayed around in the same area for several minutes causing it to look like F5 damage. On the other hand the Elie, Manitoba tornado video showed in spite of stationary rotation the tornado had already had picked up the house as it approached and threw it through the air. It made me wonder if the Greensburg tornado would have been rated an F5 on the old scale had it occured in 2005 or 2006. The Greensburg tornado was not stationary but was so wide it may have taken several minutes to pass from one side of the structure to the other. Where does NWS draw the line on whether a structure was destroyed as the tornado approached the structure or if stayed over the same area causing the damage to look much worse had it of moved along at a steady pace.

 

[Paul Austin]

My understanding is that the survey team determines the rating based solely on severity of damage, and that the benchmarks are detailed and well-established. I don't believe tornado size, movement, speed, duration, or radar presence enters into the final rating in any way at all. So a tornado with 200+ mph winds gets a low rating if it fails to impact any structures or at least some trees. Someone can correct if I'm inaccurate here.

Your observation is interesting, though. I do think duration over a point can dramatically increase damage severity, as not only do the winds continue to impact structures, but flying debris continues to pound existing structures as well.

 

[scott r currens]

I know that when a tornado is nearly stationary it may stay over the same structure for several minutes making it look like the damage was higher than it actually is. I was wondering in spite of stationary rotation if the tornado already destroyed the structure as it approached it or if it may have stayed there hitting the structure over and over again. For instance the Harper, Kansas tornado on May 12, 2004 did not receive an F5 rating because it was nearly stationary over the same area for several minutes. The guy who did this survey from the NWS in Wichita now wishes he would have rated this tornado an F5 but at the time thought since the tornado stayed around in the same area for several minutes causing it to look like F5 damage. On the other hand the Elie, Manitoba tornado video showed in spite of stationary rotation the tornado had already had picked up the house as it approached and threw it through the air. It made me wonder if the Greensburg tornado would have been rated an F5 on the old scale had it occured in 2005 or 2006. The Greensburg tornado was not stationary but was so wide it may have taken several minutes to pass from one side of the structure to the other. Where does NWS draw the line on whether a structure was destroyed as the tornado approached the structure or if stayed over the same area causing the damage to look much worse had it of moved along at a steady pace.

I'm not going to get into the peak gust vs. duration of wind debate. Both peak gust and the duration of the winds have important rolls in the degree of damage. I think it is extremely difficult to estimate peak wind speeds based on damage alone. It seems more practical to rate the degree of damage without worrying about duration or peak gust.

The damage I surveyed just SE of Harper, KS was in many respects more impressive than what I found in Bridge Creek and Moore, OK after May 3, 1999. It was on a much smaller scale but the intensity of damage was stronger than anything else that I have seen. I believe that the DOWs recorded wind speeds that are in the EF-5 range although that was before the Enhanced Fujita was implemented.


May 12, 2004 Harper County tornado damage:

All the pictures that follow are from the same farm. A waist high wheat fields surrounded the 2-story 4-bedroom farmhouse before the tornado hit.

The most intense significant removal of vegetation a soil was 200-yards NE of the home.

There were five vehicles at the farm and engine blocks were the largest remaining pieces of any of the vehicles. The vehicle parts were distributed in all different directions surrounding the farmstead.

What was left of the 2-story farmhouse.

 

[STurner]

Those are some really impressive damage photos I never saw. I remember looking at some photos and the tree debarking looked like something you seen out of a horror movie. There was also an extended cab pick up truck that literally looked like a piece of aluminum foil. The guy from NWS in Wichita said if this tornado would have happened today that it would defenitely would be rated EF5 on the new EF-scale. Another thing that house looks like it was attatched to the foundation with anchor bolts meaning it had to be relatively well-constructed.

 

[Greg Stumpf]

The guy from NWS in Wichita said if this tornado would have happened today that it would defenitely would be rated EF5 on the new EF-scale.

Ratings are not set in stone. If better evidence arises, or if the original rater feels they made an error, the record should be corrected.

 

[rdale]

The EJSSM article about tornado history (where too many deaths were recorded) seems to indicate that no changes can be made to historical tornado data... Is there a "cutoff point"?

 

[Bob Hartig]

Those photos are awe-inspiring, not to mention horrifying. I've heard of the Harper storm, but I haven't given it much thought till now. Looks like I need to read up on it, because it clearly was one baaad mutha.

 

[STurner]

Ratings are not set in stone. If better evidence arises, or if the original rater feels they made an error, the record should be corrected.

I thought after so long they could not reclassify a tornado with a higher rating. It was rated an F4 on the old scale so even if it could be reclassified then it would have to be rated an F5 since the old fujita scale was in use at the time. I asked him if there was anyway it could ever be reclassified as an F5 and he said he did not believe so because of some policy on records or of that sort.

 

[Penny Lehner]

I don't know how they rate damage from a slow-moving tornado but I do know the Providence, KY tornado was rated a F-3 on the old scale. I'm not sure of how fast it was moving but I do know it bounced around in the valley where most of the town was located. It was a clear destruction path coming down the hill then the damage path went everywhere before it climb out of the valley.

Providence, KY - April 28, 2002 - it hit at 3 a.m.

I lived about 20 miles from there in Sturgis, KY at the time and remember that night well.

 

[Greg Stumpf]

I thought after so long they could not reclassify a tornado with a higher rating. It was rated an F4 on the old scale so even if it could be reclassified then it would have to be rated an F5 since the old fujita scale was in use at the time. I asked him if there was anyway it could ever be reclassified as an F5 and he said he did not believe so because of some policy on records or of that sort.

There's no policy which determines which scale a tornado can be rated with. Any tornado, from any time can be rated with any scale (including scales not limited to F and EF, like the E-Scale).

That said, there might be an official NWS "statute of limitations" on how long a record in Storm Data can remain open for modification, but I was careful to choose my words. I said that if there is sufficient evidence or the original rater believed they were in error, the record should be changed. In other words, I advocate that there should be no such policy. While on the same topic, I also advocate that the record should include the source of the rating information, the certainty of the source information, and data about the degrees of damage to the damage indicators (since the wind speed estimates change).

 

[rdale]

I also advocate that the record should include the source of the rating information, the certainty of the source information, and data about the degrees of damage to the damage indicators (since the wind speed estimates change).

...and that's why you are our hero... That sort of info is CRITICAL to making comparisons down the road. Any idea who we might petition to have this included as part of the process?

 

[Joel Wright]

That Harper County damage is absolutely incredible! I can't imagine the tornado doing more damage to that farm than it did. The way the vehicles were literally ripped to pieces is astounding to say the least.

I wonder since the farm was in an open area, and away from other obstructions allowed for tornadic winds to maintain a higher velocity? Either way I feel horrible for whoever owned that house/farm.

 

[Kiel Ortega]

...and that's why you are our hero... That sort of info is CRITICAL to making comparisons down the road. Any idea who we might petition to have this included as part of the process?

It's being looked into by the NWS Verification Branch. Greg and myself (among others) had our brains picked about adding DI/DOD info to Storm Data.

 

[STurner]

There's no policy which determines which scale a tornado can be rated with. Any tornado, from any time can be rated with any scale (including scales not limited to F and EF, like the E-Scale).

That said, there might be an official NWS "statute of limitations" on how long a record in Storm Data can remain open for modification, but I was careful to choose my words. I said that if there is sufficient evidence or the original rater believed they were in error, the record should be changed. In other words, I advocate that there should be no such policy. While on the same topic, I also advocate that the record should include the source of the rating information, the certainty of the source information, and data about the degrees of damage to the damage indicators (since the wind speed estimates change).

I was wondering how you would you go about doing this. I tried to contact you through your office but am not sure if you would prefer to be contacted through email or other ways. I am in no means qualified to do damage surveys but have had high interests on tornadic damage for the past several years and have studied many damage photos . Would there be like a conference held on records and how to go about historic tornado data changed?