NOAA NATIONAL WEATHER SERVICE IMPROVES TORNADO RATING SYSTEM

[mikegeukes]

NOAA NATIONAL WEATHER SERVICE IMPROVES TORNADO RATING SYSTEM
http://www.noaanews.noaa.gov/stories2006/s2573.htm

Mike

 

[Scott A. Kampas]

It hit the media after the AMS National Meeting: http://www.washingtonpost.com/wp-dyn/conte...6020201165.html

That's the first coverage I've seen.

 

[Scott A. Kampas]

I found SPC has added an additional page http://www.spc.noaa.gov/efscale/ to the FAQ page which has been around for a while: http://www.spc.noaa.gov/faq/tornado/#f-scale1 --> http://www.spc.noaa.gov/faq/tornado/ef-scale.html

There's also a nice summary in the September 2004 issue of The Wind Engineer.

 

[Scott A. Kampas]

The first revision of the initial EF-Scale Report came out just over a week ago: http://www.wind.ttu.edu/EFScale.pdf

 

[Scott A. Kampas]

Another new tornado scale?

TORRO has proposed a new tornado intensity scale unifying the TORRO and Fujita scales: http://torro.org.uk/TORRO/ECSS_Slide_Show/index.html

 

[Mike Gauldin]

I think there's a lot of NWS people that are trying to get this revised more to be a little more accurate. The damage indicators have a upper bound, lower bound, and mean wind speed which can vary quite a bit (ex. 50mph difference). That's a lot of wind. Then there's some argument about overall ratings, because by this new scale the May 3, 1999 tornado might be rated more like an F4 because mainly homes were hit, even though this ranks as one of the most violent tornadoes recorded (yes, I know this scale won't change past F-ratings, but if history repeated itself, it might not have been an F5). I like the new scale in general, as it seems to be more comprehensive (i.e. adding hard-wood and soft-wood trees to help rate tornadoes in rural areas), but I think there's some people that are going to have a few words about it before it's all said and done.

 

[rdale]

"I think there's a lot of NWS people that are trying to get this revised more to be a little more accurate."

So the clear experts in this field got it wrong, and NWS mets (not engineers) want to make it better? In what way?

- Rob

 

[Mike Gauldin]

Well, the EF scale is based off of 'expert opinion'. They sat down 6 people from NWS and engineering colleges that are considered experts in their field and asked them... "How much wind does it take to tear the roof off of a house". Then of those 6 answers (which varried from ~80mph to ~115mph), they took an average and put that as the rough mph rating. If you look at the EF scale, it has upper and lower bounds for damage to a roof. So by this, a wind of 63-97mph can do <20% loss of roof covering material, yet 81-116mph will uplift roof deck. So now we have overlaping wind speeds. As a MIC at a NWS office said, "If I have some shingles missing and my neighboor doesn't, I guess it was the lower bound. So how do we measure the upper bound?". And again, these are "guesses" so to speak from different experts with different backgrounds. So my thinking is, if you have say 85mph winds, how do you determine that if it's covered by both DOD 2 and 4 with different damage ratings? There's a large gap in there that's covered by multiple DODs.

This is from the EFScale PDF under 'Table 4. One- and Two-Family Residences (FR12)'.

 

[Mike Gauldin]

...oh, another thing

If you look, F5 damage is only over like 200mph which only falls under a few catagoreys, like 'hospitals' and 'malls'. So this means something like the May 3rd tornado could be labeled as an F4 based off of damage alone since it really didn't take out a certain type of constructed building.

 

[Jeff Snyder]

Originally posted by Mike Gauldin
...oh, another thing

If you look, F5 damage is only over like 200mph which only falls under a few catagoreys, like 'hospitals' and 'malls'. So this means something like the May 3rd tornado could be labeled as an F4 based off of damage alone since it really didn't take out a certain type of constructed building.

This is nothing new -- you've never seen an F5 that stays in a field! There's no way to assume winds that are beyond the intensity needed to create a certain level of damage... If 180mph winds can destroy a house, how do you determine if the tornado had 180mph or 210mph winds? This is the same issue as we've had with the old F-scale... If a violent tornado remains in open country, that tornado can't be rated F4 or F5... This is nothing new.

Note that the document DOES note that mobile doppler radar observations CAN be used to support a given rating. This may indeed help provide a more objective rating (based on wind speeds) than what has been possible in the past.

I'll take the 28 damage indicators and 8 levels of damage of the EF-scale over the rather vague damage descriptors for "well-built homes" of the F-scale. I don't think there was a whole lot of engineering / technical analysis incorporated when Fujita developed the original F-scale, and this has been shown time and time again through the years.

As for the 'wide range' of wind speeds possible for a particular level of damage or EF-scale rating... Lowering that range would indicate precision that we just don't have. Many folks (myself included, however much that means LOL) thought it misleading that wind speeds attached to the original F-scale seemed to be so precise (e.g. 263mph). Having such a boundary implies that we can tell 260mph damage from 265mph damage... I think most people will understand that we can't do that now. Even "service station canopies" (a damage indicator in the EF-scale) are different, and different wind speeds may be required to destroy said structure. One canopy may be able to withstand 100mph winds, while the one across the street may only be able to withstand 85mph winds. Sure, even the new EF-scale wind speeds are estimates, but I certainly think they are better than what we've had in the past.

 

[Mike Gauldin]

Originally posted by Jeff Snyder

This is nothing new -- you've never seen an F5 that stays in a field! There's no way to assume winds that are beyond the intensity needed to create a certain level of damage... If 180mph winds can destroy a house, how do you determine if the tornado had 180mph or 210mph winds? This is the same issue as we've had with the old F-scale... If a violent tornado remains in open country, that tornado can't be rated F4 or F5... This is nothing new.

That's what I was trying to say. I do like that fact it has more damage indicators. I'm not trying to say this is a bad thing, just answering a question.

With time and research will come better scales. This is just a step in that direction. I couldn't do a better job if I tried, I know that much.

What I was referring to about some people wanting some changes is based on the May 3rd even where radar tagged speeds for an F5, yet damage per the EF scale would indicate F4. Granted radar will override the EF scale, but you can see the potential conflict there.