2011-04-27 MISC: AL,TN,MS,KY,OH,IN,WV,GA

[Jeff Duda]

Sounds about right. There wasn't anything terribly unique about this setup from what I can recall. Was a combination of a number of very strong parameters coming together. Think the key to the day may have been storm spacing and the isolation that was had by most supercells. I've seen some amazing parameters before "go to waste" due to the seeding of the storms and their effective competition with each other. Not the case with this outbreak.

This is what always floors me about major severe weather outbreaks. The same thing was true of the Super Outbreak, and probably of other known major severe weather outbreaks (I only know this fact about the Super Outbreak due to this paper). It's sort of frightening to realize that such a major severe weather event can be spawned from a seemingly innocent looking synoptic setup. Granted, anyone with experience watching synoptic scale patterns associated with major tornado outbreaks would've been able to recognize the potential this day had when looking at model forecasts a few days out. But really, the same ingredients that were in this event are in many severe weather events, even those that aren't major:
-a moderate amplitude trough aloft with neutral to negative tilt that was propagating east/northeast
-moderately unstable air mass with mid-upper 60s dewpoints in the warm sector and 80-90 F surface temperatures
-Strong low-level and deep layer shear
-Moderate low-level instability
-A surface boundary or synoptic scale lift to trigger storms

Think about it, how often do you see these features with other severe weather setups? Almost each one appears regularly. Yes, the degree of deep layer and low-level shear was on the extreme end of the statistical distribution, but I have seen such high levels of shear and helicity associated with instability and forcing that did not result in a major tornado outbreak in other cases. There are likely a few smaller ingredients that came together to make these storms spin like tops and drop violent tornadoes left and right. These are the ingredients that projects like VORTEX2 are trying to discover.

Let me reiterate: from a synoptic standpoint, there was little about this setup that was unusual, uncommon, or difficult for the atmosphere to achieve.

 

[Gabe Garfield]

The damage caused by many of Wednesday's tornadoes is comparable to the damage caused by the most intense tornadoes in recorded history. I've seen many, many instances of EF5 damage in images I've seen, taken from several tornado paths. My impression is that folks are a little shy about saying "EF5" because they are concerned about the "political ramifications". I think, in large part, this goes back to the La Plata, MD Tornado (2002), where an NWS forecaster initially rated the damage as F5. Later, it was found (by Tim Marshall, I believe), that the "F5" damage spots were actually caused by winds that were in the F2 range. Ever since, NWS offices seem a bit more shy about giving tornadoes higher ratings.

Personally, I believe that a tornado should be rated as objectively as possible. If the damage is consistent with a violent EF4/5 tornado, then rate it as such -- it either was, or it wasn't a violent tornado. Let the damage speak for itself. Category 5 tornadoes, though rare, do happen. What more can a tornado do but wipe a well-constructed house off its foundation, leaving nothing?

In regards to the Greensburg tornado, I believe the damage is a step down from the damage in Andover, or even Bridge Creek. While there were instances of EF5 damage in Greensburg, it did not cover a very large area. However, the Greensburg tornado was occluding when it hit town, so it's possible it was more intense outside of town, where there were very few damage indicators.

 

[Adam Atkins]

The shear and CAPE parameters were juxtaposed well, but it wasn't a never before seen setup, as Jeff said. A couple things stood out in the southeast for this particular event that I believe helped keep storms discrete:

- very dry air aloft/steep lapse rates
- a dryline that progressed well east, ahead of the cold front (relatively weak forcing)

The event was forecast extremely well many days in advance.

 

[Jeff Duda]

What more can a tornado do but wipe a well-constructed house off its foundation, leaving nothing?

I think you need to be careful when saying this. If I recall correctly, the Fujita scale was originally designed to link the Beaufort and Mach scales for wind speed. I don't believe the F5 rating was designed to explain a single family house being wiped off its foundation, although that association is frequently used. The big advantage that the EF scale has over the F scale is that it accounts for the degree of quality of construction. If you have a poorly constructed house that is wiped clean off its foundation, it might not take EF5 caliber winds to do that. I would be willing to argue that much of the construction quality in Alabama is more on the poor side of the distribution. Thus, while some buildings have been cleanly swept, unless they were well constructed, that damage may not represent true EF5 scale damage.

 

[Zack Hargrove]

Getting word that the dead chasers have been confirmed. Details are not out, but the confirmation is coming from a well respected chaser. My source is a separate source that was actually on the scene, so its is sounding as if this is true.

 

[Gabe Garfield]

I think you need to be careful when saying this. If I recall correctly, the Fujita scale was originally designed to link the Beaufort and Mach scales for wind speed. I don't believe the F5 rating was designed to explain a single family house being wiped off its foundation, although that association is frequently used. The big advantage that the EF scale has over the F scale is that it accounts for the degree of quality of construction. If you have a poorly constructed house that is wiped clean off its foundation, it might not take EF5 caliber winds to do that. I would be willing to argue that much of the construction quality in Alabama is more on the poor side of the distribution. Thus, while some buildings have been cleanly swept, unless they were well constructed, that damage may not represent true EF5 scale damage.

I'm not sure I understand what you mean. I'm arguing that, as far as we can tell, a tornado can't cause damage more intense than wiping a well-built house/building off its foundation. And, by definition, that is EF5 damage. Also, I learned today (from Greg Stumpf) that the requirements for Category 5 damage in the old F-scale are the same as the enhanced F-scale. The only difference is estimated wind speeds.

 

[jmarc Pourcelet]

2. Vous ne pouvez pas fonder une décision sur toute une infrastructure de pays sur l'état de deux poteaux de téléphone à Lansing au Michigan.

Chaque année des milliers de km de fil électrique et de téléphone ainsi que des centaines de poteaux et de pylônes dont détruits, ce qui coûte très cher alors les lignes enterréés le sont pour longtemps et ne subissent pratiquement pas de dommages. En plus le côté esthétique y gagne aussi.

 

[Jeff Duda]

I'm not sure I understand what you mean. I'm arguing that, as far as we can tell, a tornado can't cause damage more intense than wiping a well-built house/building off its foundation. And, by definition, that is EF5 damage. Also, I learned today (from Greg Stumpf) that the requirements for Category 5 damage in the old F-scale are the same as the enhanced F-scale. The only difference is estimated wind speeds.

Yes, I actually forgot to include in my previous post my point: there are multiple damage indicators now. Some of these damage indicators can tell more about the intensity of high end tornadoes than a single family house can. For example, if you expose a single family house to 300 mph and 500 mph winds, you're going to get the same result: house and all pieces completely swept away from the foundation. However, how do you otherwise distinguish the 300 mph from 500 mph winds? With a house, you can't. With some other DIs, you might be able to. For example, the maximum wind speed estimate from the highest degree of damage from the high-rise building damage indicator is 290 mph (http://www.spc.noaa.gov/efscale/19.html), while for a single family home, it is only 220 mph.

I'm aware that the same qualifications for F5 damage to a house apply with the EF-scale, except for the part about quality of construction. Unless I'm mistaken, when NWS meteorologists are surveying tornado damage, when they come to a damaged object, they find out what kind of damage indicator it is/was. From that, they determine the degree of damage, then get a wind speed value from the distribution of wind speeds that cause that degree of damage to that damage indicator (there is a range of wind speeds depending on the quality of construction). From that wind speed value they get, they determine an EF-scale rating for that damage indicator. If you look at the maximum degree of damage (10) for damage indicator #2, single family houses: http://www.spc.noaa.gov/efscale/2.html, you'll see that the range of wind speeds includes both EF4 and EF5 values. Thus, depending on which part of the distribution they decide to take a wind speed from, you could get an EF4 or EF5 from a completely swept away home.

 

[Jason Harris]

2. Vous ne pouvez pas fonder une décision sur toute une infrastructure de pays sur l'état de deux poteaux de téléphone à Lansing au Michigan.

Chaque année des milliers de km de fil électrique et de téléphone ainsi que des centaines de poteaux et de pylônes dont détruits, ce qui coûte très cher alors les lignes enterréés le sont pour longtemps et ne subissent pratiquement pas de dommages. En plus le côté esthétique y gagne aussi.

I can translate a little of this I think (until someone who knows better steps in): You can not base a decision, especially on the infrastructure of a country or what you have in one state such as with two telephone poles in Lansing then applied to (all of) Michigan.
Each year there are thousand of kilometers of electrical wires and telephone wires and so forth and hundreds of telephone poles and pylons that are destroyed, those that cost a lot in addition to the ones that are buried for a long time are not practically submitted for damages. Furthermore you have to weigh the aesthetic cost also.

 

[Todd Webb]

I'm hearing that there is debate going on between forecasters and tornado expert Greg Stumpf at Peachtree City on whether or not the Catoosa County, Georgia tornado should be upgraded to an EF5.

Here is a link to a picture of the most severe damage from the Catoosa County tornado.

http://www.srh.noaa.gov/images/ffc/catoosa088.JPG

 

[Gabe Garfield]

Yes, I actually forgot to include in my previous post my point: there are multiple damage indicators now. Some of these damage indicators can tell more about the intensity of high end tornadoes than a single family house can. For example, if you expose a single family house to 300 mph and 500 mph winds, you're going to get the same result: house and all pieces completely swept away from the foundation. However, how do you otherwise distinguish the 300 mph from 500 mph winds? With a house, you can't. With some other DIs, you might be able to. For example, the maximum wind speed estimate from the highest degree of damage from the high-rise building damage indicator is 290 mph (http://www.spc.noaa.gov/efscale/19.html), while for a single family home, it is only 220 mph.

Good point. I guess tornadoes can, theoretically, do "more" damage. My point though, was to argue that damage that is consistent with the Category 5 rating should be rated as such.

Regarding your second point, I agree that there is some ambiguity. But, for DOD 10 to a single/two-family residence, the lower bound wind speed is at the top of the EF3 wind range; whereas, the expected value and upper bound wind speeds are in the EF5 wind range. Is it fair to "cherry pick" the lower rating, when the expected value is in a particular range? Furthermore, if there are multiple instances of the same extreme damage, wouldn't that suggest that the expected value is a good estimate? I think there have already been cases where NWS surveyors took the lower bound wind speed as their estimate, when there is a preponderance of evidence suggesting that the expected value wind speed is a very good bet.

 

[Jeff Duda]

Regarding your second point, I agree that there is some ambiguity. But, for DOD 10 to a single/two-family residence, the lower bound wind speed is at the top of the EF3 wind range; whereas, the expected value and upper bound wind speeds are in the EF5 wind range. Is it fair to "cherry pick" the lower rating, when the expected value is in a particular range? Furthermore, if there are multiple instances of the same extreme damage, wouldn't that suggest that the expected value is a good estimate? I think there have already been cases where NWS surveyors took the lower bound wind speed as their estimate, when there is a preponderance of evidence suggesting that the expected value wind speed is a very good bet.

I agree. There sure seems to be some politics involved with rating top-end tornadoes, and that definitely reduces the objectivity of some of the ratings. I surely hope that outside experts that come in to help survey the damage increase the objectivity and properly rate these tornadoes. If the damage done was consistent with that of an EF5 tornado, then I hope that tornado gets an EF5 rating.

 

[Mike Johnston]

CNN is reporting the tornado that began in Mississippi, crossed through Alabama into Tennessee was a single tornado with a track of 200 miles. Not many other details and I can't find anything in print yet. Being the round number it is, the 200 miles might be an approximation.

 

[Chad Ringley]

CNN is reporting the tornado that began in Mississippi, crossed through Alabama into Tennessee was a single tornado with a track of 200 miles. Not many other details and I can't find anything in print yet. Being the round number it is, the 200 miles might be an approximation.

On the NWS BMX page, their path survey image (http://www.srh.noaa.gov/bmx/) has a break NE of Birmingham in their survey plan. That coincided with a weakening of the couplet if my memory serves me correct and occlusion. Thus, I'd bet that's where one track ends and another beings.

 

[E. Clark]

http://www.alabamawx.com/ has posted a lot of images sent in by viewers / chasers etc. It's a pretty massive collection of images at this point. A decent study in checking EF values.