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

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Jeff Duda

Resident meteorological expert
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Oct 7, 2008
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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.
 
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.
 
Mar 23, 2004
188
0
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NYC
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

Resident meteorological expert
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Oct 7, 2008
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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.
 
Mar 26, 2009
174
13
5
Bismarck, ND
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.
 
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

Resident meteorological expert
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Oct 7, 2008
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Broomfield, CO
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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.
 
Jan 7, 2008
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Bryan, TX
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.
 
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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

Resident meteorological expert
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Oct 7, 2008
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Broomfield, CO
www.meteor.iastate.edu
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.
 
Feb 14, 2005
879
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Charleston, South Carolina
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.
 
Sep 21, 2005
149
1
0
Katy, TX
mesolab.meas.ncsu.edu
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.
 
Oct 10, 2004
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Madison, WI
Let me reiterate: from a synoptic standpoint, there was little about this setup that was unusual, uncommon, or difficult for the atmosphere to achieve.
Spot on. Wasn't it just last October that the most powerful extratropical storm system (on a synoptic scale) in North American history took place? While there was indeed a tornado outbreak in the warm sector of that system, it didn't come anywhere close to the violence of Wednesday's.
 
Dec 14, 2003
327
2
6
Connecticut
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.
Yeah, I've been trying to wrap my head around this conundrum, too... What was so special about the 27th? What magic ingredient was present that was absent on, say, the 26th? The 26th had off-the-scale parameters too, but it produced mostly ragged HP storms, and yet on the next day with similar ingredients we get supercells about as classic as you can get - scads of them! Why? I've often suspected that we're missing something important concerning supercells and how and why they form. I've seen so many situations out here in CT where the numbers all look great - for example, a couple of days last summer with the supercell composite at 18 or higher - and yet storms stubbornly refuse to rotate. I've also seen gorgeous supercells on the Plains that spin for no discernible reason (e.g. supercell composite = 0). Granted, that particular index and others like it are hardly infallible, I'm just using it to illustrate a point. We may think we know all of the ingredients necessary for a tornadic supercell to form, but there are obviously still some things we are either overlooking or have yet to discover. The SPC and meteorologists in general did an absolutely fantastic job predicting Wednesday's outbreak, that's undeniable... But it's also undeniable that we still have much more to learn about these kind of outbreaks. We can say that a big outbreak is possible or even likely given a particular setup, but the exact conditions that guarantee a historic event like 4/27/11, that's something nobody really knows.
 
Dec 26, 2004
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Booneville, KY
CNN is now saying the total number of fatalities has reached 316. Depending on your source of information for the Super Outbreak (some cite 315 deaths, others 330), we may have just exceeded that outbreak in the most terrible of all statistics.

Also, the Alabama WX blog is saying that Dr. Tim Coleman was in Pratt City this afternoon and that he saw clear evidence of EF5 tornado damage there.
 
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J Allen

EF1
Mar 7, 2010
79
0
5
Palisades, NY
Dave and Jeff,
My feel on this is it just comes down to balance of those ingredients (quite strong 0-1 and 0-3 shear gave the precipitable water values enough to balance the storms back from a HP mode, when you see the image of the dynamic motion within the tornadoes there is no question that the low-level rotation was up there, the forcing was sufficiently subtle to keep it from getting messy). I was talking to a reporter regarding this yesterday, It just doesn't take as much to bugger up an event as we would wish for. If you look at the number of moderate/high risks issued by the NWS so very many of them don't verify to the extent that the NWS is concerned they might. It might be a blue-sky, it might be a dominant HP mode, it might be an ongoing MCS or linear system which peturbs the environment such that discrete supercells do not form. Seeing the radar loop from this event you got the ideal storm mode with sufficient seperation and things went crazy. The thing is, that any of the NWS high risks could do something like this, and their mission is to balance this risk assessment with the false alarm rate. As I pointed out to the reporter, just because it missed this town, or that location this time around, doesn't mean that the next time such ingredients line up over a given location that that town will be similarly spared.

I wonder whether perhaps the low layer instability is that ingredient for those stubborn storms, such that tilting of the horizontal vorticity/ingestion of rotation is countered by a scale scale outflow or just a general lack. After all we are talking a very finely balanced system here, we have all seen just how little it takes for a supercell to change complexion and stop functioning altogether. With indices, every one is imperfect, and I've yet to see a composite indice that actually impresses me with its performance. Its probably better to consider it from the basic mechanisms needed for supercell development, which are actually suprisingly low even in warm-season storms. Giving the storms too much of one ingredient or the other seems to be one of the major reasons for High risk non-events. With those days which don't look likely this commonly lets the specific indices down, and in general there is so much storm scale modification of the environment based on a host of dynamical reasons that all our indices really provide no more than a guide. A danger of approximating an approximation to the atmosphere so it would seem.

Agree completely on the rating front Jeff. I'm glad to see Greg Stumpf and other senior experts getting involved on the rating front (the 2000-2007 period did not see enough of this due to funding of QRT and hesitance to use it).
 
Jun 25, 2010
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Austintown, Ohio
Just heard Jim Cantore on NBC news say that global warming is behind this outbreak of torandoes. Who here belives that? I don't for one. I never thought I would see over 300 deaths from one day of tornadoes but sometmies you get the perfect storm. It is a natural event not something we caused.
 
Apr 2, 2005
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Norman, OK
www.chasetolive.com
Regarding the storm environment, it was not unprecedented. The tornadoes themselves were not unprecedented. However, the more unique aspect of this event (and 3 April 74) is the *number* of classic, discrete supercells *in* the volatile warm sector. No boundary augmentation of ingredients was necessary on the 27th, and nothing occurred that interfered with initiating discrete storms. In most serious tornado outbreaks similar to this, you get discrete initiation in the warm sector as a result of subtle processes (pre-frontal confluence bands, cloud streets, etc.). That was essentially what we were forecasting to occur as of 13z on 4/27, and unfortunately it occurred that way.

Convective mode and initiation is probably the toughest factor to account for in most outbreak situations. 4/27 was definitely on the upper end of the parameter distribution (STP = 12, which is 90th percentile for F4+ tornadoes), but what was more impressive was having multiple supercells *in* that environment. The prior day (4/26) in NE TX saw earlier than expected storm initiation. It was only about 1-2 hours early, but that meant the northward progress of the warm sector got cut off by the new supercells, farther south than earlier forecasts. Once you get something like that started, it can change the entire complexion of an outbreak, by changing the storm mode, location and timing.
 

J Allen

EF1
Mar 7, 2010
79
0
5
Palisades, NY
In response to the above this is a direct quote of me from an interview yesterday: "Global warming is a difficult question to answer, and a number of people are looking at this worldwide including myself. We are unable to say conclusively as yet whether storms will become more intense or frequent as such storms are produced by the balance of a number of ingredients. With the latest generation of climate models however, it is quite possible some answers and indications to this question will be available. Currently, the results of the work done suggest that thunderstorms will have the potential to occur more frequently and become more intense (producing larger hail for example). Whether that is realised or not, or we are seeing it currently is something we are unable to say for certain. By the same token, tornadoes are such small scale phenomena that we are likely not going to see the influence of global warming on that scale, however in the long run changes to the balance of ingredients may have an effect in the future. Thats the mistake people commonly make with climate change, attempting to attribute a single event, whereas the truth is all you can attribute is changes in the intensity and perhaps frequency, and even then for small scale events this is somewhat uncertain."

I won't get into the is it real debate as that will lead this waay off topic.