Our thoughts at TWC are with those who have been affected by the recent siege of severe weather including twisters, wind/hail damage, and flooding.
Here are some thoughts about last week's catastrophic tornado outbreak ...
Terrible perfection
I posted that graphic last Wednesday afternoon on my TWC Facebook page. The ingredients were "textbook." I mean, literally what I learned from a textbook more than 30 years ago. The atmosphere was explosively unstable with summerlike heat and humidity, interacting with a classic wind shear setup as a strong jet stream and upper-level trough crashed overhead. Also, dry air aloft (dark red shades on the left image below) put a lid on things and allowed the energy to build up until it blew sky high.
[Image source: NCAR/UCAR]
Not only were the elements perfect for a tornado outbreak, they were present to an extreme degree. The observed EHI ("Energy Helicity Index"), a measure which represents a combination of instability and wind shear, was extraordinary, higher than during the time of two notorious [E]F5s, the Moore, Oklahoma and Greensburg, Kansas tornadoes on May 3, 1999 and May 4, 2007, respectively.
Such a set of combustible ingredients, plus a remarkable number of supercells with hook echoes on radar and "ground-truth" observations of tornadoes, led Dr. Forbes and me to decide to up TWC's "TOR:CON" index to a 10 for northern Alabama, meaning a 100% chance of a tornado within 50 miles, the first time that's been done since the product was developed a couple of years ago.
Capricious
Many counties have been affected by the calamity, with the effects extending beyond just those people and locations struck directly by the tornadoes. Yet even with this widespread an outbreak, an extremely small percentage of the land area of northern and central AL was actually hit.
Above are screen captures from a video taken from a helicopter during Dr. Forbes' aerial survey of the damage. Tornadoes never cease to amaze me in their capriciousness, even in a situation such as this with relatively widespread devastation, leaving one part of a neighborhood unscathed while an adjacent one is in ruins.
Tornado Alley?
Last week's outbreak, and the past month of tornadoes, reinforce that "Tornado Alley" is *not* just in the Great Plains.
The first map below is one that in the past TWC has shown, and NOAA has graphics online depicting a similar area.
The second map plots all tornadoes in the official database which are [E]F 3 and higher on the [Enhanced] Fujita scale, those which account for approximately 3/4 of all tornado fatalities. IMO, true Tornado Alley is represented by the much broader area in which all those red lines are concentrated.
What about climate change / global warming?
What's needed relative to this question is an apolitical, non-reactionary, objective assessment. However, as soon as the extremity of last week's outbreak became apparent, there were, as is usually the case, reactions on two extremes. One headline on the web blared that hundreds were killed in "states represented by climate pollution deniers," while a high-ranking federal government official was reported to have dismissed climate change as a factor, quoted as saying, "Actually what we're seeing is springtime."
The former is obscene and the latter fails to represent what's happened weatherwise recently, which actually, no, has not been a typical month in spring.
On the one hand there is no decisive trend in overall tornado occurrences, and while in recent years there's been a rash of outbreaks which have been unusually far north and intense for the time of year (including the one in Wisconsin last month), the one last Wednesday was geographically consistent with April climatology.
On the other hand, this event needs to be considered in the *context* of the relentless series of severe thunderstorm and tornado outbreaks which started on April 4 and culminated on the 27th. The number of severe weather reports and confirmed tornadoes has been atypical even by April standards, shattering the previous records. Even taking into account limitations of the historical record, the numbers have been stunning.
As noted above, the combination of instability and wind shear was extreme even by classic tornado setup standards. The temperature in Laredo reached 111 degrees the day prior to the peak outbreak, the hottest on record at that location for so early in the season. Precipitation extremes have been extreme even by extreme precipitation standards, with April rainfall upwards of 20" in Arkansas and record levels on some rivers in the central U.S., juxtaposed with an exceptionally large amount of Texas being classified in extreme or exceptional drought. [Tue May 3 addendum: And the warmest April on record in the UK.]
And all of this is in the context of a relentless series of extreme weather events in the U.S. and other countries during the months preceding April, and many others worldwide during recent years which I've documented and which have had apparent a physical connection with a warmer atmosphere.
I've also read categorical statements assigning a one-to-one cause-effect attribution to La Nina. But while La Nina is present now as it was during the 1974 Superoutbreak, it was not during some of the other most notorious outbreaks of all time, such as the 2002 Van Wert / Mossy Grove, 1999 Oklahoma, 1991 Andover KS, 1984 Carolinas, and 1965 Palm Sunday outbreaks.
The atmosphere is extraordinarily complex, and ultimately what's happened the past month is probably a combination of influences, including La Nina, other natural variability, and anthropogenic global warming.
Bigger than the Superoutbreak?
Hanging on the wall of my office at TWC (low-res cellphone pic above) is an original map of the April 3-4, 1974 Superoutbreak, which I've had since the '70s (a high-res online map can be seen via this link). As Dr. Forbes said a few days ago in his blog about it, that event is the benchmark for tornado outbreaks.
As soon as the number of tornado reports from last Wednesday exceeded 148, the number of tornadoes within 24 hours in the 1974 Superoutbreak, there was talk about this outbreak being bigger than that one. I sent an email to the staff at TWC on this topic and provided commentary about it on camera, as I wanted to make sure that there was awareness of how extreme The Superoutbreak was and that the proper perspective is being applied in comparing the two.
That one had *thirty* F4 or higher tornadoes on the original Fujita scale. Communities were reeling from that level of damage in Alabama, Georgia, Tennessee, North Carolina, Kentucky, Indiana, and Ohio. Among those 30, six F5s occurred across five of those states. Tornadoes struck along a total path length of more than *2500* miles.
We'll need to await the final numbers before doing a final comparison, but with the latest information as of this writing, and even accounting for subjectivity and changes in tornado assessments over the years and decades as the Fujita Scale evolved into the Enhanced Fujita Scale, it looks like the April 27, 2011 outbreak will fall far short of some of those '74 numbers.
It's not just all about the total number of tornadoes, plus, despite definitive official statements about a total of 211 having occurred (and a report today that it's been raised to 312!), any such estimates are premature, as those are just preliminary *reports*, some of which were duplicates of the same long-track tornadoes.
That all having been said, no matter how this all shakes out in the end, the April 27, 2011 outbreak will go down as one of the biggest and worst on record. The amount of energy it unleashed is hard to comprehend.
By why do any of these such statistics even matter? Well, scientific assessments of ingredients and results help meteorologists understand the phenomena and factors and climatology involved, and apply that to forecasting future events.
But to people who have lost their lives, and their surviving family, friends, and colleagues, the meteorological statistics don't matter. Those folks are gone. And the latest death toll is now well over 300 and higher than that of the Superoutbreak and among the few highest on record in the U.S. from a tornado outbreak, and with many other people still unaccounted for.
The Katrina of tornado outbreaks
So that makes this the Katrina of tornado outbreaks, in the sense that it's a vivid and tragic reminder that although high death tolls from tornadoes and hurricanes are much less common than they were in the 19th and first part of the 20th centuries, we are not immune to them, even in this era of modern meteorological and communication technology.
Timely and accurate outlooks were disseminated in the hours and days leading up to the outbreak as well as short-term warnings once the supercells formed. This included "tornado emergency" level warnings by the National Weather Service; in fact, there were so many of those issued that it was mind-boggling.
I think I speak for all meteorologists/forecasters when I say that I/we are at peace with feeling like we did everything we could while also being heartbroken that it still wasn't enough. My heart sank as more and more reports of extreme damage came in that evening with a fatality count that has kept rising since then.
The power of the atmosphere is overwhelming, and how vulnerable we are to it has been reinforced yet again. Weather is as awe-inspiring, fascinating, mysterious, fearsome, and humbling as when I first became obsessed with it as a child. I wish it didn't have to have such tragic consequences.
[Image source: http://bit.ly/jlOpvd]
ADDENDUM 3PM EDT TUESDAY MAY 3, 2011
To follow up on some of the comments received so far ...
For those who do not understand why I chose Katrina as the hurricane to highlight, perhaps this will help. It illustrates what I was referring to in that portion of the blog entry, which is the death toll of tornado outbreaks and hurricanes in the era of modern technology vs. prior to that.
