Characteristics of Catastrophe: New Details on the April 27, 2011 tornado outbreak

Huntsville, Ala. (WHNT) — A summary of data collected on April 27, 2011 by scientists and students at The University of Alabama in Huntsville’s Earth System Science Center is now published in the Bulletin of the American Meteorological Society (BAMS). The article is available online.

The deadly tornado outbreak of April 27, 2011, was unique in several meteorological aspects, but what made it most unique wasn’t the number of powerful tornadoes spun out that day, or the death and destruction they caused.

What makes that storm system most unique from a scientific perspective was its timing and location: It was the first tornado outbreak of its size to occur at a time and in a place where modern scientific instruments could be brought to bear gathering the data scientists need to understand more about what triggers that kind of storm system and what made it evolve through the day.

Why were the tornadoes so long-lived? What about the environment that afternoon made each storm so capable of producing tornadoes? Questions like these are discussed in the UAH article.

A combination of weather instruments, including UAH’s Mobile X-band radar (MAX) and MIPS, a mobile suite of upward-looking instruments, collected data about the April 2011 storms. Published today, some findings from early analysis of data gathered that day include:

  • Roughly 90 percent of the super cell thunderstorms that day produced tornadoes, compared to the normal rate of about 25 percent. While some factors leading to tornado formation are known — and the known parameters that warn of potential violent weather were unusually high on April 27 — it is not clear what combination of weather features that day turned so many storm cells into tornado producers.
  • While the cause/effect relationships might be debated, it seems likely that several factors external to the storms — including topography, gravity waves in the atmosphere, and the presence of a thermal boundary — each interacted with different storm cells during the day at times when tornadoes were created or when existing tornadoes intensified.
  • A weather balloon launched at UAH got detailed temperature readings from the surface into the upper atmosphere, and found a temperature profile that contradicts what weather models had predicted for storms of that type. This could change the scientific understanding of the large-scale physics at work in major, violent storm systems.

Some of the numbers from that April storm system are staggering: Between April 25-28, a frontal system spun off 355 confirmed tornadoes from Texas to New York and southern Canada, with the largest number in Alabama. From midnight to midnight on April 27, a record-breaking 199 tornadoes touched down in Mississippi, Alabama, Tennessee and Georgia, including four EF5 tornadoes, the largest and most powerful tornadoes in the Enhanced Fujita scale.

There were 324 tornado-related deaths, including 238 in Alabama. Storm damages for the four-day period were estimated at $11 billion, making it one of the most damaging storms of any kind to ever hit the U.S.

An early morning “squall line” that had not been predicted produced 69 tornadoes, including several EF3 tornadoes. Some of those storms had damage paths as much as half a mile wide.

Knupp says future research will likely be done to see exactly why the computer models were not able to predict the morning round of severe weather.

A large-scale, counter-clockwise circulation formed within the northern portion of this squall line near Cullman County, Alabama, and moved northeast over mostly forested, rolling hills through Marshall, DeKalb and Jackson counties in Alabama producing about 15 smaller tornadoes, EF1s and EF2s, Knupp says.

“We’re looking at variations in topography that channel the air, that accelerate the air over ridges and change air flows,” he said. “These are things that can significantly modify low level inflow into the storms, in some cases setting up conditions that are favorable for tornadoes to form.”

Knupp and his team hope to soon begin radar modeling of several aspects of storm development, including low level air flows, gravity wave interactions and other analysis of the data from the April 2011 storms to provide greater insights into the physics that drives systems of that kind.

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