After 2011’s Destruction, 2012 Sees Far Fewer Tornadoes
In stark contrast to 2011, the number of tornadoes in the U.S. since the spring has been running well below average, putting the U.S. on course to come close to record territory. As we reported on August 1, the weather pattern responsible for the record drought that enveloped the country beginning in the spring and continuing straight through September has effectively choked off tornado activity. The month of July, for example, saw the fewest number of tornadoes since modern records began in 1954.
According to the Storm Prediction Center in Norman, Okla., part of the National Oceanic and Atmospheric Admnistration (NOAA), the preliminary total of 757 tornadoes through Sept. 21 is about 400 twisters shy of what would be expected in an average year. That is in stark contrast to last year, when the annual running total was 400 tornadoes above average at this point in the year.
Considerable year-to-year variability in tornado numbers is quite common, although this huge swing is striking. Because tornadoes require a particular combination of ingredients in order to form, the number of tornadoes is highly dependent on the prevailing weather patterns.
In order for tornadoes to form there needs to be an ample supply of warm, moist air, strong winds aloft (this is necessary to create wind shear, the most critical ingredient of all), and a trigger mechanism to spark thunderstorms, such as a cold front. This year, there were often times when some of these ingredients were present, but not all of them, or not all in the right amounts. There were some noteworthy severe weather events this summer, including the derecho event that caused heavy damage to the Ohio Valley and Mid-Atlantic states.
The ridge of high pressure, or "Heat Dome," that helped lead to the third-warmest summer on record in the lower 48 states also suppressed thunderstorm formation, as did the extremely dry soils throughout the High Plains and Midwest, otherwise known as "Tornado Alley."
The huge swing in tornado numbers from last year is not indicative of global warming-related effects on tornadoes, which is an emerging area of climate science. Studies of how the environment that gives rise to severe thunderstorms and tornadoes may change as global warming continues have found that the number of thunderstorm days may increase in parts of the U.S. — owing to an upward trend in heat and humidity — but wind shear may decrease, which could hold tornado numbers down.