Drought Helps Dry Up Tornadoes as July Sees Record Low

By Andrew Freedman

Thanks, in part, to the record-setting drought that is gripping much of the U.S., the country had a record low number of tornadoes for the month of July, and the lowest number of tornadoes for any May-through-July period since high quality recordkeeping began in 1954, according to the U.S. Storm Prediction Center in Norman, Okla. With just 24 tornado reports in July, the U.S. saw fewer tornadoes this month than Canada did, which is unusual.

A tornado captured near MooseJaw, Saskatchewan on June 26, 2012.Credit: Twitpic/@JeffAdams.

With about 231 tornadoes during the May-to-July period, this year saw far fewer tornadoes than the previous low, which was 349 tornadoes recorded in 2006. The Canadian province of Saskatchewan alone has had more than 30 tornadoes this summer, with most of them touching down in July, according to The Weather Network. “2012 has really been the most incredible tornado season anyone out here on the Prairies has seen,” a Canadian storm chaser told The Weather Network. According to The Weather Network’s website, American storm chasers have migrated north to chase severe storms.

The majority of the lower 48 states are currently in the grips of one of the most widespread and intense droughts in U.S. history, and the drought itself is inhibiting storm formation by keeping the air drier than it otherwise would be. The sinking motion caused by a stubborn “Heat Dome” of High Pressure has also acted as a limiting factor for storm formation.

The quiet U.S. tornado season and active Canadian weather pattern is related to the same weather setup that has maintained sizzling heat and drought conditions across much of the U.S. this summer. The jet stream, which is a river of air that flows in the upper atmosphere and helps steer storms, has been parked well to the north, in Canada, giving thunderstorms there a major dose of wind shear, which is a key ingredient in forming tornadoes and large hail. By contrast, the severe thunderstorms that have erupted in the U.S. during late spring into the middle of summer have occurred in environments that lack strong upper level winds, and they have mainly produced small hail and damaging straight-line winds, which do not require strong wind shear in order to form.

Tornado trends after adjusting for inflation caused by changes in tornado detection technology, and the increase in storm spotter networks. Credit: NOAA/SPC.

The low tornado numbers this spring and summer contrast with last year’s widespread and deadly tornado outbreaks. These two years are not necessarily indicative of how global warming is likely to affect tornadoes, according to Harold Brooks, a tornado expert at the National Severe Storms Laboratory in Oklahoma. “There's no expectation from the climate change/severe thunderstorm research community that large changes in tornadoes will occur as the planet warms,” Brooks said in an email conversation. Studies of how the environment that gives rise to severe thunderstorms and tornadoes may change as global warming continues shows 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.