The awful wail of the tornado warning siren is nothing new in May in Minnesota.
Most of us who go back a few decades in this state know all too well the routine we revisited on Sunday: round up the family, grab the flashlights and radios, run for shelter.
But we seem to be practicing this drill more often in recent years.
Indeed, we are.
The numbers of tornados spotted in Minnesota have risen dramatically over the decades, [PDF] according to the Storm Prediction Center of the National Oceanic and Atmospheric Administration.
During the 1980s, 199 tornadoes were detected in Minnesota, says the Prediction Center’s website. During the 2000s, the count had risen to 426.
One explanation, of course, is that we’ve gotten ever better at spotting the dreaded twisters.
Counting the killers
If you count the killer tornadoes alone, the number really hasn’t risen in recent years. These super violent storms took four lives in Minnesota during the ’80s, seven lives during the ’90s and four during the decade that ended in 2009.
Now, sadly, Minnesota added another life to the toll on Sunday. At least one person was killed in North Minneapolis — reportedly struck when a falling tree slammed atop a van. Nearly 30 others were injured and at least 100 buildings were significantly damaged.
Streets in the stricken area were sheared of trees. Downed power lines and natural gas leaks posed an ongoing danger to neighborhoods on Monday. Seven North Minneapolis schools were closed for the day as were schools in Fridley.
Even though the numbers of killer tornadoes have remained more or less level in Minnesota, we do seem to be going through a spell of rougher than usual weather nationwide.
The Prediction Center’s monthly counts support the impression you may have gotten from anecdotal reports of tornadoes roiling through Arkansas, Alabama, Missouri, Mississippi and other states this spring.
April 27 was the second deadliest day in U.S. history in terms of tornado deaths. The Associated Press reported that storms ripping across seven states killed 337 people that day. The largest death toll ever was on March 18, 1925, when 747 people were killed in storms that raged through Missouri, Illinois and Indiana, AP said. The second deadliest day had been in March 1932, when 332 people died, all in Alabama.
By the Prediction Center’s preliminary counts, more than 1,000 tornadoes threatened the nation during January through April this year. That’s up from about 200 for the same months last year. Typically we would see about 800 twisters across the United States for an entire year.
Knowing the cause but not the why
Weather experts can explain the conditions that cause tornadoes.
Here’s the explanation from a NOAA web page.
“Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.
“During the spring in the Central Plains, thunderstorms frequently develop along a ‘dryline,’ which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.
“Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows ‘upslope’ toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.
“Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.”
But that doesn’t explain why we’re seeing those conditions form more often.
Some blame global climate change, arguing that warming trends accelerate evaporation from the oceans into the atmosphere, thereby building the conditions that spawn violent weather.
Here’s what NOAA scientists say to that theory: “The best answer is: We don’t know.”
There simply isn’t enough evidence to determine whether warming has influenced certain trends in extreme weather events such as tornadoes, hail storms and severe lightning storms, says the National Science and Technology Council’s Scientific Assessment on Climate Change. [PDF]
The mystery comes down to the difference between weather and climate.
Tornadoes are short-fused weather, threatening for second or minutes at a time. And on a global scale, they are miniscule – often covering an area less than a mile across.
In contrast, climate trends take shape over many years, decades, or millennia, spanning vast areas of the globe. Climate models cannot resolve questions about a given tornado outbreak or an individual spat of severe thunderstorms. What they can do is indicate broad scale shifts in three favorable ingredients for severe thunderstorms: moisture, instability and wind shear.
But having those ingredients in place doesn’t guarantee a tornado.
From what is known so far, some of those key ingredients may increase in a warmer world while others may decrease.
“Finally, tornado recordkeeping itself also has been prone to many errors and uncertainties, doesn’t exist for most of the world, and even in the U.S., only covers several decades in detailed form,” said the NOAA scientists.
Sharon Schmickle writes about national and foreign affairs and science. She can be reached at sschmickle [at] minnpost [dot] com.