WeatherWhys Blog

American severe storms

American severe storms

America is a strong country populated by people who take risks. It is the underpinning of what has made us great. But the landform we live on is equally unique and rugged, featuring every ingredient that the severe storm recipe requires. Our US mainland is located at an average latitude of about 40 degrees north, placing it squarely in the “mid-latitudes”.  We are not exclusively in the tropics nor are we exclusively polar. In fact, the most potent aspects of both tropical heat and humidity are often composed with polar jet stream dynamics in the active spring and early summer. These drivers of weather meet in the mid-latitudes. Add to this the Gulf of Mexico source region of thermal instability and the semi-arid western high plains providing critical mid-altitude dry air, and you find the American Heartland resembles an almost idyllic laboratory setting for the genesis of strong to violent thunderstorms. It becomes a textbook example of the Perfect Storm script many times throughout the year.

And so it has been from the beginning that all the elements for powerful non-tropical thunderstorm formation are routinely present from Texas, north through the Great Plains, all the way to Canada- stretching eastward into the Mississippi and Ohio Valleys. Dynamics are also favorable for a critical twist in the atmosphere with height that promotes tornado development. The United States leads the world in tornado occurrence each year- by a large margin.

In America, the current 3-year average stands at 1382 tornadoes annually. In 2011, 1691 tornadoes hit the US boosting the annual tally up from about 1000 per year, which had been the average through much of the last century. The next busiest country is Canada with reports of about 100 tornadoes annually. The 3-year US average is 206 tornado related fatalities. That has also dramatically increased following 553 tornado deaths recorded in 2011. The deadliest year was 1925 when 794 people were killed, most of them (695) in the Great Tri-State tornado of March 18th. On the other hand, Canada has seen 67 fatalities over the last 30 years, an annual average of slightly more than 2 deaths, merely 1% of the US tornado related fatalities. And while tornadoes cause more than a billion dollars in damage each year, it isn’t just tornadoes that bring loss of life and property from severe storms.  High wind and large hail accounts for tremendous property loss in this country.

Severe storms develop at various seasons across the nation. Starting in the Gulf Coast region during February and March, the primary target area moves slowly north and west in April and May. The season pushes into the Great Plains and middle Mississippi Valley and finally by June and July the severe storm primary season reaches the Dakotas and southern Canada. In essence, storm seasons follow the polar jet steam, which retreats from south to north through late winter and spring. A second severe storm season takes place in late October and November when the polar jet stream once again returns south to the Gulf Coast region as winter descends in the northern hemisphere.

Strong thunderstorms develop due to instability in the atmosphere within an environment of strong vertical wind shear and counter clockwise spinning (helicity). From time to time all the prerequisite conditions occur simultaneously and a tornado will emerge from the base of the thunderstorm. A tornado is a rather unique meteorological phenomenon. While rain and snow falls from a cloud, it is altogether different for a cloud to strike the ground from above as a violent pendant. A tornado from the sky would seem to be as unusual as a strong earthquake on the land or a rogue wave at sea. All are exceedingly rare in the big picture and it is extremely unlikely that any particular individual in the US would experience one of the above 3 events. Yet, they all happen and all claim lives.

Of all types of weather, a severe thunderstorm packs the most immediate potential deadly threat of any weather type. There are 5 main hazards which may bring death or destruction from above: 1) tornado damage, 2) wind gust damage, 3) hail damage, 4) flash flooding, and 5) lightning. According to the NOAA Office of Climate, Water and Weather Services, detailed information on these hazards is available for the past 70+ years. But just looking at the 10-year period of 2002-2011, here is what we know about the severe storms impact in the America.

Tornado related deaths now average 108 annually, with 1288 injuries costing $1.8 billion in damage. There was a huge tornado uptick in 2011 due to the southeastern US Super Outbreak of April. The tally, again, was an astounding 553 fatalities (243 in Alabama on one day alone) with 5,483 injuries. Last year’s damage from tornadoes totaled $9.493 billion.

Severe thunderstorm wind gusts (not associated with tornadoes) now average 22 fatalities annually, with 267 injuries and $568.7 million damage.

Severe thunderstorm hail damage now averages less than one fatality annually (only 2 in the past 10 years), with 46 injuries and $858.3 million damage- in large measure to crops.

Severe thunderstorm flash floods account for an average 55 fatalities annually, 52 injuries and a tremendous $1.2 billion damage.

Finally, lightning from thunderstorms (severe or not) takes a significant toll. The 10-year average is 37 deaths annually with 225 injuries and $51.6 million damage. In 2009 there were 34 lightning deaths with only 21 tornado and 22 thunderstorm wind gust fatalities. It was the first year that lightning fatalities outpaced all other categories.

Many people have been killed by other weather related factors. Of all considerations, people die from heat exposure, winter storms, droughts and dust storms. But looking at the composite of these numbers puts what we do in the weather enterprise into proper perspective.

Here are the totals for the past 10 years from disruptive weather of all types:
Killed- 6,408,
Injured- 33,432,
Damage- $240.3 billion.

These are war statistics: nearly 40,000 casualties costing almost a quarter trillion dollars since 2002. It is a most poignant reason why we have a National Weather Service and an army of citizens who are constantly watching the weather. Without our sophisticated warning capabilities, these numbers would likely be much higher.

So here’s to those guardians of the sky in places like Oklahoma City, Kansas City, Dallas and St. Louis, Des Moines and Omaha. Working around the clock in weather offices, at TV stations and in the farthest outposts- always aware of what’s happening in the sky. Americans of the Midwest and Great Plains are made of strong material. They resemble the rugged American landform, robust and sturdy. But perhaps more amazing are the tough meteorologists who have perfected their skill through time tested method and scientific breakthrough. Season after season, their instinct to outguess nature’s next move, with help of modern technology, proves over and over to be “life sparing”. The pioneering Spirit of America that they possess has built a weather hedge around the precious population of this incredibly beautiful region of the county- beautiful for its form and its resilience. So the next time a tornado watch is issued in tornado alley, consider: these professionals are keenly attentive- both energized and in their element. These sentinels are alert, ready to sound an alarm and protect their neighbors from the violence of nature. It is their destiny. As severe storm meteorologists, they are ever vigilant “guardians of the sky”- the epitome of professionalism.