P!NPoints: April a deadly month for tornadoes

As the saying goes, April is known wet and windy weather, and is a common time for consistent thunderstorms. Last month, however, the Southeast United States saw a departure from the norm when dozens of tornadoes struck cities across the area, leading to considerable damage and loss of life. Tornadoes also made news across the Midwest and Northeast as well.

According to analysis by the The Washington Post’s Capital Weather Gang blog on April 28, “Thanks to the 139 tornadoes reported (this number will change as the National Weather Service conducts their damage assessments), April 2011 has now almost certainly seen more tornadoes than any other April on record since 1954 when an estimated 407 tornadoes descended from the heavens.”

On April 27-28, historic storms struck across the Deep South. This system created more tornadoes in a single day than any recorded in U.S. history. According to varying measures appearing in news reports, as many as 300 tornadoes formed during the outbreak across Alabama, Mississippi, Tennessee and Georgia between April 27-28. The previous record, according to the National Oceanic and Atmospheric Administration, was 148 tornadoes formed over April 3-4, 1974.

The month was notable not only for the frequency of tornadoes, but in the severity of tornadoes hitting states from Missouri to North Carolina. They included, on April 27, “rare, mile-wide plus beasts, causing hideous damage, including denuded trees, flattened buildings, and pancaked cars,” according to the Post.

The April 27-28 storms were also the most deadly in a single day in more than 85 years. Last month’s storms led to more than 320 deaths from one system, compared to a storm system in March 1925 that led to 747 fatalities across seven states. The previous weeks showed further evidence of an unusually high rate of tornadic activity.

On April 22, the St. Louis area was struck by a series of tornadoes, including one that tore the roof off the main terminal of the Lambert-Saint Louis International Airport, shutting it down for two days. The storm was so strong, it shattered plate glass at the terminal and rocked airplanes on the ground, causing a few non-fatal injuries. A tornado warning for the storm was issued about 35 minutes before the tornadic supercell struck the airport. The storm was reported to have reached level EF4 on the Enhanced Fujita Scale, on which EF5 is the most powerful rating. According to the EF4 rating, the tornado struck with winds of up to 200 miles per hour.

On April 16, more than 70 sightings and 28 confirmed touchdowns were reported as a major storm cell struck in 19 North Carolina counties, concentrated mostly in the eastern part of the state. Twenty-four North Carolinians perished in the storms, many of whom lived in mobile homes. The 28 confirmed tornadoes were the most recorded in the state in one storm, according to National Weather Service forecasters.

Experts have considered several explanations for the outbreak of tornado activity. The Washington Post’s Capital Weather Gang blog speculated that the perfect combination of a warm and humid atmosphere, strong jet stream winds and “atmospheric wind shear” – winds that vary in speed and direction with height. Combined with a cold front, these factors may have led to the high incidence of severe storms that left millions of dollars of damage across several states.

For other perspectives on these record tornadoes, you can read more here and here. For more information about how tornadoes form, visit the National Weather Service’s informative guide here.