Temperature, moisture play role in tornados

Last week’s tornado outbreak in Kansas was considered to be the worst in more than 50 years.

More than 95 percent of the town of Greenburg was destroyed Friday night by a monster twister. In a town of 1,500 people, piles of rubble towered as high as 30 feet. The huge F-5 tornado, the highest level of classification, was reported to be 1.7 miles wide and tracked for 22 miles with winds estimated at approximately 205 miles per hour.

“One small tornado took out trees within 20- to 30- feet of us,” Ed Petrowsky, who lives about 30 miles from Greensburg, told me. “A friend was storm-spotting a half-mile west of us when another funnel took out the poles and put power lines across his pickup. There were at least one big twister and three smaller ones. I have never seen a twister plow up a road until now.

“Most of the houses and businesses were wiped off their foundations,” Petrowsky said. “People who had lived here all of their lives did not recognize their own street.”

The worst outbreak of tornadoes in U.S. history occurred on April 3 and 4 in 1974, the so-called “Super Outbreak.” A total of 148 twisters touched down in 13 states from Illinois and Indiana southward into Mississippi and Alabama. Three hundred and thirty people were killed and 5,484 people were injured over a period of 16 to 18 hours.

The property damage estimate was $600 million. The most deadly and damaging tornado ever occurred on April 4, 1974 at Xenia, Ohio. Half of the town was destroyed and property damage exceeded $100 million.

In an average year, approximately 1,200 tornadoes are sighted in the United States, more than any other place in the world. More than 60 percent of all U.S. tornadoes each year occur in what is called, “Tornado Alley,” which stretches from Texas and Oklahoma northward through Kansas and eastern Colorado into Nebraska and Iowa.

Wind-measuring instruments are often destroyed by tornadic blasts exceeding 250 mph. Pieces of straw can penetrate wood at wind speeds exceeding 230 mph. The strongest wind speed ever measured in a tornado was 280 mph in Kansas in 1997.

Usually beginning in early spring, low-level winds from the south/southeast bring large amounts of moisture and subtropical air northward from the Gulf of Mexico into the central and southern Great Plains and the lower Midwest. At the same time, much cooler and drier air spills down the eastern slopes of the Rockies, crashing headlong into the warm, moist flow. This is a recipe for meteorological disaster. As the colder, heavier air tries to push toward the ground, the warm air attempts to rise through the sinking currents. The huge differences in both temperature and moisture at various levels often triggers strong winds, torrential rains, crop-damaging hail and, occasionally, deadly tornadoes.

Fortunately in the calmer Inland Northwest, the Cascade Mountains to the west and the Rockies to the east usually protect us from extremely powerful thunderstorm activity. But, every spring season, and sometimes during the hot summer months, we do see an occasional period of extreme weather conditions.

For example, last spring on June 4, hail “the size of hen’s eggs” caused widespread agricultural losses in parts of Eastern Washington, North Idaho and Western Montana.

The deadliest outbreak of tornadoes in our part of the country occurred on April 5 and 6 in 1972. There were six fatalities and more than 300 injuries alone in Eastern Washington. Damage exceeded $50 million.

On May 31, 1997, four F1 tornadoes hit Stevens and Spokane counties. Another F1 tornado was sighted near Athol in North Idaho. Baseball-sized hail damaged property in the Lewiston area. Fortunately, there were no deaths or injuries.