We may see a slightly above normal season for tornado activity this spring in the U.S. Most of the twisters are expected to form east of the Rockies, especially in Tornado Alley, from Kansas southward into Texas.
Since early this year, we’ve been in a La Nada, the sea-surface temperature pattern in the south-central Pacific Ocean that is between the cooler La Niña and warmer El Niño. Some computer models indicate that ocean temperatures could cool into a new La Niña later this year. We could see an outbreak of twisters sometime in May or June if ocean temperatures suddenly become colder.
In the U.S., active tornado seasons almost always are caused by unusually cool air persisting over the Midwest. It was during extremely chilly La Niñas that we saw the so-called “super outbreaks” of tornadoes in 1974 and 2011. During these strong La Niña springs, arctic cold fronts advance further south, providing ample opportunity for cold air masses and warm air masses to collide, causing more tornados.
The U.S. has about 1,200 tornadoes each year, about four times more than Europe. Twisters mostly strike from March to August. On Jan. 30, 2013, several strong tornadoes were reported in Georgia, Mississippi and Alabama.
In our part of the country, tornadoes are not as common because the Rocky Mountains block the moist, unstable air from the Gulf of Mexico that is needed to spawn twisters. However, on April 5, 1972, there were six fatalities and more than 300 injuries around Vancouver, Wash., from an F-3 tornado. An elementary school, a grocery store, a bowling alley and about 100 homes were damaged or destroyed. Smaller tornadoes touched down in Grant, Lincoln and Stevens counties that day.
On May 31, 1997, four F-1 tornadoes hit Stevens and Spokane counties. Another F-1 tornado was sighted near Athol in North Idaho. Baseball-size hail damaged property in the Lewiston area. Fortunately, there were no deaths or injuries.
It’s possible that we may see a twister in the Inland Northwest during the spring or early summer season as thunderstorm forecasts are expected to be higher than normal.