Weathercatch: Why tornadoes ramp up in springtime – even here

As millions of Americans hunker down to quell the spread of the novel coronavirus, a different kind of trouble looms: tornadoes.

Although they can occur any time of year in the United States, their peak season is April, May and June. And with April not yet over, a high number of these violent storms have been carving paths of destruction across the South and Southeast.

Just last Sunday, a series of tornadoes struck Mississippi, Alabama and Texas – exactly one week after a deadly outbreak on Easter Sunday when more than 40 twisters spanned 1,200 miles from Texas to South Carolina.

While tornadoes aren’t common in the Pacific Northwest, they’re most likely to occur in springtime. In June 2004, three small twisters touched down in Washington state. This year on March 31, a small tornado struck near Richland. On April 5, 1972, the deadliest tornado recorded west of the Rocky Mountains ripped through Vancouver, killing six people and injuring 300. Later that afternoon, two smaller funnels touched down 50 miles west of Spokane.

Whether in Washington state or Washington, D.C., why is the threat of tornadoes greatest in spring?

Mother Nature’s fury stems from a clash of changing weather conditions. Pockets of the season’s increasingly moist, warm air collide with lingering cool air. Also this time of year, high-altitude winds known as the jet stream begin to dip southward, pulling cold air with them. Sometimes the resulting instability combines with wind shear – a difference in wind speed or direction at different atmospheric levels, which in turn causes the battling air masses to spin in columns and become tornadoes.

An average of two tornadoes touch down in Washington state each year, according to the National Oceanic and Atmospheric Administration. Compare that tiny number to Texas, which gets an average of 146 each year and Kansas, 92, and Oklahoma, 65.

A key ingredient missing in our region is low-level moisture, or humidity, necessary to kick-start thunderstorms that produce twisters. Because air blowing off the Pacific Ocean is cool, it holds less moisture than the warm, humidity-laden air flowing off the Gulf of Mexico.

As the calendar slips deeper into peak tornado season during the COVID-19 pandemic, more communities in the eastern half of the nation will be confronted by two overlapping crises.

Here in the the Inland Northwest, we can thank the Pacific Ocean for providing us a more temperate climate where late-season frost is a bigger concern than tornadoes.