Rainy Summer Brought on by Winter-Like Jet Stream, Says UA Professor

TUSCALOOSA, Ala. – Everyone knows it’s been a rainy summer, but do the experts know why it’s been rainy? Yes…well, sort of, anyway.

“We know exactly what’s causing the rain,” said Dr. David Shankman, a professor of geography in The University of Alabama’s College of Arts and Sciences who teaches climatology classes. “One reason is what the jet stream is doing. The jet stream is in an unusual pattern for the summer, and it’s maintaining that pattern.”

During the summers, the jet stream — a band of strong winds, some 8-10 miles above the surface, that blow from west to east — typically flows across the northern portions of the United States in a relatively straight line. Occasionally, the jet stream plunges down, forming a u-shaped trough across the Southeast during the summer, bringing with it unstable atmospheric conditions, leading to rain.

This occurs because two distinct air masses are on either side of the jet stream, the UA geographer said. “These air masses are not distinguished so much by temperatures but by humidity levels,” Shankman said. On the northern side of the boundary is a dry air mass. On the southern side, is a humid air mass, incorporating moisture from the Gulf and the rest of the Atlantic Ocean. Near the trough site, the warm humid air cools as it comes into contact with the drier, cooler air of the other mass. The warm air condensates, as it cools, bringing rain.

Typically, the jet stream then returns to its northern location, and the Southeast returns to it standard summer parching. This summer, more often than not, the trough has stayed in place across the Southeast, and so has the rain. From Knoxville, Tenn., to Mobile and from Atlanta to Meridian, Miss., record and near-record summer rainfall amounts have soaked the region.

It’s not uncommon for jet stream troughs to linger over the region during the winter, but it’s unusual for them to linger during the summer. So, why has what’s been a relatively consistent summer pattern for the jet stream been so inconsistent this summer?

“That’s what we know less about — the behavior behind the jet stream,” Shankman said. “We still have many very basic questions about atmospheric processes that are unanswered, and one of those is the jet stream.”

Clearly, technological advancements and better understanding of weather and climate have brought many advances in weather forecasting, said Shankman, who teaches in UA’s College of Arts and Sciences. Predicting and projecting tornadoes and their likely paths has grown more precise, and weather predictions up to three days in advance are fairly reliable. However, because of the many unknowns, such as why the jet stream behaves as it does, accurate, long-term forecasting is tough, Shankman said.

So, what does Shankman think about the rain chances for the region for the remaining three weeks of the summer? He’s uncertain, but he doesn’t foresee an immediate change.

“As for now, there is no evidence this wet weather pattern is going to change.”

The College of Arts and Sciences is the University’s largest division and the largest public liberal arts college in the state, with approximately 5,500 undergraduates and 1,000 graduate students. The College has received national recognition for academic excellence, and the College’s students have been selected for many of the nation’s top academic honors, including 13 Rhodes Scholarships, 14 Goldwater Scholarships, seven Truman Scholarships, and 15 memberships on USA Today’s Academic All-American teams.