Historic floods, heavy snow, rising temps: The science behind Montana’s catastrophic weather

Nora Mabie

Recent catastrophic weather events have left many wondering: What just happened?

Areas of southwest Montana are flooding, the northwest region received heavy snowfall in mid-June, and temperatures statewide are expected to rise into the 80s and 90s on Thursday and Friday.

Brian Tesar, a National Weather Service meteorologist based in Billings, said the flooding in southwest Montana was primarily a result of rainfall on heavy snowpack.

Recent winter storms and cooler May temperatures contributed to normal and above-average snowpack sitting in mountains above 6,000 feet. Warmer June temperatures softened the snow, which Tesar said “was primed to melt real fast.”

Then an atmospheric river — a long, narrow region of the atmosphere like a river of water vapor in the sky — moved in from the Pacific. When it dropped heavy rainfall on the Beartooth Mountains, the heavy, wet snow melted, and melted fast.

“We had 7 inches of water come off the mountains in 24 hours,” Tesar said. Normally, 7 inches of water would melt off over the course of several weeks. In a normal year, rivers gradually rise as the snowpack melts, but the combination of heavy rainfall and wet snow brought more water than channels could manage, making

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