From dry to deluge, how heavy snow, rain flooded Yellowstone

RED LODGE, MONTANA >> Just three months ago, the Yellowstone region like most of the West was dragging through an extended drought with little snow in the mountains and wildfire scars in Red Lodge from a year ago when the area was hit by 105-degree Fahrenheit heat and fire.

Rivers and creeks this week raged with water much higher and faster than even the rare benchmark 500-year flood. Weather-whiplashed residents and government officials raced to save homes, roads and businesses.

Mostly natural fleeting forces with some connections to long-term climate change combined to trigger the switch from drought to deluge, scientists said.

It was a textbook case of “weather weirding,” said Red Lodge resident and National Snow and Ice Data Center deputy lead scientist Twila Moon. Her cropped hair was up in a sweat band and she was covered head to toe in mud from helping residents clear out flooded areas.

But these were conditions unique to the northern interior West, scientists say. Most of the West doesn't have much snow and will keep struggling with drought.

In the Yellowstone area, after a winter with light snow, it finally accumulated a couple of months ago, wet and cold, likely thanks to the natural weather event La Nina, building the snowpack in the mountains to above normal levels.

Snow fell so hard on Memorial Day weekend people had to abandon camping gear and get out of the park while they could, said Tom Osborne, a hydrologist who has spent decades in the area.

Things looked good. The drought wasn't quite busted — in fact Thursday's national drought monitor still puts 84% of Montana under unusually dry or full-fledged drought conditions — but it was better. Then came too much of a moist thing. Heavy rains poured in thanks to a water-laden atmosphere turbocharged by warmer than normal Pacific water. And when it poured, it melted. The equivalent of nine inches (23 centimeters) of rain flowed down Montana mountain slopes in some places. Half or more was from the melting snow, scientists said.

All the rivers and streams reacted the same: “They shot up to levels far beyond anything ever recorded,” Osborne said. “Hydrologists know that there's nothing that causes higher magnitude flooding in the West more so than a rain-on-snow event.”

One gage on the Stillwater River near Absarokee, where Osborne lives, normally flows at 7,000 feet (2,100 meters) per second during a moderate flood and races at 12,400 feet (3,780 meters) per second in a 100-year flood, he said. A once-in-500-year flood would mean water raging at 14,400 feet (4,390 meters) per second. Preliminary numbers show that on Monday, it crested at 23,700 feet (7,225 meters) per second, the equivalent of stacking three moderate floods on top of each other, according to Osborne.