Worst river flooding in decades to threaten lives, property in Montana

By Alex Sosnowski, AccuWeather senior meteorologist
May 11, 2018, 10:22:01 AM EDT

Ongoing flooding along streams and rivers is likely to worsen across parts of the northern Rockies this week.

Surging temperatures are causing deep snow cover over intermediate and high elevations from this past winter to rapidly melt. Temperatures have surged into the 60s, 70s and 80s F since late April.

In some cases, temperatures reached 20 to 30 degrees above average, following a winter with well below-average temperatures and well above-average snowfall.

"Rounds of showers and thunderstorms anticipated into this weekend may add to the runoff that has pushed streams and rivers out of their banks," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.

Static Rockies Flood 3 pm

Local rainfall of 0.50 of an inch to 2 inches may fall into Saturday.

Some of the worst conditions are expected along the Clark Fork River in Montana. Water levels are projected to reach major flood stage at Missoula, Montana. Flooding is likely be the worst since May 24, 1981, when the river crested at 13.38 feet at Missoula. The flooding could end up being the second worst on record, perhaps rivaling that of June 1975.

Static Missoula River Level Forecast

On Monday, Missoula County Sheriff's Office deputies notified residents in flood prone areas that they are under an evacuation warning. Ravalli County commissioners issued an emergency proclamation as officials prepared for potential flooding.

On Tuesday, mandatory evacuations were ordered for the Orchard Homes area due to the flood danger.

Clark Fork River in western Montana reached major flood stage on Thursday, and is forecast to crest 14 feet on Saturday. This would be the first time water has reached that level in 100 years, according to the National Weather Service.

The risk of stream and river flooding also extends into parts of Idaho and northwestern Wyoming, including Yellowstone National Park.

In addition to the Clark Fork River, portions of the Yaak, St. Joe, Moyie, Bitterroot, Blackfoot, Swan and Flathead rivers will be out of their banks this week.

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People are urged to take precautionary measures if they live along small streams and rivers in the region.

Motorists are urged to not drive through flooded areas. The water may be much deeper than it appears, and the road beneath may have been washed out. This advice includes, but is not limited to, low water crossings along small streams.

Motorists should also be on the lookout for rock and mudslides along secondary roads in the mountains.

People should avoid the banks immediately adjacent to streams and river that are running high. The banks could give way without notice.

"A break of dry weather is likely after this weekend," Pastelok said. "However, more warmth during the middle of the month may cause additional snowmelt in the high country that leads to significant runoff down below."

Cool and wet conditions are expected for the first part of the summer before conditions dry out and warm significantly during midsummer for the region.

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