Winter Storm Warning: Up to 16 Inches of Snow and 30 MPH Gusts Hit Idaho and Montana Mountains, Slush Possible on I-90 at Homestake Pass

Two National Weather Service offices now have winter storm warnings up across the Lemhi, Bitterroot, Anaconda Pintler and Big Hole high country, with slush possible on Interstate 90 at Homestake Pass.

Peak Driving Danger Window

Now through 9 AM Monday in Idaho and the western Montana ranges, and through noon Monday in the Big Hole country, with snow heaviest overnight and early in the morning.

A Summer Storm Now Over The High Country

The late-June storm forecasters warned about has moved over the Northern Rockies, and the snow is piling up on the high passes of Idaho and Montana. Earlier in the week the low valleys ran in the 80s and low 90s. By Sunday morning the high country was under winter storm warnings from two weather offices, with more than a foot of snow expected on the tallest terrain through Monday.

The Missoula office kept its winter storm warning in force for Western and Eastern Lemhi County, the Bitterroot and Sapphire ranges, and the Butte and Blackfoot region above 6,500 feet, running until 9 AM Monday. Totals of 4 to 8 inches are expected, with 8 to 16 inches across the Anaconda Pintler Mountains near Georgetown Lake and up to 12 inches on the highest Lemhi peaks. Forecasters added that west winds of 10 to 20 mph, gusting to 30, will drive the wet snow and deepen the chill.

Farther east, the Great Falls office raised Northwest Beaverhead County to a winter storm warning of its own, in effect until noon Monday. That zone, which takes in Chief Joseph Pass, Big Hole Pass, Wisdom, Wise River and Dewey, can expect 2 to 6 inches at pass level and more than a foot in the higher terrain.

Where The Warning Hits The Road

For drivers, the storm reaches several named routes. The warning covers Idaho Highway 28 between Tendoy and Lone Pine, Lemhi Pass and Bannock Pass, and US 93 over Lost Trail Pass in the Bitterroot zone. Lemhi Pass, an unpaved Continental Divide crossing at 7,373 feet, is among the first to turn to mud and snow. Lower down, the Missoula office said snow levels could briefly drop far enough to leave minor slushy accumulation on Interstate 90 at Homestake Pass and on US 12 over Macdonald Pass, both near Butte and Helena. The office advised that “persons should consider delaying travel across the backcountry and higher elevations.”

The Yellowstone gateways sit on the lighter side of the storm. Targhee Pass on US 20 and Monida Pass on Interstate 15, the main approaches into the West Yellowstone entrance, stay under a winter weather advisory for up to 4 inches, with localized amounts over 8 inches only on the highest nearby peaks.

What Drivers Should Do

• Check Idaho 511 and Montana 511 for live pass conditions before you leave.
• Carry chains or traction devices if you plan to cross any of these passes.
• Pack a winter kit even in summer: blanket, water, flashlight, shovel and warm layers.
• Slow down on bridges and shaded curves, where wet snow freezes first.
• Build in extra time, and be ready to turn back if a pass closes.

Low valleys, including Salmon and the towns along US 93, are seeing rain rather than snow. The heavy snow is a high-country story, focused on the passes and peaks, and totals can still shift through Monday. Anyone crossing these routes should check the road first.

This is a developing storm. We will update this story as the National Weather Service revises snow totals, adds or drops alerts, and as road conditions change through Monday.