While Helena proper is under a Winter Weather Advisory this weekend, the mountains ringing the valley are under a more serious Winter Storm Warning, with the National Weather Service in Great Falls forecasting heavy snow and dangerous travel conditions through 6 a.m. Monday. The warning covers the Big Belt, Bridger, and Castle Mountains, as well as the Gates of the Mountains area, the Upper Blackfoot and MacDonald Pass corridor, the Southern High Plains, and the Southern Rocky Mountain Front.
Snow totals under the warning vary significantly by location. The Big Belt, Bridger, and Castle ranges can expect six to thirteen inches with winds gusting to 40 mph. The MacDonald Pass area — which sits on U.S. Highway 12 west of Helena and is one of the main routes over the Continental Divide — is in a zone forecast to receive ten to twenty inches, with gusts up to 45 mph. The Lincoln valley is expected to see a somewhat lower total, up to seven inches, but remains under the warning.
Forecasters flagged the wet and heavy nature of the snow as a particular hazard. That kind of snow loads up on tree branches quickly, and the warning notes that isolated power outages are possible as a result. Anyone planning to travel MacDonald Pass or into the mountain drainages east of Helena this weekend should check conditions before departing and be prepared to turn around.
The warning went into effect at noon Friday for the most exposed areas and at 6 p.m. Friday for the mountain ranges. It runs through early Monday morning across all zones. Montana 511 is the best real-time source for road conditions on the passes.