A significant snowstorm is set to unfold over the northern Rockies, less than a week after the official start of fall. (TropicalTidbits.com/NOAA)

Great Falls, Mont., has had temperatures in the 60s and 70s all week. It’s also under a winter storm watch for “snow accumulations of mostly 10 to 18 inches” between Saturday and Monday.

The Treasure State is bracing for “historically significant early-season snow” amidst a storm that the local National Weather Service says could bring “extreme” impacts. In one bulletin, the agency wrote that this storm “has the potential to set a new benchmark.”

Three to four feet — with locally higher amounts — is possible in the higher elevations. Totals of one to three feet are likely along Highway 15 and Route 87, with more than a foot expected in Great Falls. Helena could see eight inches, with a couple inches as far south as Bozeman. A dusting is even possible in Yellowstone National Park, where scattered snow showers accompanied by thunder are possible to start the workweek Monday.

Montana is no stranger to off-season snow. The state’s highest one-day snow total -- 16.5 inches -- came in a late April storm in 1973. Eight inches fell on June 7, 1950. The half-foot-plus total it got on Aug. 23, 1992 trumps any calendar-day snow totals from September or October. And in fact, Great Falls has recorded snow in every month of the year.

But in September and October, it’s usually measured in inches — not feet.

Many are comparing this storm to one in September 1934 that brought 13.3 inches to Great Falls in three days. This storm has the potential to top that.

“Regardless of specific totals, the impacts don’t change,” said Francis Kredensor, a meteorologist with the National Weather Service in Great Falls. “With leaves still on the trees and the potential for this to be a heavy, wet snow, we’re looking at lots of tree damage, fairly widespread power outages, and travel concerns.”

The instigator isn’t your typical Northern Tier or Upper Midwest storm. There’s no rapidly intensifying low-pressure zone, common in many storms. This storm isn’t especially powerful. But it has lots of moisture to work with.

A zone of low pressure east of the Rocky Mountains near the Four Corners region will be energized beneath a pronounced dip in the jet stream. Meteorologists call that mountain-assisted development “lee cyclogenesis.” A 120 mph high-altitude jet stream will aid in boosting the fledgling storm.


A powerful low-level jet will aid in impressive moisture transport, setting the stage for bigtime snows in Montana. (TropicalTidbits.com/NOAA)

That jet is also helping copious tropical moisture to stream unusually far north, overrunning the cooler air banked in the northern High Plains and the Rockies. That conveyor belt of moisture, coupled with chilly air at the lower levels and enhanced uplift from the mountains, will become a virtual snow-making machine as the ingredients gel Saturday night and Sunday.


Impressive moisture flows northwards to fuel the snowmaker. (TropicalTidbits.com/NOAA)

Around the immediate Great Falls area and adjacent nearby mountains, "appetizer” precipitation arrives in the form of patchy showers Friday evening, becoming steady with bouts of light rain throughout the day Saturday. It is expected to flip over to mixed precipitation and eventually all snow as temperatures drop Saturday evening. Falling at first as a heavy wet snow, the flakes will become fluffier with time.

The snow should ease late Sunday night or early Monday, but strong winds in the storm’s wake may present the danger of blowing snow and reduced visibility Monday.

North to northeasterly winds of 20 to 30 mph in the valleys and the lower elevations will knock wind chills into the single digits. Higher gusts up from 40 to 50 mph in the higher elevations and summits could produce brief blizzard conditions.


A pocket of upper-level energy approaching from the west will enhance surface disturbance, setting the stage for a real-deal snowstorm in Montana this weekend. (TropicalTidbits.com/NOAA)

Despite the impressive snowfall rates, the snow won’t stack up to its full potential on the ground due to some initial melting and compaction. After all, ground temperatures are commensurate with what you’d expect for September! But as Kredensor mentioned, the presence of fully leafed trees will set the stage for significant damage to vegetation and, by extension, for power outages.

Amid the somewhat premature visit by Old Man Winter, it’s not all bad news. “Making a snowman in September would be something,” Kredensor said with a laugh.