Updated Sep. 26, 2019 10:16 AM
While fall is not even a week old yet, some parts of the northern United States and the Canada Prairies are bracing for a major snowstorm that weather officials are warning could be historic.
The general accumulation of snow, likely to be measured in feet in the highest elevations, will occur along with a substantial drop in temperature this weekend.
Because of the early-season and severe nature of this storm, people in the backcountry will be at risk for becoming stranded for days with well-below-freezing temperatures at night.
Rain showers are forecast to occur Thursday into early Friday over portions of southeastern British Columbia, southwestern Alberta, northern Idaho and western and central Montana.
However, cold air will surge southward across western Canada around the same time a storm will track eastward from the Pacific Ocean, and a change to snow will take place from north to south and from higher elevations to lower elevations later Friday and Saturday.
The change in temperature could leave some people with a case of weather whiplash. Temperatures are forecast to plummet 25-50 degrees Fahrenheit (12-25 degrees Celsius) from the middle of this week to this weekend. Highs in the 50s and 60s F (15-20 C) at midweek will be replaced with highs in the 20s and 30s (5 below zero to 4 above zero C).
"This is the type of setup where it will snow not only in the mountains but also down to the valley floors and away from the foothills," Brett Anderson, AccuWeather senior meteorologist, said.
In the mountains and intermediate elevations, a general 1-2 feet of snow is forecast to fall with an AccuWeather Local StormMax™ of 40 inches over the east-facing slopes of the Sawtooth, Flathead and Lewis and Clark ranges in northwestern Montana.
A few inches of snow may fall in the valleys and over parts of the High Plains and Prairies.
On Wednesday, the National Weather Service (NWS) in Missoula, Montana, warned that the storm has the potential to be "historic" due to its occurrence so early in the season. While snowfall accumulations will be lower across the valley, there will still be life-threatening backcountry conditions, hazardous travel, wind damage to docks and marinas and agricultural damage from record low temperatures, the NWS warned residents in the area.
Snow is also likely to spill west of the Continental Divide. Portions of northern Idaho, northeastern Washington and south-central British Columbia are likely to see a few inches of snow over the mountains. A small accumulation of snow is also likely on the eastern slopes of the Washington Cascades.
One factor forecasters are notably concerned about is the amount of foliage still on trees.
"It's a very big concern here; we're just beginning to turn our colors, so trees are fully loaded with foliage here," LeeAnn Allegretto, a meteorologist with the NWS in Missoula, told AccuWeather. "Snow will have a huge impact and will likely cause downed trees and potentially plenty of damage/power outages."
Allegretto added that she and her coworkers believe the amount of snowfall on foliage-laden trees will have the biggest impact, but the strong winds and record-challenging cold will also be dangerous.
Cities likely to be hit with accumulating snow include Calgary and Lethbridge, Alberta; Regina, Saskatchewan; and Great Falls, Butte and Missoula, Montana.
Missoula is forecast to experience a high temperature in the upper 30s on Saturday; on Tuesday, highs flirted with 70 at Missoula International Airport.
Snowfall is likely to cause substantial travel delays through the mountains and over the passes, and could be heavy enough to shut down travel altogether.
"There will be heavy accumulation on the roads over the passes with this storm," Anderson said.
Travel along many secondary roads, as well as Interstate 15, I-90 and Canada's Highway 1 may be adversely affected and dangerous in some locations.
A substantial amount of snow is forecast to fall on Glacier National Park and Yellowstone National Park.
The September snowfall in Missoula has the chance to be a record-setting event. Allegretto said it's been 36 years since September snow was recorded in Missoula. With up to 2 inches forecast in the Missoula Valley, it could turn out to be the greatest one-day September snowfall total since 1934 when 1.5 inches fell.
Enough of an east to northeast wind may accompany the snow to cause blowing and drifting with poor visibility over the higher elevations.
Over the lower elevations, where snow initially makes roads wet, subfreezing temperatures at night can create icy spots during and following the storm.
During Sunday night and Monday night, nighttime temperatures may challenge record lows.
Snow this early in the season is not unheard of in this part of North America .
For example, on Sept. 8-10, 2014, Calgary received about 10 inches (25 cm) of snow. Just last year, a little snow fell on Calgary, Great Falls and other locations in the region from Sept. 27-30.
Earlier this September, a few small pockets of snow fell on the high country of the Rockies.
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