CNN
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The first big snowfall of the season is headed for the US Northwest and parts of the Northern Plains, and many high-elevation areas will be buried under more than a foot of fresh snow until the last snowfall.
A wave of cold air and deep moisture from Washington and Oregon will sweep across much of Montana Tuesday before a strong storm moves northwest Tuesday night.
Flakes will begin to fly Monday night or Tuesday morning as moisture from this storm moves across the Cascades into Washington. The higher the altitude, the greater the chance of troublesome blizzards and difficult travel. Several important high-elevation mountain passes in this part of the Cascades may be affected, including Stevens Pass, Snoqualmie Pass, and Stampede Pass.
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Snow will begin to fall across the rest of the northwestern United States Tuesday afternoon and Tuesday night. Parts of Idaho and Montana, as well as the Cascades in Oregon, will begin to accumulate snow during this window as temperatures struggle to rise above the freezing mark.
Temperatures will drop Tuesday night, forcing many high-elevation locations to drop below freezing. Temperatures in northern Idaho will drop into the 20s Wednesday morning, while parts of northwestern Montana will drop into the 20s.
A combination of cold air and abundant moisture will set the stage for heavy snowfall Tuesday night. Fifteen or more inches of snow could quickly accumulate at traffic levels in the Cascades by Tuesday night, with amounts approaching a foot in areas above 7,000 feet.
Snow accumulation will increase Wednesday in the Northwest and Northern Rockies. Winds will also pick up and snow will fall during this period, significantly reducing visibility and making travel worse.
Although total snowfall from this storm will occur in the highest elevations, some lower elevation areas in Washington, Montana, and South Dakota will not fully experience winter.
Parts of Washington are expected to get a few inches of snow up to 1,000 feet on Wednesday, but Seattle will see only cold rain.
Snow will begin to taper off Thursday in the Cascades, but will spread across the Northern Plains as the storm moves east. Parts of South Dakota will get more than a foot of snow Thursday and Thursday night.
When snow falls in low-lying areas, another danger will arise: melting and freezing. Any snow that melts during the day only refreezes at night, creating treacherous ice on roads and sidewalks.
By Friday, significant amounts of snowpack will disappear from much of the northern United States, but a few flakes will still fly along the US-Canada border before the storm completely crosses southern Canada.
