A computer model depiction of this week’s snowfall from the storm as of Monday afternoon.
CNN
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The first major snowfall of the season is on its way to the U.S. Northwest and parts of the northern Plains, and by the time the last snowflake falls, many high-altitude areas will be buried in more than a foot of fresh snow.
A wave of cold air and deep moisture from Washington and Oregon will spread across much of Montana on Tuesday before a strong storm moves northwest Tuesday evening.
Flakes will start flying early Monday night or Tuesday morning as moisture from this storm moves across the Cascades in Washington. The higher the altitude, the greater the chance of annoying snowfall and difficult travel. Several major high-altitude mountain passes in this part of the Cascades could be affected, including Stevens Pass, Snoqualmie Pass, and Stampede Pass.
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Snow will begin falling across the rest of the northwestern United States Tuesday afternoon into Tuesday evening. Accumulating snowfall will begin covering parts of Idaho and Montana as well as the Cascades in Oregon during this window as temperatures struggle to rise above the freezing mark.
Temperatures will drop Tuesday night and will force many high-altitude locations to drop below freezing. Temperatures in northern Idaho will drop into the low 20s by early Wednesday morning, while parts of northwest Montana will drop into the low 20s.
A combination of cold air and abundant moisture will set the stage for heavy snowfall Tuesday night. Six or more inches of snow could accumulate quickly at traffic levels in the Cascades Tuesday night, with amounts likely approaching a foot in areas above 7,000 feet.
Snow accumulations will increase Wednesday across the northwest and northern Rockies. Wind speeds will also increase during this time and snow will fall, which may significantly reduce visibility and worsen travel.
While the total snowfall from this storm will be at the highest elevations, some lower elevation areas in Washington, Montana and South Dakota will not completely escape the winter weather.
A few inches of snow is expected to accumulate up to 1,000 feet in parts of Washington on Wednesday, but Seattle will only endure cold rain.
Snow will begin to taper across the Cascades on Thursday, but will spread to parts of the northern Plains as the storm moves east. More than a foot of heavy snow will bury parts of South Dakota on Thursday and into Thursday evening.
Another danger will arise when snow falls in low-altitude areas: melting and refreezing. Any snow that melts during the day will only freeze overnight and cause treacherous ice to form along roads and sidewalks.
By Friday, significant amounts of accumulating snow will be gone across much of the northern United States, but some flakes will still blow along the US-Canada border before the storm fully crosses into southern Canada.
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