After a modest round of wintry weather over the weekend, more snow, including a potentially historic winter storm will target the Pacific Northwest this week, with heavy accumulations expected, even in Seattle.
In the Cascades of Washington and Oregon, snow will be measured in feet and along some western-facing slopes, it could take multiple yard sticks to total up the powder!
A cold Arctic outbreak thanks to frigid high pressure centered over southwestern Canada is providing the cold air for the snow, while an onshore flow from the Pacific is adding in the moisture.
With several rounds of moisture poised to move ashore, the snow will continue to come in phases as indicated by Edwards. The first round delivered more than 2 inches of snow to Seattle on Sunday, marking the first measurable snow of the season.
Another round will continue through today, with much of western Washington and Oregon poised to experience accumulating snow. In the Portland area, the snow will mix with rain, while plain snow is on tap again for Seattle and Bellingham.
By tonight, at least 3 to 6 fresh inches of the white stuff will be on the ground around the Emerald City, with heavier amounts along the hills to the east.
The snow map above is available in a larger version at the AccuWeather.com Winter Weather Center.
The worst conditions will arrive across the Northwest later tonight and Wednesday as a moisture-laden and powerful storm slams ashore.
There are still some questions concerning how much warm air the storm will push inland, but the potential exists for an historic amount of snow in some areas, including the greater Seattle area, where more than a foot is a strong possibility. Eventually, a changeover to rain is expected on Wednesday.
Warming air should spare Portland and Eugene of significant snow totals altogether.
Characteristically, snow totals will be much higher in the Cascades and will threaten travel through major passes. As Ken Clark points out in his blog, several feet of snow is possible even at pass levels.
Snoqualmie and Sherman passes are at a relatively high risk of being closed during these snow events, especially with the storm expected to hit early on Wednesday.
Heavy snow amounts will also extend across inland and mountainous areas of eastern Oregon, Washington, Idaho and western Montana by midweek.
Back along the I-5 corridor from Portland to Bellingham, travel will be treacherous this week, and could turn out to be dangerous for a time midweek.
Though heavy snow is relatively rare, Seattle is not immune to significant snowstorms.
"Single day snowfall of 6 inches or greater has occurred on only 15 days since 1950, none since 1996," said Climatologist Jim Rourke.
"The top Seattle snowstorm was Feb. 1, 1916 when 21.5 inches piled up," added Rourke.
Senior Meteorologist Kristina Pydynowski points out another potential threat: major flooding.
"Despite the exact outcome of the snow in the middle of the week, a parade of warmer storms late next week that follow will deliver heavy rain, putting the Seattle area at risk for major flooding," says Pydnowski.
The risk of drenching and locally gusty thunderstorms will expand northwestward over the balance of the week, reaching parts of Southern California, Arizona and Nevada.
While the heat wave and high humidity will recede in the Northeast to finish out the week, 90-degree F air may linger in many areas into August.
A cold front swinging across the northeastern United States will bring the threat of heavy, drenching thunderstorms Thursday afternoon.
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