A powerful Pacific storm will barrel across the western US early this week on a cross-continental journey to southeastern Canada. The storm will have become a powerful cyclone by the time it crosses the Rocky Mountains midweek. It will be capable of producing damaging winds, heavy snowfall, torrential downpours, and severe weather over the Plains and the Upper Midwest. But the storm’s worst impacts will occur long after skies clear as snow melt drains into an already flooded watershed.
The Pacific storm was already inundating northern California and Oregon Monday with torrential rain and high winds. Flooding has been an ongoing issue for parts of Oregon’s Willamette Valley Monday afternoon after up to a foot of rain fell over high elevations. The flooding is expected to worsen as runoff drains into tributaries of the Willamette River. The rain will continue through early Monday night, by which point up to an additional 1.5″ of rain will have fallen along the Coastal and Cascade ranges.
Rain and snow will engulf the Great Basin and Rocky Mountains Tuesday as cold air from the Canadian prairies is drawn southwestward. Up to two feet of snow will fall over the northern and central Rockies through Wednesday as this cool air is forced up steep mountain sides. The cold will not be strong enough for appreciable snow for most low-lying foothill cities like Boise and Helena. However, enough flakes could fall late Tuesday night and Wednesday to accumulate a few slushy inches in Salt Lake City.
The boundary separating the cold air mass responsible for Tuesday’s mountain snow will stall against encroaching subtropical air flowing from the Gulf of Mexico. The collision of the two air masses will initially generate heavy rain across eastern Montana, eastern Wyoming, South Dakota, Nebraska, and Iowa between Tuesday afternoon and Wednesday evening. But the rain cannot sustain its state for long.
The Pacific low will emerge from the Rockies into the Plains Wednesday. Invigorated by the mountains and freed access to the Gulf of Mexico’s moisture tap, the cyclone will become a force to be reckoned with.
Winds strong enough to topple trucks, down power lines, and blow debris will extend far from the storm’s center across the Central and Southwestern US.