A surge of moisture aimed at the Pacific Northwest will generate a bout of flooding rain before a massive Arctic air mass plunges into the West next week.
Rain will increase in coverage and intensity across much of western Washington late Saturday night, including in the cities of Seattle, Olympia and Vancouver.
The moisture will interact with the Cascade Mountains, effectively leading to dangerous rainfall rates of around 1 inch per hour throughout most of the passes.
For residents or visitors traveling through Snoqualmie or Stevens Pass on Sunday, be prepared for slick roadways and potentially blinding downpours.
Rainfall amounts in the central Cascade passes could reach 3-6 inches through Sunday night as snow levels remain on the high side above 6,000 feet.
Mountain rain of this magnitude can lead to sharp rises on area rivers with moderate flooding possible in some locations.
Meanwhile, for folks traveling back home from holiday festivities along Interstate 5 between Everett, Seattle and Olympia, steady to locally heavy rain will develop late Saturday night and continue through Sunday. Rainfall amounts through Sunday will average 1-2 inches, which can lead to some flooding of low-lying and poor drainage areas.
Arctic Air Blasts Southward Sunday Night
After the heavy rainfall that occurs through Sunday, temperatures will turn sharply colder Sunday night across the Pacific Northwest and northern Rockies as a strong cold front slides southward.
Snow levels across the Washington Cascades will fall from around 5,500 feet on Sunday to 2,500 feet Sunday night and end up just above the ground on Monday.
Heavy rain in the Cascades will transition to heavy snow Sunday night with several inches of accumulation likely. Along with the transition to snow, a rapid freeze is likely across the passes of the Cascades as temperatures quickly fall into the 20s late Sunday night. Interstate 90 through the Cascades will become covered with ice and snow and extremely dangerous.
The air will be so cold that the rain could even mix with some wet snowflakes in places such as Seattle and Portland along the I-5 corridor Monday and Monday night. High temperatures by Tuesday and Wednesday won't get out of the middle 30s in Seattle which is nearly 15 degrees below average for the time of year.
According to Meteorologist Michael Doll, "Several locations, including Seattle and Portland, will flirt with their record low temperatures both Tuesday night and Wednesday night."
Farther east, blizzard conditions will develop Monday from central Alberta into northern Idaho and northwestern Montana as the Arctic air spills southward.
Travel will become extremely difficult Monday in Calgary and Edmonton, Alberta, as wind-whipped snow combines with temperatures falling into the single digits.
Travel along Interstate 15 will also turn dangerous across the northern Rockies as heavy snow falls and the air becomes bitterly cold. Temperatures on Monday in cities such as Great Falls and Helena will start out in the 40s before plummeting into the 20s then falling close to zero Monday night. Highs by Tuesday will struggle to get out of the teens.