Forecasts call for a storm to move through Western Washington late Thursday. Snow in the Cascade Mountains already prompted a temporary closure of the North Cascades Highway.
Rain in the Seattle area on Thursday has been light, but meteorologists expect a drenching late in the evening as a storm moves over the region, according to the National Weather Service.
Forecasters expect heavy rain to begin Thursday around 10 p.m. that will last into the morning, with heavy precipitation again Friday night, said Johnny Berg, a meteorologist for the service. Meteorologists expect the rain to last through Saturday, totaling up to 2 inches in accumulation, he said.
“The really heavy stuff is expected tomorrow evening where we might get 1/2 inch [accumulated] in the Seattle area,” he said. “Right now, we’re not going to see too much.”
Meteorologists and transportation agencies are advising Puget Sound commuters to take extra precautions.
The service issued a flood watch that began Thursday evening and lasts until Sunday morning for much of Western Washington. Forecasters say flooding is likely to be more widespread Friday.
With heavy rains, there is a possibility for some urban flooding, Berg said. In the Cascades and Olympic Peninsula, forecasters say an “atmospheric river,” or a warm stream of moist air with tropical origins, will burst at high peaks, dumping rain and snow that could cause lowland rivers across the area to flood.
“Because the mountains stick out, they act as a barrier and they squeeze out the moisture,” Berg said. “All that is going to flow into rivers, and since it’s going to be a warm system with moisture, it’s going to be mostly rain that falls into rivers.”
Because of the weather, maintenance crews closed the North Cascades Highway (Highway 20) on the west side of the mountains at milepost 134, east of Diablo, Whatcom County, and on the east side at milepost 171, 14 miles west of Mazama, Okanogan County, according to a notice from the state Department of Transportation (WSDOT).
WSDOT closes the highway in colder months due to heavy snow. The transportation agency said Thursday’s closure is temporary and crews will reassess the highway on Monday for avalanche and mudslide potential, according to the notice.
“There’s 2 feet of snow in the avalanche chutes now, and up to another foot forecast today, followed by heavy rain on Saturday,” the department’s website reads. “Avalanche potential will be very high if the weather matches the forecast.”
The transportation agency urges drivers to prepare for fast-changing weather conditions, regardless of elevation.
Meteorologists expect most mountainous areas to get between 6 and 8 inches of precipitation. Some areas in the Cascades, though, could get up to 10 inches, and portions of the Olympic mountains could get up to a foot, according to the weather service.
In King County, Berg said, meteorologists are watching several rivers, including the Skykomish, Snoqualmie and Tolt, for flooding potential.
Despite the advisories, Berg said the forecast is nothing out of the ordinary for the area in November.
“This is kind of what we get this time of year,” Berg said. “It’s normal for people that have grown up here, but for people who just moved here, it could be a shock.”