Atmospheric river to hit western WA on Halloween

An atmospheric river is expected to bring heavy rain to western Washington, beginning Friday evening — the night of Halloween.

“The main threat with this system is the heavy rain, though wind gusts in the 20–35 mph range can also be expected for anyone outside as the system moves through,” KIRO 7 meteorologist Morgan Palmer said. “This will not be a repeat of last weekend’s destructive wind event, and now that trees have already been tested by stronger winds this fall, the scope of power outages is expected to be much more limited.”

While this system is expected to be much weaker than last weekend’s storm, one to three inches of rain is estimated to occur.

Washington is home to some of the most atmospheric rivers in the country, most of which occur along the Oregon coast and in the Pacific Northwest. Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky, according to the National Oceanic and Atmospheric Administration (NOAA) – that transport most of the water vapor outside of the tropics.

The average atmospheric river carries roughly the amount of water vapor as the average flow of water at the mouth of the Mississippi River, with exceptionally strong ones transporting up to 15 times that amount.

The record for Halloween rainfall at Seattle-Tacoma International Airport is 1.18 inches in 2014.

The strongest downpours should be clear by Saturday morning.

“Rainfall totals of a quarter- to a half-inch around the lowlands can be expected by early Saturday morning, with higher amounts across the northern islands, far northern sections, the coast, and the Strait,” Palmer stated.

A Winter Weather Advisory is currently in effect for the Cascades above 4,500 feet in Whatcom and Skagit counties through 11 a.m. Wednesday.

Contributing: KIRO 7

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