It’s the first of the cool-season rains, which arrive like clockwork every fall and last through the winter. A delayed onset to the rains in recent years, largely catalyzed by climate change, has made for more severe wildfire seasons that last deeper into autumn.
This round of heavy rain will offer a welcome reprieve from fire weather conditions and briefly inhibit already-burning fires, but it also raises the risk of localized flooding and debris flows.
Extremely dry, hot conditions helped spur fire activity this summer. In June, the Pacific Northwest bore witness to a “thousand-year” heat event with unprecedented temperatures obliterating long-standing all-time records by historically large margins. Seattle soared to 108 degrees, Portland hit 116, and Canada broke its national temperature record three days in a row, hitting 121 degrees June 29. The town of Lytton, British Columbia, where the record was set, burned to the ground days later as extreme wildfires scorched the sunbaked territory.
The upcoming system that will instigate the heavy rainfall was located over the Aleutian chain of Alaska on Thursday afternoon, marked by a lobe of chilly air at the mid-levels of the atmosphere. This high-altitude pocket of bitter cold and spin will shift east-southeast Friday, energizing a surface disturbance as it approaches western Canada from the Gulf of Alaska.
The first tendrils of rainfall will snake ashore during the overnight hours into early Friday along the coast from roughly Vancouver northward before arriving in northwest Washington late Friday. The rain will expand down the coast and increase in areal coverage and intensity in western Oregon and extreme northwest California, particularly west of the Cascades.
Rain will begin to taper Sunday, but dousing areas only with a general one to two inches; a few spots may wind up with totals approaching four inches. The highest totals may occur in the higher elevations, since the greatest moisture will reside several thousand feet above the ground.
The setup consists of an “atmospheric river,” or plume of deep tropical moisture originating from north of Hawaii, that will stream toward North America and lap at the coast. The high terrain of the Rockies and Coastal Range juts up into that corridor of moisture, forcing it to condense and form bands of heavy precipitation. That rainfall will be focused particularly on the windward side of any mountains.
In some places, snow will fall, particularly above 6,000 feet.
The Center for Western Weather and Water Extremes ranks this upcoming event a Level 5 out of 5 on its rating scale for atmospheric rivers. That number is a reflection of a system’s integrated water vapor transport, or a metric that calculates how efficiently moisture is shuttled from the tropics to land in the conveyor-belt-like feature.
Atmospheric rivers are a staple of the weather on the West Coast. Some areas see upward of 80 percent of their annual precipitation from perhaps half a dozen high-end atmospheric river events.
Jerry Schneider, a meteorologist at the National Weather Service in Seattle, said that the upcoming rain event could be mostly good news.
“It’s essentially going to end our fire weather season in western Washington,” he explained. “We still have a bit of [time] when it could dry out again, but we’re expecting two to four inches in the mountains, which should put a damper on the fire weather conditions.”
Where fires have already burned, the water could make for sporadic debris flows, but that’s not an issue in western Washington.
“The fires we have don’t tend to burn down into the soils,” said Schneider. “It’s more down in the Columbia Gorge. Our fire season was pretty benign.”
In Seattle, the fall rains can arrive any time in autumn, but the average first date on which 0.75 inches or more falls tends around early to mid-October.
“It fluctuates a lot [year to year], but [the first big rain is] usually somewhere in mid- to late September, so we’re just maybe a little early,” said Schneider.
However, the rain may not bring as much fire-quenching help to Northern California. While showers are possible in the region Friday, climate scientist Daniel Swain indicated the rain will probably not be a season-ending event like in Oregon or Washington. Strong onshore winds over the weekend and offshore winds at the beginning of the week could then help drive fire growth.