A rare atmospheric river is unfolding across the Northwest to end the week, with over a month's worth of rain possible in some parts of Oregon and Washington, forecasters said.
The rain event should tamp down wildfire danger but will soak anyone outdoors.
The stream of moisture is unusual for the Pacific Northwest at this time of year, which typically experiences drier Augusts. It's forecast to bring 1 to 3 inches of rain to the Cascade Mountains and coastal areas, with varying rain totals elsewhere in both states.
“It’s pretty unseasonal – it does happen from time to time but not very often in August,” Tanja Fransen, a meteorologist with the National Weather Service in Portland, told the Salem Statesman Journal, part of the USA TODAY Network. “We certainly need it.”
The rain began the evening of Aug. 14 and is expected to pick up in a second round the evening of Aug. 15 and morning of Aug. 16, before gradually giving way to sunnier skies on Aug. 17. Between 0.5 and 0.75 inches already fell in the last 12 hours with the heaviest amounts along the coast, the weather service in Seattle reported. In western Washington, 1.5 to 2.5 inches is expected in the coming days.
"The storm would be considered formidable even during the middle of the wet season in the winter months," AccuWeather reported.
How much rain will Washington get? Over a month's worth could fall
Rainfall totals from this rain event could surpass usual rainfall amounts for the whole month of August in parts of Washington and Oregon in just a few days, AccuWeather reported.
"The rain will ebb and flow, so expect some periods that are heavier rain, then lighter rain, it may totally stop and then start up again," the National Weather Service office in Portland told USA TODAY.
Through Tuesday, Aug. 19, rainfall will vary widely across Washington, with the heaviest totals in the mountains and along the coast.
Cascade Mountains: Significant accumulation of 2-6 inches, with localized higher amounts along western slopes. Higher elevations could also see snow or rain-snow mixes.
Western Washington and Coastal Areas: 1.5–2.5 inches, especially on the western slopes of the Olympic Mountains.
Seattle / Puget Sound: Approximately 0.75–1.25 inches, with northern areas lighter due to the rain shadow effect.
Eastern Washington: Mostly light rain, 0.1–0.5 inches, though higher terrain may reach 0.75 inches.
The atmospheric river will also impact Oregon, though rainfall is lighter there. The greatest totals are centered in southern and coastal British Columbia, near Vancouver and the Lower Mainland — marking the first significant rainfall in weeks for the region, which is experiencing a growing number of wildfires in what is now the second-worst wildfire season on record.
A light rain may also impact northern California.
When will the rain hit?
The storms should start to taper off by midday Saturday, lingering as scattered showers through Sunday afternoon, Aug. 17.
Forecasts show clear skies on Monday, Aug. 18, and Tuesday, Aug. 19.
What is an atmospheric river?
On the West Coast, atmospheric rivers approach from the Pacific Ocean, carrying tremendous amounts of water. They occur most often during certain weather patterns, including El Niño, which can increase their frequency or intensity, though they can occur in other years as well.
These long, narrow corridors of air have been responsible for as much as 65% of the West’s extreme rain and snow events, USA TODAY formerly reported.
According to the National Oceanic and Atmospheric Administration, an atmospheric river — described as a “river in the sky” — transports water vapor outside of the tropics. While they vary in size and strength, the average system carries an amount of water vapor equivalent to the average flow of the Mississippi River at its mouth.
Past atmospheric rivers have caused widespread impacts. For example, earlier this year, two systems drenched Southern California, triggering severe flooding, tornadoes, avalanches and mudslides. Cities including San Francisco and Redding received up to a month’s worth of rain, prompting evacuations and rescues from southern Santa Barbara to Orange County.
How to prepare for an atmospheric river
The weather service in Portland said residents should take these steps to prepare for the heavy rain:
Check your home's gutters and clean them out if there are leaves from recent winds.
Practice caution around low water crossings, where street drains may back up and cause rain to pool.
Slow down while driving off interstate and bridge ramps and when making turns. Recent heat has caused oil in the road to come to the surface, making roads slick when rainfall is added.
Expect creeks to rise and have more flow in the Coastal Range, Willapa Hills and Cascades.
Check your local weather forecast for expected rainfall amounts and timing.
This article originally appeared on Kitsap Sun: Atmospheric river over PNW. How much rain will Washington get?
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