Rare weather phenomenon could dump a month's worth of rain on the Northwest

USA Today

10 hours ago

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 camping. 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.
Over a month's worth of rain 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.
Portland, Oregon, gets an average of just over half an inch of rain in August, and Seattle sees about 0.97 of an inch. Totals could be one to two times this amount, according to the outlet. The western slopes of the Olympic and Cascade mountains could see 4 inches of rain in some spots, AccuWeather said.
Some of the rain could dip as far south as Northern California.
"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 weather service in Portland said.
Rain a much-needed relief for dry Oregon
The rain comes just as Oregon's drought has rapidly expanded and wildfire danger reaches its seasonal peak.
More than half of Oregon is now classified as being under a moderate drought and most of the Willamette Valley and northwest Oregon is under a severe drought, according to the U.S. Drought Monitor.
'It's going to help with the drought and wildfire danger, but it's also not a 'season-ending' event,' Fransen said.
Oregon's weather is expected to heat back up again by next weekend and wildfires will remain possible throughout late August and September.
Rain is coming starting after midnight tonight through midnight Sat/Sun am. It can be heavy at times. More details in this thread. (1/x) pic.twitter.com/UCa9CJCnI7
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.
Atmospheric rivers are the largest transport mechanisms of freshwater on Earth, according to NASA. These long, narrow corridors of air heavy with moisture are sometimes called "rivers in the sky."
How to prepare for an atmospheric river
The weather service in Portland said residents should take these steps to prepare for the heavy rain: