In the higher elevations, significant mountain snow is likely. Some peaks in the Cascades could wind up with two feet or more of accumulation.
The weather system responsible is termed an atmospheric river, describing the narrow channel of exceptionally humid air extending from the deep tropics to the mid-latitudes. Such conveyor belts can stretch for hundreds of miles and can last for days where they intersect the coast.
Atmospheric rivers are sorted on a 1 through 5 scale, with 5 being the most significant. The coming atmospheric river is anticipated to peak at level 4 or “extreme” status. Events this intense are considered “mostly hazardous” due to their flood potential, “but also beneficial” due to their contributions to the water supply, according to the Center for Western Water and Weather Extremes, which devised the scale.
The scale is a product of the atmospheric river’s duration and moisture transport, in essence gauging how much water the feature trucks into a region over a given interval.
The atmospheric river, which will start to blast the Pacific Northwest on Monday evening, has origins in the tropics well west of Hawaii. Meteorologists sometimes refer to this channel of moisture as the “Pineapple Express.”
Flood watches are up along and west of the Cascades from Oregon’s middle coast north through Washington state. A widespread two to four inches of rain are likely in the lowlands from Oregon’s Willamette Valley northward, with four to eight inches in the higher terrain. A few localized 10-inch totals cannot be ruled out where the mountains help focus moisture. The best chance will be on the windward, or western, side of the slopes.
The heavy rain comes after a wet several weeks tied to a prolonged unsettled pattern. Seattle has recorded 10.8 inches of rain since the start of December, with nearly eight inches falling in Portland. The ground is saturated, meaning any additional rainfall will be more prone to causing flooding.
The National Weather Service Weather Prediction Center, which specializes in precipitation forecasts, has already drawn a level 3 out of 4 “moderate risk” of flooding into the forecast for the Pacific Northwest on Tuesday. That bull’s eye includes Highway 101 between the Siuslaw National Forest and Cannon Beach. That is where the core of the “strong to extreme” atmospheric river will be pointed, resulting in the highest rainfall rates.
Precipitation in “the Northwest Oregon Coastal Range has been running about 150 to 300 percent of normal,” writes the Weather Prediction Center, stating that very high runoff rates are likely. That will “result in rapid-rises along streams and flooding along large main-stem rivers. Additionally, mud/rock slides are possible.”
The atmospheric river will be reinforced by a pair of upper-air weather systems enhancing lift, or upward motion in the atmosphere, and rainfall rates. Warm mid-level temperatures will allow rain to fall as high as 6,000 feet in elevation, contributing to snowmelt and exacerbating the risk of flooding.
Above 6,000 feet in Oregon, predominantly snow is predicted through Wednesday, when precipitation will gradually come to an end north to south. Snow levels in northern Washington may dip as low as 3,000 feet, with up to 24 inches of new accumulation possible through Tuesday.
Atmospheric rivers are a major contributor to the annual water budget of the Pacific Northwest. In many cities in Washington and Oregon, roughly half of annual precipitation comes down in just three months’ time in the winter.
La Niña regimes, like the present one, tend to favor more robust and frequent atmospheric river events in the Pacific Northwest. Conversely, El Niños tend to feature atmospheric rivers making landfall farther south, into central and even Southern California.
Unfortunately, all of this event’s incoming water will not help residents in Southern California, where the snowpack in the Sierra Nevada sits at half its typical extent. The drought-stricken and moisture-starved region is likely to remain largely dry through the weeks ahead.