A barrage of atmospheric rivers in recent weeks has drenched the Pacific Northwest and Northern California, bringing widespread 30-day rain totals between 15 and 30 inches, debris flows, heavy mountain snows and mudslides. Another atmospheric river is soaking coastal Washington and northern Oregon, the latest in a relentlessly wet pattern that doesn’t look to budge anytime soon.
Flood watches remain west of the Cascades and include cities such as Seattle, Tacoma and Portland. More than a dozen flood warnings are in effect as rivers overflow their banks, swollen from a month of copious to prolific rainfall.
In Neskowin, Ore., near the Pacific Coast about 90 miles southwest of Portland, 50 people were being evacuated by helicopter from an RV Park Friday due to rising waters along the Neskowin Creek and the threat of mudslides, according to the U.S. Coast Guard.
Another 2 to 6 inches of rain are possible out of the ongoing atmospheric river, but totals may approach a foot or more after yet another bout of storminess drags an atmospheric river into the region toward the middle of next week.
The heaviest rain as of Friday morning was falling from just south of Seattle to near the Oregon border, with rainfall rates around a quarter-inch per hour. A second batch of steady rainfall was lurking just off the coast, and will be overspreading the region through about noon local time before a gradual decrease in precipitation intensity in the afternoon.
“We’re still under it,” said Miles Higa, a meteorologist at the National Weather Service in Portland. “Our precip rates aren’t quite as high as they were last evening, but it was enough to bring some of our coastal rivers to flood stage.”
Because atmospheric rivers carry the bulk of their moisture at the mid-levels, the heaviest rainfall is expected on the windward, or west-facing, side of any mountains — particularly along the Coastal Range and the Cascades. Those same areas have seen up to 2½ feet of rain in the past month.
“Some of our favored sites on the coastal mountains had briefly about half an inch per hour, but most of the area is seeing 0.15 to 0.25 inches per hour for several hours,” he said.
Flash flooding hasn’t been a significant concern — yet. Higa explained that though total amounts have been heavy, rainfall rates are more modest. That allows the ground to absorb water at roughly the same rate it falls.
“Some spots in the south Washington Cascades saw up to about four inches, but they can absorb quite a bit of water,” he said.
Rainfall will continue throughout Friday evening primarily west of the Cascades; air forced up the mountains from the west will lose moisture before sinking, warming and drying to the east. Rain should wind down in the evening, leaving totals of 1 to 4 inches in the lowlands and 3 to 7 inches in the higher elevations.
The plume of moisture has set records, including in Salem, Ore., where Friday morning’s weather balloon launch recorded a record PWAT, or precipitable water index. That’s a measure of how much moisture is present in a column of atmosphere. Salem measured a 1.45 inch PWAT; that’s the most ever observed this late in the season, and also beats out anything between mid-November and the end of May.
“Through the winter, these tend to be our heavier bursts of rain,” said Matthew Cullen, lead meteorologist at the National Weather Service in Seattle. “We get the progressive systems that dump some rain and move on, but in terms of sizable rainfall, it’s always the atmospheric rivers.”
The current atmospheric river corresponds to a Level 4 out of 5 on a scale devised by the Center for Western Weather and Water Extremes. A new atmospheric river is expected to lash the Northwest this weekend, but will be narrower, briefer and less intense than its predecessors.
The atmospheric rivers bring welcome rainfall to the Pacific Northwest, but it’s not terribly advantageous for water resources in western Washington and northwest Oregon.
“We don’t rely quite as heavily on our snowpack to sustain our water throughout the spring as perhaps other parts of the West,” Higa said.
The atmospheric rivers are more beneficial when they extend farther south into Oregon and California, which are suffering a serious drought. They are particularly helpful when they produce snowfall in the mountains that can be stored through the winter months.
The stormy conditions in the Pacific Northwest have a strong chance of continuing through the winter months, but how far south into California precipitation extends is more uncertain. During La Niña events, cooling waters in the tropical Pacific Ocean set off a chain reaction, which often steers storms toward the Pacific Northwest but don’t always offer a lot of precipitation farther south. The Weather Service declared the arrival of La Niña in October and forecasts that it will persist through the winter.
La Niña has helped the stormy season off to a fast start.
“It’s not unusual to see some back-to-back, but we’re only in the first half of November. It’s been one after another after another. It feels busier,” Cullen said.
While there isn’t a firm link between climate change and the frequency of atmospheric river events, an increase in atmospheric temperatures allows the air to carry more water. Sacramento recorded more than 5.4 inches of rain in one day during a recent event, nabbing the city’s wettest day on record. Salem’s record-moist atmosphere was also consistent with influences from human-caused climate warming.