Parts of western Washington state and British Columbia have received more than two feet of rain in the past 30 days, and flood watches are up again ahead of the next round of heavy rainfall. At least two more atmospheric rivers are on the way, part of a conga line of storminess that has kept the regions waterlogged for more than a month.
The first of three atmospheric rivers in November’s final week bombarded the region Wednesday night and Thursday. The next arrives Saturday, lasting through the night, before narrowing and losing intensity on Sunday.
The third atmospheric river will approach from the northwest on Monday evening, lasting into Wednesday before easing.
“Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations,” wrote the National Weather Service regarding the weekend flood threat in northwest Washington.
Autumn ordinarily features considerable atmospheric river activity off the northwestern United States and southwestern Canada, but the 2021 season has proved particularly busy, with two or three events per week since October.
Vancouver was isolated by road after nearly 10 inches of rain caused flooding on Nov. 15, prompting officials to evacuate nearly 7,000 residents from the town of Merritt, B.C. Residents were just beginning to return early this week, some finding they had “nothing left.”
This week’s most recent atmospheric river unloaded another 1 to 2 inches of rain in western British Columbia from Wednesday night into Thursday. More than 7.5 inches drenched Tofino, a town on Vancouver Island.
In parts of northwest Washington, the rain was heavy enough to set daily records Thursday. Bellingham, which was flooded Nov. 15, received another 1.43 inches, and Quillayute posted 3.16 inches, both records for Nov. 25.
The next two systems
Atmospheric rivers are narrow strips of deep tropical moisture thousands of miles long. Some in the Pacific can originate as far away as Hawaii, their juiced-up air mass riding a conveyor belt northeastward toward the western United States and western Canada.
The two upcoming atmospheric rivers will feature an atmospheric squeeze-play of sorts, with a clockwise-spinning high pressure system to the south and a counterclockwise swirling low up north. Like two gears meshing, that mechanism will drive a ribbon of humid air eastward.
The upcoming atmospheric rivers do not rival that which brought disastrous flooding to western North America two weeks ago but still will carry an abundance of moisture.
The Center for Western Weather and Water Extremes estimates the upcoming rivers will reach a Level 2 out of 5 on its ranking scale, judging by “integrated vapor transport.” That’s a measure of how much moisture each atmospheric river is trucking. Estimates call for around 1,650 pounds of water per second to be moving over every meter (3.3 feet) slice of the atmospheric rivers at their peak.
Another #AtmosphericRiver is aimed at British Columbia Canada & western Washington state today. Per the @NWSSeattle forecast discussion: "at least 3 distinct atmospheric river events" will impact the region this week, raising concerns for #flooding. https://t.co/LA5VIMvZ8w #WAwx pic.twitter.com/naVXFGg83I— UW-Madison CIMSS (@UWCIMSS) November 25, 2021
Since atmospheric rivers carry the bulk of their moisture a mile or so above the ground, the heaviest rain will fall on the western slopes of the Coastal Range and the Cascades, particularly where the air is forced upslope. Rainfall of up to six inches is possible through the weekend in the mountains of northwest Washington.
At lower elevations in northwest Washington, the Weather Service projects 1 to 2 inches, with up to four inches at the north coast over the weekend. Minor to moderate river flooding is anticipated.
Southwest British Columbia is predicted to see 2 to 4 inches from the weekend system, with 6-inch totals possible in the mountains.
Several more inches are likely with the event Tuesday in British Columbia. While the heaviest precipitation from this event may remain in Canada, northern Washington could still see a couple more inches.
Atmospheric rivers are inherently warm systems, given their tropical connection, which is why they ordinarily do not deliver snow much below 9,000 feet of elevation or so.
In British Columbia’s Fraser Valley, “strong warming will accompany this system causing snow levels to rise well above the mountain tops Saturday afternoon,” wrote Environment Canada, the country’s Weather Service. “Snowmelt will contribute to run off, increasing the risk of flooding and possibly impacting vulnerable landscapes and infrastructure.”
Where snow does fall in the highest elevations of British Columbia’s coastal range and the Canadian Rockies, however, a staggering 5 feet of snowfall is possible through the middle of next week.
Seemingly relentless rains
With the recent barrage of seemingly relentless rains, soils are saturated and can’t necessarily handle much additional water. According to the Climate Prediction Center, soil moisture levels are running about 3 to 5 inches above average for this time of year, pushing the 99th percentile when compared with historical norms.
Seattle has seen 9.11 inches so far in November, nearly four inches above average. October also came in ahead of normal.
Along the western slopes of the coastal range in Western Washington, a foot and a half of rain has fallen in the past month, including in Grays Harbor County, which recorded 18.18 inches west of Aberdeen. A whopping 25.48-inch total was observed in adjacent Mason County, and 27.10 inches west of Forks in far northwest Washington.
A map of the United States over the past month (above) shows just how copious the rains have been. The Pacific Northwest has received more rainfall than anywhere else in the country, save for the coastal temperate rainforests of Alaska.
Jason Samenow contributed to this report.