Parts of western Washington state and British Columbia are dealing with renewed flooding as the latest in a series of atmospheric rivers bombards the region.
Atmospheric rivers, or narrow jets of atmospheric moisture often originating from the tropics, have pointed at this area for more than six weeks, leading to historic amounts of precipitation and, at times, serious flooding. The National Weather Service reported Tuesday that Seattle posted its wettest meteorological fall (September through November) on record, with more than 19 inches of rain.
Now the same areas are being hit hard again by a third atmospheric river in a week. After some areas were engulfed by high water over the weekend, flood watches are up again across northwestern Washington, where an additional two to four inches of rain or more are possible. In western British Columbia, at least six inches of rain could fall in the high terrain.
The heavy rain is also bringing an elevated concern for dangerous mudslides and landslides, which could further sever routes.
“Persistent rainfall over the last few weeks has dramatically increased soil moisture to high levels across Western Washington,” wrote the National Weather Service in Seattle. “The increased threat of landslides will continue.”
Atmospheric river slams western Washington with record rains, flooding and high winds
As of early afternoon Tuesday, light to moderate rain was falling along the coast of Washington as well as across Vancouver Island. The Vancouver, B.C., metro area was under a rainfall warning from Environment Canada, its equivalent of the National Weather Service, with snowfall and wind warnings also hoisted across western British Columbia.
Because atmospheric rivers carry the bulk of their moisture a mile or so above the ground, the heaviest rain ordinarily falls in the mountains. Atmospheric rivers, which are characterized by strips of deep tropical moisture, also transport warm air masses with them. That will melt mountain snow in parts of British Columbia, further exacerbating the flood risk.
“Rising freezing levels and snowmelt may contribute to increased runoff,” Environment Canada wrote.
The atmospheric river will remain pointed at the coast through at least midday Wednesday before tapering back in intensity during the afternoon and evening hours. Precipitation totals through Wednesday night could exceed half a foot locally in the higher elevations on Vancouver Island and in western British Columbia, and three or more inches in the Coastal Range and Cascades of Washington.
Seattle proper, nestled comfortably within a valley, may see only a quarter- to half an inch, as will neighboring communities in the lower elevations. They’ll still have to deal with river flooding, though.
“Whatcom County is the main county with the flood concerns,” said Jacob DeFlitch, a meteorologist at the Weather Service office in Seattle. “Our focus will be there and toward the northern Olympics as yet another atmospheric river comes in.”
Mount Vernon, the seat of Skagit County, Wash., was also preparing for flooding. The Skagit River is expected to hit major flood stage by midweek.
This latest atmospheric river presents a renewed flood threat just after the atmospheric river over the weekend sent river levels soaring above flood stage, displacing residents. The Nooksack River flooded and poured water into the city of Sumas.
The Whatcom County Sheriff’s Office tweeted that an emergency shelter had been opened up at the Mount Baker Rotary Building in Lynden on Sunday, with another shelter available for displaced residents in Ferndale. Dozens of road closures were listed on the county’s website.
“Stay away from flooded creeks, as the water is rushing swiftly and currents are extremely strong,” read a bulletin on the city of Bellingham’s website Sunday. “Also avoid trail bridges crossing creeks or streams during flood conditions, as they could be undermined by floodwaters.”
In the past 30 days, rainfall totals have exceeded 30 inches in the high terrain of western Washington. Seattle was up to 18.62 inches of rain since the start of September as of Monday night, marking its wettest meteorological autumn on record. Some places in the mountains have seen 50 inches within that window.
While this atmospheric river marks the last before a brief pause, it will be quite some time before the floodwaters recede and their impacts vanish.
“It’s been just nonstop consistent rounds of active weather with strong winds, landslide concerns and flooding over the past month or so as a continuous stream of [atmospheric rivers] moved in,” DeFlitch said. “At this point it doesn’t take much rainfall to lead to river concerns, let alone rainfall rates” if there are heavier downpours.
NOAA’s Climate Prediction Center notes that the ground in the Pacific Northwest is wetter than anywhere else in the country, sitting in the 99th percentile for soil moisture. That means the ground can’t handle much additional rainfall.
The region should dry out for much of Thursday and Friday before another rainy system moves in this weekend. Long-range models show the storm parade continuing next week, with wet weather coming through every few days.