An unusually powerful force is fueling weather hazards in the northwestern United States and Canada, including an intense atmospheric river headed to California. The record-strong low-pressure system is developing offshore of Oregon and Washington, causing low air pressure that is more typical of a major hurricane.
By Ben Noll
As of early Wednesday morning, more than 650,000 customers were without power at the peak of outages in Washington state as winds gusted as high as 101 mph nearby. The National Weather Service in Seattle reported that its office “took a power hit”, causing weather radio transmissions to go down.
Among the multiple hazards that will last into the weekend, an atmospheric river of moisture will extend some 2,000 miles across the Pacific, from Hawaii to Northern California, where it made landfall on Tuesday night. It is expected to deliver 10 to 20 inches of rain to California’s northern Coast Ranges and very heavy snow to the Klamath Mountains.
Heavy rain, mountain snow and strong, potentially damaging winds will affect Oregon, Washington and British Columbia. Snow will also extend into Idaho, Montana, Alberta and Saskatchewan.
Why is this atmospheric river unusual?
Atmospheric rivers are long, relatively narrow, concentrated plumes of water vapor that transport heat and moisture out of the tropics and toward the poles.
These rivers in the sky are ranked based on their level of moisture transport and duration. The one that made landfall in Northern California on Tuesday night was forecast to qualify as a Category 4 (out of 5), strong atmospheric river — most notable for its long duration.
But what’s most unusual about this atmospheric river is the intense low that’s driving it.
On Tuesday, a low-pressure system off the Northwest coast strengthened so rapidly that it qualified as a meteorological bomb. This happens when a storm’s central pressure drops by more than 24 millibars in 24 hours, although the intensification rate needed depends on latitude.
This storm was forecast to deepen from around 984 millibars Tuesday morning to 943 millibars in the evening, easily becoming a bomb cyclone in half the time required to qualify.
The image below shows the location of the storm during what was expected to be its peak intensity Tuesday night, denoted by the purple shading offshore Washington state.
Record-breaking low pressure, more typical of a major hurricane
An analysis of atmospheric pressure dating back to the 1940s shows that this should be the most intense November storm on record in the region by about 15 millibars. The intensity of the storm, according to its minimum central air pressure, is consistent with that of a Category 4 hurricane — which speaks to the ferocious nature of its expected impacts.
The image below shows the areas, shaded blue, where the storm is forecast to break air pressure records for November.
What’s causing this intensity?
The storm will deepen as a very loopy jet stream is forced to flow around a strong ridge of high pressure centered over the Bering Sea before taking a big dip southward off the Northwest coast.
This pattern is occurring on the backdrop of a North Pacific marine heat wave, which has contributed to unusual jet stream patterns this fall. Marine heat waves are expected to become more common amid a warming world.
The slow-moving storm’s intensity is expected to be reinforced when a secondary low-pressure system strengthens offshore from late Thursday into Friday, causing another surge of drenching rainfall and heavy mountain snow.
A blizzard warning is in effect for the Cascade Mountains of Washington above elevations of 2,500 feet, where total snowfall may exceed a foot with winds gusting to 60 mph. Similarly heavy snowfall is forecast across the border in British Columbia, such as across the Vancouver Island Ranges and in the mountains to the east of Vancouver.
The animation below shows the atmospheric river as it flows into the region from Tuesday night through Friday.
The rain and snow should turn less intense across the region over the weekend before giving way to a drier pattern next week.
However, this may just be the start of what could be an active winter season across the Northwest, bolstered by a La Niña-like pattern and an active northern branch of the jet stream.
