A bomb cyclone and an atmospheric river will combine to dangerous effect on the west coast

CNN
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A powerful “bomb cyclone” will combine with an atmospheric river to release more than a month’s worth of rain, hurricane-force winds and feet of mountain snow to parts of the Pacific Northwest and northern California.

A storm system off the Pacific Northwest is expected to rapidly intensify on Tuesday in a phenomenon called “bombogenesis,” giving it the designation “bomb cyclone.” It will intensify so much so quickly that it could become a “triple bomb,” tripling the criteria needed to be considered a bomb cyclone, the National Weather Service in San Francisco said.

Bomb cyclones are formidable and load heavy snow and strong winds in winter. This could be among the most intense on record for its location, a storm that only occurs “about once every ten years” and will generate “some of the strongest winds we’ve seen in several years” hitting up into “very dangerously mountainous” seas of 30 to 35 feet,” the National Weather Service in Medford, Oregon, said.

This bomb cyclone will work with an atmospheric river, a long plume of water vapor that moves like a river through the atmosphere, to churn out heavy rainfall and significant snowfall in the mountains of the Pacific Northwest and Northern California starting Tuesday. The pair stalls along the coast and hammers the area with dangerous conditions throughout the week and into the weekend.

What are atmospheric rivers?

Parts of northwestern California could record 16 inches of rain or more in 48 hours. More than a month of rain is expected in the northern San Francisco Bay area, primarily north of the Golden Gate Bridge, the weather service there said. Rains of this magnitude are expected to cause significant urban flooding, debris flow on roads and river flooding.

Conditions begin to intensify on Tuesday afternoon. A Level 2 out of 4 threat of flooding rain is in effect for parts of northwestern California and southwestern Oregon, where 2 to 5 inches of rain could fall, the Weather Prediction Center said.

The heaviest rainfall is expected to begin Wednesday and peak Thursday in northwestern California. A level 3 of 4 risk of flooding rain is in place there for Wednesday and a rare level 4 of 4 high risk is in place for Thursday, according to the WPC.

It’s hard to overstate how big of a deal these high risks are. They are issued on fewer than 4% of days per year on average, but are responsible for more than 80% of all flood-related injuries and 40% of all flood-related deathsshows research from WPC.

Three to 6 inches of rain could fall Wednesday with some spots getting up to 8 inches. Thursday’s precipitation could meet or exceed Wednesday’s totals, especially in the high-risk area.

Heavy snowfall is expected across higher-lying areas where winter weather warnings are in place. Blizzard warnings are in effect across parts of the Washington Cascades, with snowfall over a foot and gusty winds gusts up to 60 mph are possible Tuesday afternoon through Wednesday morning.

“Travel may be very difficult to impossible. Strong winds may cause extensive damage to trees and power lines,” the National Weather Service office in Seattle warned.

One to 4 feet of snow is possible Tuesday through Wednesday across the Cascades and northern Sierra Nevada. Snowfall may create impossible travel conditions on Interstate 5 and Highways 31, 36, 66, 89, 97 and 140.

Strong winds will also escalate Tuesday and peak Tuesday evening across the region. They will be especially strong off the coast and rare hurricane strength warnings has been issued for much of the coastal waters of the Pacific Northwest.

Widespread winds of 35 to 50 mph are possible inland with tropical storm force gusts of 70 mph. Isolated gusts in higher elevations and across capes and headlands could hit 85 mph or hurricane force.

These strong winds can cause potentially widespread power outages and damage to buildings and create difficult travel, especially for high-profile vehicles.

Conditions will begin to improve over the weekend, but light rain may continue into next week.