West Coast braces for simultaneous 'bomb cyclone' and 'atmospheric river

A "bomb cyclone" with hurricane-like strength and a chart-topping "atmospheric river" will coincide Saturday night into Sunday to unleash flooding rains, wet snow, strong winds and coastal surf across the western US.

This will be the third and strongest in a series of storms to strike the West Coast this week.

A bomb cyclone is a system that drops at least 24 mb in pressure 24 hours or less -- and typically the lower the pressure, the stronger the storm. Atmospheric rivers are narrow bands of concentrated moisture that cruise more than 2 miles above the ocean and release rain or snow when they hit land -- and this one rates level 5 of 5.

"By Saturday night, a rapidly intensifying Pacific cyclone directing a powerful atmospheric river squarely at the West Coast delivers a fire hose of rich subtropical moisture into California," the Weather Prediction Center said Friday.

These two simultaneous events will result in the season\"s first heavy, wet snow across the Sierras and periods of soaking rains for the coastline and valleys across central and Northern California. Areas of Washington and Oregon will also see impacts through the weekend.

This system is forecast to rapidly strengthen on approach to the Pacific Northwest, becoming a "bomb cyclone" over the next 24 to 48 hours. This system could be one of the lowest pressure storms to ever develop across the northeastern Pacific Ocean, with a forecast pressure comparable to major hurricanes in the Atlantic.

"I don\"t think a lot of people realize that these cyclones in the northern Pacific can be equivalent to a hurricane in the Gulf of Mexico or Atlantic," Carly Kovacik, a lead meteorologist for the National Weather Service office in Seattle, told CNN. "A lot of these storms produce very strong winds and high seas, but since they are mid-latitude storms, we just don\"t call them hurricanes even though the impacts can be very similar." 

Like a hurricane, the track of this particular system matters in terms of what impacts will be felt this weekend across the Pacific Northwest. 

If the system takes a more northerly track -- like the storm earlier this week did -- the impacts will be lesser across Washington and Oregon, Kovacik explained. But if the center of the storm takes a more southerly track toward Vancouver Island, the impacts to the coast and inland areas will increase, meaning stronger winds and higher swells.