NorCal Braces For 'Bomb Cyclone,' High Winds And Copious Rainfall

SAN FRANCISCO, CA — Cloudy skies and mild temperatures hung over Northern California on Saturday, but forecasters warned that it was merely the calm before a furious storm expected to arrive Sunday — and residents are urged to prepare.

The northern Pacific endured a parade of storms this week, which were classified as "bomb cyclones." This weekend, another storm of a similar magnitude was headed for the northern reaches of the state, as well as a Category 5 atmospheric river system, which was expected to bring more rain to the Bay Area than any day since 2010, weather officials said.

A bomb cyclone or bombogenesis, is a rapidly strengthening storm that can form when a cold air mass collides with warm air mass, which typically happens over warm ocean waters, according to the National Ocean Service.

Several weather advisories came down for various parts of Northern California this weekend, including flash flood warnings, high surf advisories and warnings of debris flows. Strong winds were also expected Sunday, with gusts of up to 60 mph at the windiest spots.

Officials were especially concerned about areas that fires recently tore through, as burn scars in the region's topography could lead to flash flooding due to poor drainage.

In Santa Cruz County, evacuation warnings were issued near where the CZU Lightning Complex once burned, leaving massive burn scars in its wake. Officials warned residents that the Sundays storm could send debris in that area flying — mud, rocks, trees and sometimes cars or homes.

READ MORE: Evacuation Warning Issued In Santa Cruz Co. As Huge Storm Looms

"The Storm Door is officially open," the National Weather Service said last week.

The storm was predicted to wallop much of Northern California and aside from fire impacted areas, residents who live on or below hillsides will be vulnerable to debris flows or mudslides this weekend, according to the California Department of Conservation.

Debris flows tend to manifest during copious rainfall, transforming topography into a treacherous river of mud, weather experts said. These mud rivers can flow rapidly, striking with little or no warning at avalanche speeds — faster than a person can run, according to Ready.gov, a website created by the U.S. government to help citizens cope with emergencies.

READ MORE: What Are Debris Flows And How Can NorCal Residents Cope?

"Ahead of the event, we recommend taking today as a day of preparation," National Weather Service officials said in a forecast discussion on Saturday. "Today is a perfect day to make sure you are ready for the strong winds, moderate to heavy rain, and the possibility of power outages.

Residents were urged to charge up cell phones and battery back ups, tie down outdoor furniture and trash cans. Emergency kits or "go bags" should also be prepared ahead of the story, officials said.

Californians celebrated when rain started falling earlier this week for the first time in any significant way since spring. The San Francisco International Airport set a record rainfall for Thursday, with 0.44 inches of rain tallied. The old record was 0.13 inches on the same day in 1970, the National Weather Service in the Bay Area tweeted.

Satellite view of the developing bomb cyclone and extreme atmospheric river, being pushed east by a powerful Pacific jet extension. #CAwx pic.twitter.com/1BuhrY1R8m
— US StormWatch (@US_Stormwatch) October 23, 2021

Rain and snow will continue to drench central and Northern California before spreading into Southern California on Monday, although Angelenos woke up to a bit of drizzle on Saturday morning.

The incoming heavy rainfall is welcome news in the Golden State, where exceptional drought conditions and another monstrous fire season has plagued residents and authorities working in treacherous burn zones.

Southern California will receive some rain from the storm out of the north, but officials don't expect that precipitation to quell fire season or drought concerns there. But there's a possibility that this storm ends fire season in Northern California.

"This rainfall is coming about a month ahead of average and will be very welcome in fighting the remaining fires, particularly in Northern California," according to Jon Porter, AccuWeather Chief Meteorologist. "Repeated storms delivering such copious rainfall will have a major effect on the wildfire season – effectively ending the wildfire season from Northern California northward."

Wildfires have ripped through 2.4 million acres in California this year, which have destroyed or damaged more than 3,600 structures and killed three people, Cal Fire reported.

And while the storms have helped to corral fire season in the state, they won't end drought. The state's 2021 water year, which ended Sept. 30, was the second driest on record and 2020's was the fifth driest water year on record.

The Associated Press, Bay City News and Patch Editor Kat Schuster contributed to this report.