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Update:
Evacuation warnings have been issued for areas of San Mateo County after the National Weather Service issued a flash flood watch and wind advisory Saturday afternoon for portions of the CZU Lightning Complex burn area.
The weather service's flash flood watch and wind advisory now increase the potential for debris flows, downed trees and power outages in the affected areas. The evacuation warnings are effective immediately, according to a joint statement by Cal Fire, the San Mateo County Sheriff's Department, the San Mateo County Fire Department and San Mateo County.
The evacuation warnings are for the following areas:
South Coast Zone SMC-E019, which includes Ano Nuevo State Park, Whitehouse Canyon Road and areas south of Gazos Creek Road east of Highway 1; South Coast Zone SMC-E038, which includes Butano State Park and the community of Barranca Knolls, west of Cloverdale Road; The communities of Loma Mar and Dearborn Park in Zone SMC-E018; The community of Butano Canyon in Zone SMC-E098.
These areas will be upgraded to evacuation orders at 8 a.m. Sunday. According to officials, as Sunday morning progresses, and winds and rain increase, downed trees, tree branches and mudslides could impact evacuations. Electricity could also be impacted.
Half Moon Bay High School will open at 8 a.m. Sunday as a temporary evacuation point (TEP). The school is located at 1 Lewis Foster Dr., Foster City.
A live interactive evacuation map is available at community.zonehaven.com.
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The National Weather Service has issued several weather advisories for various parts of the Bay Area, including Redwood City, beginning Sunday, Oct. 24.
There is a Flash Flood Watch in effect Sunday for the North Bay from 5 a.m. to 11 p.m., for the Santa Cruz Mountains from 1 p.m. to 11 p.m. and for Monterey County from 5 p.m. to 4 a.m. on Monday.
Heavy rainfall could lead to debris flows and flash flooding in areas recently burned by fires due to poor drainage. There may be potential for mudslides and washouts in steep terrain in these areas, according to the National Weather Service.
There are also Wind Advisories in effect Sunday in the North Bay from 2 a.m. to 5 p.m., in San Francisco, the East Bay and the South Bay from 5 a.m. to 8 p.m. and in Monterey County from 11 a.m. to 11 p.m.
Southerly winds are expected to be 15 to 25 mph with occasional gusts up to 35 to 45 mph. Some areas might see gusts up to 60 mph.
Residents are advised to secure loose outdoor objects and structures and be aware tree limbs could be blown down, resulting in some power outages.
Driving may be difficult for vehicles with high profiles.
A High Surf Advisory will be in effect for coastal parts of the Bay Area from 11 p.m. Sunday to 11 a.m. on Tuesday.
There will be an increased risk of sneaker waves, large breaking waves, rip currents and coastal run-up.
But what is debris flow, and why is it so worrisome?
The question is particularly relevant for Bay Area residents who live on or below hillsides, especially those impacted by recent wildfires, according to the California Department of Conservation.
Colloquially, a debris flow is often identified as a mudslide. To be more exact, it is a fast-moving mass of material — a slurry of water, rock, soil, vegetation, and even boulders and trees – that moves downhill by sliding, flowing and falling.
Debris flows develop during intense rainfall, changing the earth into a flowing river of mud. They 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.
Debris flows range from a few square yards to hundreds of acres in area, and from a few inches to 50 feet deep.
"Even smaller ones can be locally dangerous: Imagine trying to walk through a 3-inch deep mass of wet concrete moving at 30 mph," according to the California Department of Conservation website, https://www.conservation.ca.gov/.
The speed of debris flows is what distinguishes them from large, slow-moving landslides. It can take weeks or months for detectible movement to start as a landslide. Debris flows are more dangerous because they are fast-moving – triggered by short, intense periods of rainfall, like the period the Bay Area is currently experiencing.
Debris flows can cause both property damage and loss of life, and residents should take precautions to prepare for and survive them.
Residents should pay close attention to official forecasts, the conservation department said. Debris flows can spring up anywhere, according to the department.
Many Bay Area residents, especially those in the North Bay, already have "go bags," also described as emergency kits, as well as family communications plans. Both are recommended by Ready.gov, as well as making a plan for the household and the pets.
"Get out before the storm arrives," the conservation department advises, adding that "if you wait to see if a debris flow is coming your way, it will be too late to leave safely."
Aside from debris flows, storms that cause them can also cause more common flooding dangers, and residents should be on the lookout for these as well, according to the department.
"Never drive, walk, or bicycle through a flooded road or path. Even a few inches of water can hide currents that can sweep you away. Also, the water level can rise before you finish crossing," according to the conservation department.
