San Francisco ? Authorities on Friday urged homeowners with hillside houses to collect valuables and identify escape routes in case mudslides force them to evacuate as Northern California prepared for heavy weekend rain and flooding.

Rainfall totals for areas north of the San Francisco Bay area averaged about 4 inches for the 24-hour period ending Friday evening, according to the National Weather Service. Some mountainous areas near the Humboldt County coast got as much as 6 inches.

As the storm gradually moved south, federal and state forecasters warned that parts of Sonoma, Sacramento, Shasta and Tehama counties were ripe for their worst flooding in years.

In the Sonoma County town of Guerneville, for example, where flood levels start when the Russian River reaches 31 feet, officials prepared for the water to rise as high as 43 feet by late today or Sunday. The warning prompted the Fife’s Guest Ranch resort to cancel its New Year’s Eve celebration.

The storms could add as much as 6 inches to the already water-logged region by the end of the weekend, said Rick Canepa, an NWS meteorologist based in Monterey.

“Both storms are going to add up to quite an amount of rainfall, some of the heaviest rains we have seen so far this season,” Canepa said.

Pedestrians cross a rainy Ninth Avenue on Friday in San Francisco. San Francisco residents snapped up free sandbags as the latest in a string of powerful storms began moving in Friday, days after brutal downpours drove Northern California rivers to their highest levels in seven years.

Hilly coastal areas such as Sonoma and Mendocino counties were especially at risk of mudslides, according to Andy Morin, an NWS hydrologist based in Sacramento.

Interstate 5 was closed both ways over the Siskiyou Summit near the Oregon border between Hilt and Ashland, Ore., first because of flash flooding, and then from three separate landslides. U.S. Highway 101 was closed by fallen trees and mud south of Crescent City.

Wet runways and poor visibility caused delays of two to three hours on most flights into and out of San Francisco International Airport on Friday. More than 2 feet of snow was predicted to blanket the Sierra Nevada from Yosemite National Park to Kings Canyon, making road travel nearly impossible this weekend, said officials at the San Joaquin Valley Weather Forecast Office.

In Sacramento, officials concerned about flooding in the Sacramento and Feather River basins prepared to release more water from Oroville Dam, which would keep “flood space” in the reservoir, said Michael Miller, spokesman for the Department of Water Resources. Officials can then regulate the water’s release downstream at safer levels, he said.

Meanwhile, Southern California prepared for the arrival of moderate to heavy rain today and a second, stronger storm Sunday and Monday. Forecasters issued flash flood watches for large areas burned by the year’s wildfires in Santa Barbara, Ventura and Los Angeles counties.