Blustery storms heading to Northern California

A pair of blustery, soaking storms will stream into Northern California beginning Friday, bringing as much as 10 inches of rain south and east of the Bay Area by Christmas Day and setting up potentially difficult travel conditions for those escaping for the holidays.

Though computer weather models differ on the exact trajectory and strength of the two separate weather systems, meteorologists are bracing for sizable amounts of rain and the possibility that some rivers might overrun their banks.

"No matter how you slice it, it's going to be a significant weather event," said Rob Hartman, lead hydrologist with the National Weather Service's California-Nevada River Forecast Center. "If it comes in heavy and warm, it will be a bigger flood event. But the real wild card is the freezing level."

Watching the rivers

If temperatures plunge and much of the precipitation falls as snow in the Sierra Nevada, Hartman said, most rivers should be able to handle the flows. But if the mercury creeps higher, rains could overwhelm already-full waterways such as the Cosumnes River south of Sacramento and the Merced River in Yosemite National Park.

Hartman doesn't anticipate flooding on the Napa or Russian rivers, but said an especially robust storm cell could push the South Bay's Guadalupe River above flood levels. Urban creeks and roadways could also spill over in neighborhoods where soil is already saturated and leaves and other debris clog storm drains.

The first storm is expected to move into the region Friday evening, as a cold low-pressure system off the California coast collides with a wet surge of air from the tropics. Experts at the National Weather Service's office in Monterey said low-lying parts of the Bay Area could receive about an inch of rain by Saturday, with more like 2 inches in the hills.

'A good shot of rain'

"We're going to get a good shot of rain here, and it may last awhile," said meteorologist Diana Henderson.

There's less consensus on the second system, which will shift into the region early next week. Some forecasts point to as much as 10 inches of precipitation for portions of the Sierra and the Monterey Peninsula, with about 4 inches falling in the Bay Area.

Conditions don't bode well for getting out of town, with San Francisco International Airport gearing up for the busiest travel day of the season on Wednesday

"It's going to be a really challenging week," said airport spokesman Mike McCarron. "Right now, we're making sure all the drains are working and we're asking the airlines to tie down all their loose equipment."

Reminders of floods

To some, the current patterns are reminiscent of December 2005 and January 2006, which delivered driving rains to the region and flooded downtown San Anselmo, causing millions of dollars in damage and cutting power to thousands.

"If we get some good, wetting rains over the weekend and additional strong rain next week, we will start to see the rivers respond - which is something we haven't seen in five years," said Steve Goldstein, forecaster with the National Weather Service in Sacramento.

Goldstein said next week's forecast will firm up over the next few days; but it appears that the final shopping days before Christmas will be wet and windy.

In the meantime, the federal weather agency is holding regular briefings with state and local authorities to plan for any major snafus.

"These storms are on our radar," said Jay Alan, spokesman for the California Office of Emergency Services. "When the weather folks say we might have significant storms like this, we pay attention."

E-mail Kelly Zito at kzito@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle