Big storm means a wet weekend for SantaCon, other SF events

More rain in the forecast for the Bay Area. More rain in the forecast for the Bay Area. Photo: Getty Images Photo: Getty Images Image 1 of / 32 Caption Close Big storm means a wet weekend for SantaCon, other SF events 1 / 32 Back to Gallery

San Francisco's SantaCon, where thousands of drunken people dressed like the bearded man descend on the city's bars, is always a mess. Throw in rainfall, and things are bound to reach a new level of mayhem.

A potent winter storm is taking aim at the San Francisco Bay Area this weekend when dozens of holidays events are taking place. If you're planning to go to a tree lighting or holiday shopping fair, leave your house wearing a proper rain coat.

A brief break from the rain is expected Thursday. "I think we have a slight chance of rain everywhere, but it's unlikely," said Suzanne Sims, a meteorologist with the National Weather Service Bay Area. "It's going to be a break day."

A storm the National Weather Service describes as a "potent cold front" is forecast to drop down from the Northwest on Friday and deliver a mix of high winds, bouts of heavy rain and maybe even some thunderstorms. The most severe storm activity is expected Friday evening and overnight.

"The models are showing the storm arriving later than it was showing yesterday," said Sims. "It looks like the timing of the front is more Friday evening and then overnight, and then the post-frontal rain continuing on Saturday. The heavier rain and stronger winds will be Friday night, by like 8 p.m. in the North Bay and then spreading down into the greater Bay Area closer to midnight."

Hourly rainfall rates could exceed half an inch an hour, leading the NWS to issue a flash flood watch in the North Bay were the Kincade Fire burned. The watch is in effect Friday afternoon through Saturday morning.

"Enough instability will be present with the frontal passage and with the upper trough that a slight chance of thunderstorms will remain in the forecast Friday and Saturday," according to the NWS forecast.

By Saturday morning the cold front will have pushed through much of the Bay Area, but an associated trough of low pressure will still need to pass through, keeping showers across the Bay Area Saturday into Sunday.

"Sometimes after the front goes by, you can have a day of rain but the winds will have died down," said Sims. "On Sunday the rain will taper off."

Rainfall totals from Friday into early Sunday range from .5 to 1.25 inches in most urban areas, except 1.5 to 2.5 inches in the North

Bay valleys. In the hills and mountains, rain totals are forecast to range from 1.5 to 4 inches in the North Bay mountains, 1.25 to 1.5 inches for the East Bay hills, 1.5 to 3 inches in the Santa Cruz Mountains, and 1 to 3 inches across Monterey and San Benito County Mountains.

On Monday, a ridge of high pressure is forecast to build over the region, blocking storm activity off the Pacific Ocean. Sims expects the Bay Area will be dry Monday and into at least early next week. "It's this next storm and then we'll have time to let everything soak into the ground," she said.

Amy Graff is a digital editor for SFGATE. Email her at agraff@sfgate.com.