Big storm expected to unleash thunderstorms: 'People may see funnel clouds'

Screengrab from a video shared by NWS Bay Area on Sunday, January 6, 2019 of a waterspout spotted near Santa Cruz. Screengrab from a video shared by NWS Bay Area on Sunday, January 6, 2019 of a waterspout spotted near Santa Cruz. Photo: By DropAndTurnShow Via NWS Bay Area Photo: By DropAndTurnShow Via NWS Bay Area Image 1 of / 44 Caption Close Big storm expected to unleash thunderstorms: 'People may see funnel clouds' 1 / 44 Back to Gallery

A so-called atmospheric river storm system is carrying a deep layer of clouds into the Bay Area with the potential for lightning this week.

National Weather Service meteorologist Drew Peterson says there's potential for extreme temperature differences in the atmosphere known as convection that triggers thunderstorms.

When this supercharged system slams into the Bay Area Wednesday evening, likely between 6 p.m. and 10 p.m., Peterson says hail, thunder, and lightning are all likely. The unsettled atmospheric conditions could also whip up funnel clouds over the Pacific Ocean.

"This is going to be a potent storm system and we're going to see all the ramifications of a big storm including water spouts," Peterson says. "People may see funnel clouds."

In the Bay Area, funnel clouds most often form in Monterey Bay and rarely reach over F0 on the tornado scale of F0 to F5. With an F0 reading, wind gusts rage between 40 mph to 72 mph.

Earlier this month, two funnel clouds reached landfall in Santa Cruz, tearing the roof off a restaurant. (See photos above.)

Peterson says hail storms are of more concern with this incoming storm and are more likely to people, especially motorists.

"We can get that really small hail coming down," he says. "The storm could dump buckets and buckets of tiny marbles of ice. It's really dangerous. It's like driving on ice."

ALSO: One Bay Area mountaintop could see 7 inches of rain in 24 hours

The National Weather Service has issued several winter storm warnings for the upcoming event including a flash flood watch warning of pooling on roadways and flooding on urban creeks and a wind advisory. Valley areas could see gusts up to 40 mph and coastal areas and mountaintops up to 60 mph. Downed trees and power lines causing power outages are possible.

More weather stories:

Life-threatening blizzard to pummel Sierra Nevada: 'Zero visibility in whiteout conditions'

One Bay Area mountaintop could see 7 inches of rain in 24 hours

Powerful storm to deliver 40-foot waves to Bay Area beaches

People will be saying 'atmospheric river' a lot on Wednesday. Here's what that means

Send photos, videos and stories from the storm to SFGATE news producer Amy Graff at agraff@sfchronicle.com