San Francisco Bay Area weather: flood watch issued ahead of storm

Unseasonably warm, storm ahead

It's going to be unseasonably warm today with a storm brewing for Wednesday and Thursday.

OAKLAND, Calif. - Don’t be deceived by the sunny skies and unusually warm winter temperatures to grace the Bay Area the last few days. Things are about to change. What’s being billed as potentially our biggest storm so far this season, was set to slam the region early Wednesday, with a flood watch issued for the entire Bay Area.

The warning was set to go into effect on Wednesday at 4 a.m. and last through Friday at 4 a.m.

The storm system promised not only copious amounts of rain, but powerful winds as well.

Updated rainfall totals showed the highest peaks of coastal mountains in the North Bay and Santa Cruz County would be hit the hardest, with more than 5 inches of rain forecasted, according KTVU Meteorologist Roberta Gonzales.

Northern and coastal areas of the North Bay were expected to receive 3 to 5 inches.

San Francisco, the San Mateo Coast, Oakland, the East Bay Hills, San Mateo County, lower elevations of Santa Cruz County, southern portions of the North Bay, and the Big Sur Coast were set to see 2 to 3 inches.

Santa Clara Valley, the interior East Bay, interior Monterey County and San Benito County were forecasted to receive 1 to 2 inches of rain.

The main rainfall threat will be urban and small stream flooding, according Roberta.

She said that while larger, main rivers were expected to see their waters rise, the rainfall accumulation from smaller streams and higher slopes would tribute into those rivers, which can increase the threat of flooding even after the storm has passed.

There was also concern the rain would lead to mud and rock slides.

The winds were expected to precede the rain. On Tuesday evening, they were forecasted to turn south and become breezy. Roberta said gusts would build through the night with dangerously strong winds expected early Wednesday morning.

A high wind warning has been issued for the Santa Cruz Mountains from Wednesday at 4 a.m. through Thursday at 4 a.m., with gusts of up to 55 mph.

Roberta warned the strong winds could cause widespread damage and create dangerous conditions on the road. She said expect the conditions to bring down trees and maybe even down power lines, which could lead to outages.

There was potentially some good news when it came to the prospects of storm damage. Roberta said the updated forecast on Tuesday showed that the system may be on a more accelerated path than previously observed.

Atmospheric Rivers: What are they and how do they form?

With an atmospheric river headed towards the Bay this week, we asked KTVU meteorologist Mark Tamayo to give us a break down on what exactly they are and how they form.

"May be moving through a bit quicker, which is good," she explained. "We do not want the storm to stall!"

As for the longer range forecast, the region could see shower activities on Friday and Saturday, which at this point appeared, would bring much lighter rain.

Longer term models suggested another rain event could hit on Sunday and stretch into early next week, though it's still early to tell for sure, and the system could move farther south.

"But any additional rain will cause runoff with additional mud or landslides through the weekend," Roberta said, and she added, "These winter storms are always considered our ‘Weather Super Bowl!’"