Update: Rain spreading across Bay Area; Flood watch issued as atmospheric river targets NorCal

SAN FRANCISCO -- Driven toward the Bay Area by a surging jet stream, a winter storm system -- carrying with it a Cat. 4 atmospheric river -- began pelting the Bay Area with rain Monday amid a variety of alerts and warnings.

Among the alerts issued by the National Weather Service was a flood watch from late Monday night through Tuesday afternoon for cities including San Francisco, Watsonville, Pacifica, Santa Cruz, Scotts Valley and Boulder Creek.

By Tuesday morning, excessive runoff could bring flooding to rivers, creeks, streams, and low-lying areas, the weather service said in an advisory. 

"Localized flooding will likely occur, particularly in urban areas where ponding on roadways or poor drainage is a common issue," weather service forecasters said. "Excessive runoff may result in flooding of rivers, creeks, streams and other low-lying flood-prone locations. Commuters should plan on a wet trip on Tuesday and allow for extra time to arrive at their location."

Monday evening's wet weather was causing delays of up to 20 minutes on BART system-wide, the agency reported. The weather service said that rain was developing in the vicinity of the San Francisco Airport around 5:50 p.m. 

The weather service has also issued a wind advisory for late Monday night to Tuesday morning for the coastal areas from Sonoma County to Santa Cruz County.

"South winds 20 to 30 mph with gusts up to 50 mph expected," the weather service said. "Local gusts up to 60 mph over the ridges and peaks."

The wind forecast had officials warning of falling trees and limbs and possible power outages.

A beach hazard statement and a high surf advisory were also issued for coastal beaches for "dangerous swimming and surfing conditions."

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How much rain are we talking about?

"Rainfall totals will range from 1 to 3 inches, with higher elevations receiving upward of 3 to 5 inches," the National Weather Service predicted. "Locally up to 7 inches are possible over favored peaks and higher terrain of the Sonoma Coastal Range where prolonged moderate to heavy precipitation and higher rain rates are currently forecast."

"Last but not least, if that was not enough," forecasters added. "There is a slight chance of thunder which has expanded southward to just around San Francisco. Not expecting much more than a rumble of thunder here and there."

The weather service predicted that Monday-Tuesday's storm will be the start of a wet week.

"Like the Jelly of the Month Club, a stout jet across the entirety of the Pacific Basin will be the gift that keeps on giving as it delivers additional waves of moisture across the West Coast," forecasters at the weather service's Reno office predicted. "Potentially up to three additional waves of moisture will be possible before the New Year with systems possible again on Thursday, Friday, and  Saturday and additional waves arriving through the first week  of January."

"Timing of these upcoming storms are still to be determined, although it's becoming likely that they will be cooler and wetter than our upcoming storm on Tuesday." 

Forecasters were particularly focused on "the next upper-level trough moving in around December 30th, providing the next healthy dose of rain for the Bay Area."