As the start of the Christmas week beckoned, National Weather Service forecasters already were eyeballing the intense rain that was expected to soak the holiday in the Bay Area and Northern California.
Now, their focus has changed. Atop the list now are developing southerly winds and a clash of cold air and warmer air, which could lead to thunderstorms — and a slight chance of tornadoes — that the weather service called “severe.”
“Essentially, rain is now third on the list of priorities,” NWS meteorologist Dylan Behringer said Tuesday. “There’s still gonna be a decent amount, but that’s not what we’re most concerned about.”
The hit the Bay Area is expected to take early Wednesday is spurred from the most powerful wave of an atmospheric river storm.
Behringer said the primary worry is the southerly winds, which are likely to begin blowing Tuesday night and last into Wednesday morning. Those winds are expected to take a break on Wednesday night, before building again on Thursday morning. The coastal areas of the Bay Area and the Central Coast are most at risk; Behringer said winds there could blow as high as 60 mph.
Thunderstorms are expected to be severe from Point Arena north of Santa Rosa to south of Big Sur and are likely to affect areas of Alameda, Contra Costa, San Mateo, Santa Clara and Santa Cruz counties, according to the weather service. In the advisory, they said there is a 2% chance that at least one tornado may develop.
The weather service issued a high wind warning Tuesday afternoon for the coastal areas of the region and a wind advisory for the interior areas that lasts until 4 p.m. Wednesday. Forecasters expect the winds to be powerful enough to snap power lines, bring down trees and result in property damage.
“When we get into Thursday morning, there will be more of a chance for really strong thunderstorms,” Behringer said. “That will bring lightning strikes, and high rain-rates. Lots of rain coming down really fast can cause flash floods. The good news is that the winds will make the storm move fast, so we don’t anticipate that hard rain coming down in one spot for very long.”
That said, the rain is supposed to pound the region heavily. At least an inch of rain — maybe as much as 1½ inches — is expected everywhere, while high-elevation areas along the North Bay coast could get between 2-3 inches from this wave.
A flood watch for the North Bay began Monday, while a similar warning went into effect for areas south of San Francisco — including San Jose, Santa Cruz and Monterey — early Tuesday. That flood watch for all areas lasts until Friday at 10 p.m.
The weather service also issued an warning for extremely dangerous marine conditions along coastal waters along the Northern California coast that runs through Friday morning. A beach hazards statement for all the beaches on the Pacific Coast was set to last from Tuesday afternoon until 4 a.m. Wednesday. Waves are expected to break at 20 feet high, visibility is expected to be low and people are advised to stay away.
In the Sierra Nevada, a winter storm warning was set to go into effect Tuesday night and last through 10 p.m. Friday. The Sierra and Southern Cascades above an elevation of 4,500 feet are expected to get at least 1 foot of snow, while 4 to 8 feet of snow are expected above 5,500 feet.
One foot of snow also is expected in Shasta County along the Coast Range above 3,500 feet. As much as 3 feet of snow could fall there in the elevations above 4,500 feet.
The weather service said near white-out conditions are expected while the snow falls and urged drivers not even to attempt to navigate the storm. Road conditions are expected to be miserable and chains or snowtires are likely to be required on any highway that remains open.
By Friday night, the wave of nasty weather is expected to have moved into Southern California, and a period of dry, cold time is expected to follow, Behringer said. The forecast is expected to be without rain from at least Saturday until the following Tuesday, and Behringer said forecasters “expect a decent period of drying out.”
“It’s gonna get chilly, too,” he said of the dry period. “The overnight lows will start to drop into the 30s.”
