What the National Weather Service called a “moderate scale” atmospheric river storm — the second such storm already in November — continued to make its approach toward the Bay Area on Wednesday, but the storm’s real power is likely to show early Thursday, according to the National Weather Service.
A 24-hour stay is still expected, but then again, the storm development is moving more slowly than originally forecast, so it may be longer.
“The main front is developing in place, but it’s taking longer than we anticipated,” NWS meteorologist Roger Gass said Wednesday afternoon. “It’s slowing everything down.”
So instead of rain Wednesday night, Gass said it was far more likely precipitation would begin to fall between 4 a.m. and 10 a.m. Thursday. The main part of the storm front is expected to pass right through downtown San Francisco.
The weather service said the storm will bring with it a strong cold front that will mix with the high pressure that’s created the dry conditions of the past few days. Combined, that will produce mighty strong winds, rain, and the possibility of thunder and lightning for some areas of the region, while others covered in a rain shadow may stay relatively untouched.
That so-called “rain shadow” often occurs in atmospheric rivers, according to the weather service. The phenomenon causes the lifting of the system as it moves over mountains and hills and leaves less precipitation for areas that surround it.
No such element will happen in the North Bay above Santa Rosa, where the weather service said there’s nearly a 50% chance that at least 2 inches of rain will fall.
According to the weather service, the center of the storm was developing rather than advancing, and the mixture of the lower cold-air pressure mixing with the higher warm-air pressure had created an unstable atmosphere that is expected to continue.
As a result, the weather service said it anticipates southerly winds that are expected to reach 15 to 25 mph and gust up to 50 mph in Alameda, Contra Costa, San Mateo and Santa Cruz counties, according to the weather service. In areas of Sonoma and Napa counties, those winds could rise to a steady pace of 20-30 mph with gusts reaching 60 mph.
The weather service issued a high-wind warning for San Francisco, coastal San Mateo County and the North Bay that was set to begin at 10 a.m. Wednesday and run through 10 a.m. Thursday. The rest of the region was under a wind advisory for the same time period.
The rain that comes with the wind is expected to be heavy in most places, with downpours in others. The weather service said most of the region has at a 50% chance for at least an inch of rain — though that percentage decreases to about 20% in areas of the Santa Clara Valley such as San Jose and in Concord and Livermore, which are in the shadow of mountains and hills.
Signs of the pending storm were apparent Wednesday on the water. The weather service said hazardous marine conditions were developing Wednesday and were likely to continue into Thursday, thanks to winds and waves that were expected to run between 7-15 feet high.
The storm’s effect also will have an impact on the Sierra Nevada, where the weather service issued a winter storm advisory above 8,000 feet and for the Greater Lake Tahoe area beginning at 1 a.m. Thursday and lasting until 7 a.m. Friday. Travel disruptions through the Sierra passes is expected Thursday. Up to 4 inches of snow is expected between 7,500 and 8,000 feet and 6 inches at the 8,000-foot elevation.
Wind gusts near Lake Tahoe are expected to reach 30 to 40 mph but could gust up to 90 mph in the upper peaks, the weather service said.
