A day after scientists announced that La Niña will be a reality this year — a claim that often brings the assumption that a dry winter is ahead — a storm that the National Weather Service said will be more in keeping with the excessive rain of an El Niño continued to brew off the Pacific Coast and remained on track to reach the Bay Area early next week.
First, however, is expected to be a mild, pleasant weekend with temperatures staying in the low 70s and high 60s in the hottest spots and sun peering in through scattered clouds.
After that calm comes the storm.
“This looks to be pretty heavy,” NWS meteorologist Dial Hoang said of the looming rainfall. “It’s a big system. Looking at the total rain forecast, we anticipate that there will be at least an inch across the Bay Area and there could be up to 2-3 inches in the mountains and higher elevations. … This is probably going to be a pretty notable early season storm.”
The weather service’s prediction center said the storm will be strong enough that there is at least a 5% chance it will cause flash flooding in the region. At least 1½ inches are expected to fall in the driest spots — including near San Jose and other areas that sometimes miss out because of the rain shadow of the Santa Cruz Mountains. In the wetter places, including Mount Tamalpais and the Santa Cruz Mountains, as much as 3 inches of rain may fall.
“That’s not unusual for an atmospheric river in the winter,” NWS meteorologist Dylan Flynn said. “It’s a little unusual to get it in (October). The trend has been increasing, too. In the initial forecast, we expected 1½ inches in the heaviest spots. And it could go up from what we are projecting (Friday afternoon).”
That the storm is due so soon — after federal scientists on Thursday announced that La Niña has begun and is expected to continue into winter — also made its pending strength noteworthy. La Niña occurs when Pacific Ocean waters along the equator off South America are cooler than normal, leading to conventional wisdom that a dry winter is on its way.
“The normal amount of rain for the entire month of October in San Francisco is nine-tenths of an inch,” Flynn said. “So to get an inch, and inch-and-a-half, maybe more in one storm tells you that’s it’s a pretty heavy storm. I don’t want to overhype it. If it was January, it would be more standard.”
The storm also will do one other thing: It should put an end to fire season.
But thunder and lightning are also a possibility. The weather service said the low pressure that will bring that storm also will leave cold air aloft and could create some instability in the atmosphere Monday into Tuesday. Those thunder-and-lightning conditions are expected to be gone by Tuesday when the weather service said additional cold air behind the initial cold front will make its influence felt.
Hoang said the system also could bring some snow to the Lake Tahoe area at 6,500 or 7,000 feet. Temperatures are expected to reach the upper 50s in the region, so any snow is not expected to stick for long, according to the weather service.
Staff writer Paul Rogers contributed to this story.
