70% of Northern California expected to see rain over the weekend

Labor Day weekend is the last hurrah of summer — when Californians like to soak up some sun at beaches, at lakes and in the mountains. Not this year.

An unseasonably cold low-pressure system had reached California by Friday morning, bringing chilly temperatures across the state and a chance of rain and thunderstorms at least through Saturday, mainly to locations in interior Northern California.

After several days of hot weather, the system will bring “a major pattern change” to California, UCLA climate scientist Daniel Swain wrote on social media. Swain said that much of California “will stay mostly dry” — including the Central Coast, San Francisco Bay Area and coastal Southern California — but precipitation chances are in the forecast for inland Northern California from Friday into Saturday. “A few thunderstorms are possible, and locally heavier rain could fall in Northern Sierra!” Swain wrote.

Brent Wachter, a predictive service fire meteorologist with the U.S. Forest Service, told SFGATE on Thursday that an estimated 70% of Northern California will see some rain associated with the system, from light drizzle to heavy, scattered showers.

This is good news, as multiple wildfires are burning in far Northern California. “It’s going to be quite impactful,” Wachter told SFGATE. “We’re expecting widespread wetting rain across most of the fires in Northern California. In some instances, there could be quite a bit of rainfall.”

Afternoon highs across Northern California will generally be in the high 50s to low 70s on Friday, according to the weather service. This is a dramatic change from recent days when the state was in the grips of a heat event. Sacramento soared to 95 degrees on Wednesday and is forecast to reach only 74 on Friday. Downtown San Francisco was a balmy 84 Wednesday but is likely to hit only 68 degrees Friday. In the Tahoe area, Truckee was 79 on Wednesday and is forecast to reach 61 degrees Friday. 

Unseasonably cool weather is also in the forecast for Saturday and Sunday across Northern California, though temperatures will likely be a little warmer than they were Friday. Monday is forecast to be warmer than Saturday and Sunday, and many inland valley areas could get into the 80s.

Light showers fell “over portions of the eastern Sacramento Valley and northern Sierra” early Friday morning, the weather service said, while the Bay Area received some drizzle. The rain chances continue in Northern California through the day and into Saturday, potentially Sunday in some spots.

For Friday through Sunday, the weather service is predicting that Eureka will receive 0.10 to 0.25 inch of rain, Weaverville 0.50 to 1 inch, Redding 0.25 to 0.50 inch, Susanville 1 to 2 inches, Chico 0.10 to 0.25 inch, Grass Valley 0.50 to 1 inch, South Lake Tahoe 0.25 to 0.50 inch, Sonora 0.25 to 0.50 inch, and Yosemite 0.10 to 0.25 inch. San Francisco, Santa Rosa, Ukiah, Sacramento, Stockton and Modesto are all expected to see less than 0.10 inch of rain. 

Some of the highest chances of rain and thunderstorms in Northern California are in the foothills and the mountains, including the northern Sierra Nevada and the Cascade, Klamath, Siskiyou and Coast ranges, as SFGATE reported Thursday. Wildfire burn scars in these areas are at risk of debris flows if high rainfall rates develop. 

The weather service’s Sacramento office issued flash flood watches in effect from 11 a.m. to 7 p.m. Friday for the million-acre August Complex burn scar that straddles seven counties — Mendocino, Humboldt, Trinity, Tehama, Glenn, Lake and Colusa — and also the 123,000-acre McFarland Fire burn scar in Shasta, Trinity and Tehama counties. The Eureka office issued a flash flood watch for Friday, from 11 a.m. to 11 p.m., in Del Norte County, where the Smith River and Slater burn scars are at risk of debris flows.

In the San Francisco Bay Area, the chances of any significant rain are slim. “Main story is cooler temperatures this weekend along with occasional drizzle and low end potential (~10-20%) for light rain showers today and Saturday,” Sean Miller, a meteorologist with the weather service’s Bay Area office, wrote in a message. “Regardless we’re not expecting anything meaningful in the rain department, but there could be enough to wet the ground in spots.”

In Southern California, the weather service’s Los Angeles office said there will be “rapid cooling” on Friday “with a growing chance of showers Friday thru Sunday.” The agency said, “Any rain will generally be light, but isolated moderate rain possible. Small chance for thunderstorms.”

The San Diego office, which monitors the forecast in the greater San Diego area, Orange County and the Coachella Valley, also said to expect cooler weather and precipitation chances — though the chances are low, with a 19% chance in San Diego, a 20% chance in Anaheim and a 17% chance in Palm Springs.