Temperatures are set to take a dramatic plunge across Northern California this week, with some neighborhoods expecting highs nearly 10 degrees below normal by Thursday. The cooldown comes as a series of weather systems moves through the region, bringing cloud cover, isolated showers, and a renewed threat of thunderstorms in the Sierra.
The National Weather Service in Sacramento has issued a Red Flag Warning for Thursday from 5 a.m. to 9 p.m. for the Sierra Nevada of El Dorado, Amador, and Northern Alpine counties above 5,000 feet, including Highway 50. The warning also covers the Sierra Nevada of Calaveras, Tuolumne, and Southern Alpine counties above 5,000 feet. Forecasters warn that thunderstorms and strong outflow winds could combine with dry fuels to spark wildfires.
“The National Weather Service in Sacramento has issued a Red Flag Warning for thunderstorms and strong outflow winds, which is in effect from 5 a.m. to 9 p.m. Thursday,” the agency said. “The Fire Weather Watch is no longer in effect.”
While the red flag warning is specific to the higher elevations, the broader forecast paints a picture of a week that starts mild but turns unsettled. Tuesday morning brought a pleasant breeze through the Valley, but clouds are expected to build overnight and linger into Wednesday and Thursday. Those clouds will act like a lid, trapping cooler air and keeping afternoon highs in the low 80s across the Valley on Thursday—a far cry from the typical August heat.
In the Sierra, isolated storms are possible Tuesday afternoon on the eastern slopes south of Highway 50, with better chances arriving Wednesday and Thursday, including around Lake Tahoe. Storms are expected to remain below severe limits, but lightning could still spark fires, especially given the dry conditions. A few light showers and sprinkles were spotted early Tuesday morning in Mono County, though those quickly faded.
The cooling trend is welcome news for many, especially after a stretch of scorching temperatures. By the weekend, Valley highs should settle into the upper 80s, with the foothills seeing more comfortable values as cooler overnight air returns. But the muggier conditions are expected to creep back by midweek, adding a sticky feel to the air.
For parents, it might be wise to have kids grab a hoodie or long sleeves for the early morning bus stop Tuesday, though by afternoon, t-shirts will be the norm. Highs in Sacramento are expected to be near normal Tuesday, but they’ll drop below normal for the rest of the workweek.
The red flag warning carries specific precautions. The NWS advises that if burning is permitted, burn barrels should have a weighted metal cover with holes no larger than three-quarters of an inch. Cigarette butts should be disposed of carefully—never tossed from a moving vehicle. Outdoor fires must be fully extinguished with water and stirred to ensure they’re cool to the touch; charcoal should be submerged until completely devoid of heat. And no fire should ever be left unattended, as even a small spark can ignite dry grass.
Sierra thunderstorm chances will taper off after Thursday but could linger through Saturday. Meanwhile, Valley highs are projected to stay in the upper 80s to low 90s through next weekend, and potentially into the start of the following week.
