The National Weather Service on Thursday put the hills of western Sonoma County and the mountains of Sonoma and Napa counties under a heat advisory for Saturday and Sunday. Valley towns forecast to reach the upper 90s fall outside it.
The advisory, issued at 9:43 a.m. by the weather service’s San Francisco office, runs from 11 a.m. Saturday to 10 p.m. Sunday. In the North Bay, it covers two forecast zones: the Sonoma Coastal Range and the North Bay Interior Mountains. It also covers the East Bay, the South Bay, the Santa Cruz Mountains and parts of Monterey and San Benito counties.
Across that whole area, the weather service expects highs from the mid-90s to near 103. In its forecast discussion, it said lows in the higher elevations would stay in the upper 60s to mid-70s, “limiting overnight recovery from the warm conditions.”
Who is in it
The advisory is drawn by forecast zone. Based on the weather service’s zone maps, the Sonoma Coastal Range takes in Cazadero and Annapolis. The North Bay Interior Mountains zone takes in Angwin, Pope Valley, Mount Veeder and Sugarloaf Ridge.
Santa Rosa, Healdsburg, Cloverdale, Guerneville, Calistoga, St. Helena, Sonoma and Napa sit in the North Bay Interior Valleys zone, which isn’t under the advisory.
Weather service point forecasts pulled Thursday call for Calistoga to reach about 98 on Saturday, Healdsburg 97, Cloverdale 96, Santa Rosa 95 and Napa 94.
Inside the advisory, Pope Valley is forecast near 97, Cazadero and Mount Veeder near 96, and Angwin and Sugarloaf Ridge near 92. Annapolis is forecast near 88.
Saturday night, Sugarloaf Ridge is forecast to drop only to about 70, and Cazadero, Angwin and Mount Veeder to about 66. Santa Rosa and Healdsburg are forecast to fall to about 61, and Napa to about 63. Calistoga is forecast to fall to about 66, and Cloverdale to about 67.
How long it lasts
The advisory ends Sunday night, but the heat may not. The weather service’s forecast discussion calls for temperatures 10 to 20 degrees above normal across the interior from Saturday through Wednesday, with the heat peaking Saturday.
It said a heat advisory “will likely be for a majority of the interior,” with a chance for it “to linger into early-to-mid next week.” It also said weaker-than-normal onshore flow “will bring less relief than normal” at night.
The weather service also flagged elevated fire weather conditions across the higher terrain Thursday, with gusts of 25-35 mph and up to 45 mph on exposed peaks such as Mount St. Helena. It said overnight humidity above the marine layer could drop as low as 20-30%.
Outdoor workers
Under California’s heat illness standard, Title 8, Section 3395, employers must provide shade when it’s hotter than 80 degrees in the work area.
At 95 degrees, employers in agriculture, construction, landscaping and several other industries must follow high-heat procedures. For farmworkers, that includes a minimum 10-minute cool-down rest every two hours.
The weather service said the heat “affects anyone without effective cooling and/or adequate hydration.” Its instructions in the advisory begin: “Hydration is a critical piece to surviving the heat.”
Cooling centers
As of late Thursday morning, neither Sonoma County nor Napa County had posted a heat notice or a cooling center list on its county news page. Sonoma County posts emergency information at SoCoEmergency.org.