110 degrees in Livermore, 105 in Big Sur; scorching Labor Day weekend ahead

SAN FRANCISCO -- The National Weather Service has issued an excessive heat watch for the entire Bay Area over the  Labor Day weekend, predicting temperatures soaring to as high as 112 degrees and warning local residents to include cooling activities in their holiday plans.

Just how hot will it get? The weather service issued a menu of what Mother Nature has in store.

• Livermore/Concord/Discovery Bay: 105-110 degrees
• Morgan Hill/Gilroy: ~105 degrees
• Parkfield/Pinnacles National Park: 108-112 degrees
• Napa/Santa Rosa: 99-102 degrees
• San Jose: 95-100 degrees
• Big Sur (higher terrain): 90-105 degrees
• Santa Cruz: ~90 degrees   

"While confidence in dangerously hot temperatures is greatest across the interior, coastal areas are included in the watch as just a little offshore flow could bring temperatures up significantly in these locations," the weather service said.

Forecasters warned the threat of heat-related illness will be high from Thursday through Tuesday.

"Keep in mind the upcoming heat will be a multi-day event and that heat-related illness and its impacts is a cumulative effect," the weather service said.

The state's power grid operator issued an order restricting maintenance operations from Aug. 31 through Sept. 6 to help ensure that all generators and transmission lines were online.

The California Independent System Operator did not issue a call for voluntary conservation of electricity but said it would be monitoring the weather through the weekend. Its most recent call for voluntary conservation was on Aug. 17.  

The heat wave and low humidity levels will also lead to worsening wildfire conditions in hills, forests and fields already bone-dry from months of drought.  

"After several days of strong heat, conditions will be much drier which may introduce some level of fire weather risk," the weather service said. "Drier fuels make for easier fire starts related to human activity."

It's not just Northern California that will be sweltering in the oppressive heat. Excessive heat watches went into effect Wednesday morning and will remain in effect through Sunday evening in a large swath of Southern California, including much of the normally temperate coastline, forecasters said.

Residents were warned that the conditions could produce unhealthy levels of smog.

"Elevated temperatures increase emissions of chemicals leading to ozone formation and enhance ozone formation rates," the South Coast Air Quality Management District said. "This long duration and intense heat wave will likely cause unusually high and persistent levels of ozone pollution."