SAN FRANCISCO (CBS SF) — A high pressure system building off the Northern California coast will send both temperatures and the threat of wildfires soaring during the upcoming weekend, forecasters predicted.

The National Weather Service said Saturday would be hottest day with inland temperatures climbing into the 90s and triple digits. The stifling heat will be joined by winds gusting to 30 mph and humidity in the teens — a set of conditions most feared by firefighters.

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A fire weather watch has been issued for the Bay Area by the weather service for late Saturday morning through Sunday.

“The areas of greatest concern will be located in higher terrain of northern and eastern Napa and Sonoma county as well as throughout the Diablo Range,” the weather service warned. “Any fire starts may spread rapidly in these conditions.”

The weather service asked residents to “avoid activities that may lead to fire ignition, including lawn work, driving on grass, tossing cigarettes, lighting campfire or fireworks, especially during Saturday and Sunday afternoons.”

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The stagnant air mass forced the Bay Area Air Quality Management District to issue the first and second Spare the Air smog alerts of the year for Friday and Saturday.

Light winds combined with high temperatures and exhaust from motor vehicles are expected to cause unhealthy ozone accumulation in the Bay Area, according to the Air District.

“Emissions from the millions of vehicles on Bay Area roadways in combination with hot temperatures will create unhealthy air quality in our region,” Jack Broadbent, executive officer of the Air District, said in a statement.

Broadbent called for commuters to find alternatives to driving alone to help the area reduce the amount of transportation-related pollution.

Spare the Air Alerts are issued when ozone pollution is forecast to reach unhealthy levels. Ozone, or smog, can cause throat irritation, congestion, chest pain, trigger asthma, inflame the lining of the lungs and worsen bronchitis and emphysema.

Meanwhile, the weather services issued an excessive heat watch for Sacramento and the Central Valley. Temperatures as high as 108 were forecasted for over the weekend.

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“There is potential for heat-related illness to humans, pets, and livestock with long outdoor exposure,” the weather service said.