The chilly mornings that greeted the Bay Area this weekend won't last. The National Weather Service office in Monterey says a ridge of high pressure will take over by midweek, sending temperatures 10 to 15 degrees above normal from Thursday, Oct. 15, through Saturday, Oct. 17. Inland valleys are forecast to reach the 80s and 90s, and coastal spots the 70s to low 80s.

Cool through Tuesday, then a fast warmup

Sunday felt like "the first real breath of fall," forecasters wrote in their Monday morning discussion, with highs in the 60s to low 70s, about 5 to 10 degrees below normal. Monday looks similar, and overnight lows are mostly in the 40s, with a few higher-elevation spots dipping into the 30s. Tuesday warms only 2 to 4 degrees as the upper-level low drifts east.

Wednesday is the transition day, near or slightly above normal. The real heat arrives Thursday and holds into the weekend. By zone, the weather service's fire weather forecast calls for:

  • Santa Clara Valley, including San Jose: 80s to low 90s Thursday, and 80s to mid 90s Friday.
  • East Bay interior valleys: 80s Thursday, 80s to low 90s Friday.
  • North Bay interior valleys: 80s to low 90s Thursday.
  • Bay shoreline: upper 70s to mid 80s Thursday and Friday.
  • San Francisco: 70s to low 80s Thursday and Friday.
  • Santa Cruz Mountains: mid 80s to low 90s by Friday.

What it means for fire danger

The bigger concern is how dry the air gets. Forecasters expect daytime humidity to drop into the teens late in the week, and they say elevated fire weather conditions are likely as the heat builds. They also note there are no major signs of strong, gusty winds during the hot stretch, which should limit some of the risk. More pronounced offshore flow is possible later in the week as high pressure builds, the fire weather forecast says.

Even before the heat, Tuesday brings weak offshore flow that will keep higher elevations dry, with poor humidity recovery overnight. The forecast has the East Bay Hills at 20% to 30% minimum humidity Tuesday with north and northwest gusts around 30 mph, and similar gusts in the North Bay interior mountains.

"Fuels are very dry/flammable," the weather service warned, urging anyone working with fire or towing a trailer to keep sparks from spreading. It also expects moderate HeatRisk from late in the week into the weekend and recommends shade breaks, plenty of water and less time outdoors during the hottest part of the day.

King tides and surf this week too

A Coastal Flood Advisory for San Francisco, the Bay shoreline and the Monterey Bay coast runs until 3 p.m. Tuesday and could be extended into Wednesday, depending on how high Monday's and Tuesday's tides run. A separate advisory for the North Bay interior valleys runs until 4 p.m. Wednesday. A Beach Hazards Statement remains in effect through Monday evening for sneaker waves, rip currents and large breaking waves, and long-period swell is forecast to build to 16 to 18 feet by early Tuesday. Stay off jetties, piers and rocks along the coast.