SAN FRANCISCO (CBS SF) — A weather system stalled over the Southwest began sending waves of excessive heat into Northern California Wednesday, leading Cal ISO to issue a Flex Alert for Thursday starting at 5 p.m.

The Cal ISO Twitter account posted about the Flex Alert Wednesday afternoon.

The Flex Alert urges voluntary conservation between the hours of 5 p.m. and 10 p.m. Thursday to relieve stress on the grid. Residents can go to FlexAlert.org for conservation measures.

The California Independent System Operator earlier issued a heat bulletin, warning that the state-wide heat wave will challenge the power gird.

“An abnormally strong ridge of heat is forecast to bring temperatures as high as 115 degrees to the California interior that could last until the weekend. Because of the extreme heat and nighttime lows expected to cool off only between 78 and 83 degrees, the state’s electric grid will be straining to meet evening demand when air conditioners are in heavy use and solar energy generation is waning,” the agency said in a statement.

Cal ISO said its own projections show demand possibly exceeding available supply for several days this week, with the biggest deficit projected between 8 and 9 p.m. on Thursday.

Residents are urged to pre-cool their homes, use major appliances, close window coverings and charge devices and electric vehicles beforehand.

Over the next 72 hours, temperatures will top 100 degrees to as high as 107 in Bay Area communities inland from the coast. Further east, it will be even more stifling with temperatures soaring to record levels of 110 degrees or higher in the Sacramento Valley.

The National Weather Service has issued an excessive heat warning from 11 a.m. Thursday until 9 p.m. Friday for cities that include Angwin, Lagunitas-Forest Knolls, Woodacre, Concord, Antioch, Livermore, Walnut Creek, Pleasanton, Pittsburg and San Ramon.

“Temperatures will increase on Wednesday, with the hottest day of the week on Thursday,” weather service forecasters warned. “While temperatures cool slightly on Friday near the coast, they will still be hot across the interior.”

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The Bay Area Air Quality Management District has also issued a Spare the Air alert for Thursday because of excessive levels of smog expected.

The air district issues a Spare the Air day when weather conditions, like Thursday’s forecast of light winds and unusually hot temperatures, will combine with vehicle exhaust to make an excessive amount of smog. Thursday’s will be the second Spare the Air alert of the season.

On Spare the Air days, drivers are encouraged to find alternatives to driving alone to work or other destinations to limit the number of vehicles on the road. Young children, seniors and people with respiratory and heart conditions should also avoid prolonged exposure to smog, and outdoor exercise should only take place in the early morning when concentrations are lower, air district officials said.

The hot, dry weather will continue to worsen local drought conditions. The San Francisco Bay Area is already in the grips of an excessive drought and the heat wave will only suck even more moisture out of the fire-prone, tinder-dry hills and forests and evaporate precious water out of depleted reservoirs.

Health officials urged local residents to take plenty of precautions if possible including:

  • Drink plenty of fluids
  • Stay in an air-conditioned room
  • Stay out of the sun
  • Check up on elderly relatives and neighbors
  • Young children and pets should never be left unattended in vehicles
    under any circumstances.
  • When possible reschedule strenuous activities to early morning or
    evening.
  • Know the signs and symptoms of heat exhaustion and heat stroke.
  • Wear lightweight and loose fitting clothing when possible.
  • To reduce risk during outdoor work, the Occupational Safety and Health Administration recommends scheduling frequent rest breaks in shaded or air conditioned environments.