The National Weather Service has issued an excessive heat warning for parts of Southern California including the Santa Clarita Valley as the worst heat wave of the year continues to intensify through the weekend.

Meanwhile, the California Independent System Operator, which oversees the state’s wholesale electrical power grid, has issued a flex alert statewide that takes effect Friday and extends through Sunday, asking consumers to use less electricity, especially between 11 a.m. and 6 p.m.

The heat wave is the result of a strong high pressure system and a weak onshore flow. Humidity readings are relatively high, in the mid-teens.

The excessive heat warning is in effect now through at least Friday early evening as the National Weather Service forecasts temperatures will top out at 109 degrees on Friday, 107 Saturday and 104 Sunday, before dipping to 100 on Monday. Don’t miss a thing. Get breaking news alerts delivered right to your inbox.

“The combination of hot temperatures and high humidity will combine to create a dangerous situation in which heat illnesses are possible,” the service warns.

Drink plenty of fluids, stay in an air-conditioned room as much as possible, stay out of the sun, check up on relatives and neighbors, and never leave children, the elderly or pets in the car for any reasion or any length of time, the service urges.

To reduce the risk for people who work outdoors, the Occupational Safety and Health Administration (OSHA) recommends scheduling frequent rest breaks in shaded or air-conditioned environments. Anyone overcome by heat should be moved to a cool and shaded location. Heat stroke is an emergency; call 911.

Cooling centers supported by the City of Santa Clarita and County of Los Angeles are open at the Valencia and Canyon Country libraries through the weekend. The SCV Senior Center on Market Street in Newhall has a cooling center open Monday-Friday but not on the weekends.

To help save energy during the statewide flex alert, power grid operators ask customers to keep air conditioner thermostats set at 78 degrees or higher, turn air conditioners off when not at home, use major appliances at night, and turn off all unnecessary lights and appliances.