San Jose thermometers reached 99 on Monday, after a high of 95 on Sunday – and today will be hotter.

At 3 am today, the National Weather Service issued an Excessive Heat Warning through Thursday for the entire Bay Area, plus the Santa Cruz Mountains, Central Coast, Salinas Valley and San Francisco, as well as areas of San Bernardino County and Death Valley in Southern California.

Today’s forecast high temperature in San Jose? 103, dropping slightly to 99 on Thursday, and slipping to 93 on Friday. The weather service warned that thermometers could soar to a blistering 113 today in the Santa Cruz Mountains.

This week’s unusual and brutal heat wave in California had been expected to cool off starting today, but the forecast was extended early today until 11pm tomorrow. Daytime high temperatures won’t reach back into the 80s until Monday.

Besides climate change, which intensifies heat waves, the National Weather Service pins the blame for the high temperatures on “offshore winds, dry air, and high pressure.”

Several Southern California cities smashed records Tuesday. Already, Sacramento, Stockton and Modesto have set marks for the number of triple-digit days within a calendar year.

The sweltering heat is also lowering humidity levels, which can make wildfires more powerful, including the Line Fire that firefighters have been battling for nearly a month in San Bernardino County. It has burned more than 43,000 acres and is still only 76% contained. So far this year, about 1 million acres have been burned in California. (Visit CalMatters’ wildfire tracker for live updates on active fires.)

But wildfires aren’t the only concern when temperatures soar.

Lost wages and agricultural disruptions cost the state billions of dollars. Students in K-12 public schools, due to a lack of updated air conditioning systems, can fall ill or learn less effectively. And farmworkers, who are also vulnerable to heat illness, can lose their jobs trying to protect themselves from sweltering temperatures.

Gov. Gavin Newsom vetoed a handful of key bills that would have addressed some of these issues.

That includes ones that would have created a plan to make schools more climate resilient; made it easier for farmworkers to file for workers’ compensation for heat illness; and allowed victims of 2021’s Dixie Fire and 2022’s Mill Fire to avoid paying state taxes on their settlement money.

The weather service warned that extreme heat will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities.

It advised the public the take precautionary actions: Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun and check up on relatives and neighbors. Young children and pets should never be left unattended in vehicles under any circumstances.

Also: Take extra precautions if you work or spend time outside. 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.

San Jose Inside contributed to this report.