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A high-pressure ridge that built over southwestern deserts over the past few days is responsible for the unusually blistering heat this early in the year, National Weather Service meteorologist Karleisa Rogacheski said.
"Today last day of seasonable weather in California," Rogacheski said.
California saw balmy weather on Monday, with temperatures in the upper 80s and low 90s Fahrenheit (30-35degC), but forecasts called for warming on Tuesday, spiking into the triple digits 3by Thursday and lasting several days.
In a tweet, the NWS warned temperatures could go as high as 120 Fahrenheit (49degC).
It has issued an excessive heat warning for parts of southwest Arizona, including Phoenix, on Monday, predicting "dangerously hot conditions" at last through Saturday.
"Very High Heat Risk. Increase in heat-related illnesses, including heat cramps, heat exhaustion, and heat stroke. Heat stroke can lead to death," the NWS said in the advisory.
California's dry winter left forests and brush parched, prompting worries that the heat wave could touch off wildfires.
Wildfires scorched more than 6500 square miles (17,000 square km) of land in 2020, destroying hundreds of Californian homes during a particularly fierce fire season.
The baking weather could also strain California's power grid as residents crank up air conditioning units across the state.
Experts say the heatwave forecast for this week, brought on by the early high pressure system, could not be blamed directly on climate change.
"It difficult to tie any one particular event to climate change," said Eric Schoening, a meteorologist in the Salt Lake City office of the National Weather Service.
"But studies show that as the climate changes and it gets warmer, we will see more of these anomalous events over time."