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After a weekend of dramatic lightning storms, fire tornadoes, power interruptions and possibly the highest temperature recorded in Death Valley, California regulators warned of more rolling blackouts as an extreme heat wave continued to cause the most severe power shortage in the state in nearly two decades.

The state initiated blackouts both Friday and Saturday nights, and officials said more could come this week if conservation measures didn’t work.

The California Independent System Operator issued a statewide flex alert Thursday, recommending air conditioning thermostats be set to 78 degrees, unnecessary lights be turned off, electrical devices not in use be unplugged, blinds and drapes be closed, and fans used, when possible.

The ISO, which runs the electric grid for most of California, then declared a Stage 3 emergency Friday evening and ordered utilities to shed about 1,000 megawatts, prompting rolling blackouts across the state. A single megawatt powers about 750 homes, spokeswoman Anne Gonzales said.

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