(Bloomberg) -- California is asking homes and businesses to cut back on electricity use Wednesday afternoon as heat grips the West Coast and a years-long drought squeezes the region’s hydroelectric-power supplies.

The state’s main grid operator expects power demand to soar as residents crank air conditioning to cope with temperatures exceeding 100 degrees Fahrenheit (38 Celsius). The conservation call is scheduled for 4 p.m. to 9 p.m. local time, according to a Tuesday statement from the California Independent System Operator.

“We are anticipating a high peak load today, possibly the highest so far this year,” Anne Gonzales, a CAISO spokeswoman, in an email. The grid operator “called a flex alert to give us some cushion in our supply margin.”

Read: California swelters with power demand poised to hit summer high

The worst dry spell in 1,200 years has gripped nearly every inch of California with drought this summer, leaving rivers and reservoirs perilously low. That has significant implications for a state that generates about 10% of its electricity from hydroelectric dams and has aggressively closed natural-gas power plants in recent years.

California’s grid is facing a confluence of factors Wednesday that threaten supplies. Monsoonal conditions in parts of the state could diminish solar output, according to Gonzales. Fossil fuel power plants are likelier to fail during multiple days of heat. And while California’s electric demand Wednesday is expected to be lower than Tuesday’s high, supplies from neighboring states that are also gripped by stifling heat may be limited Wednesday.

Wednesday’s high in Sacramento, California’s capital, could reach 103 degrees Fahrenheit and go even higher Thursday and Friday. Stockton, in the state’s Central Valley, is set to remain at 100 degrees or more for the rest of the week, while further Fresno could hit 106 on Friday, according to the National Weather Service.

Day-ahead power prices for Wednesday surged to an average of $129 a megawatt-hour in Northern California, the highest since September.

Heat advisories and excessive heat warnings stretch from Washington State and Idaho through Oregon and into the heart of California. Red flag fire warnings have also been posted across southern Oregon and Northern California.

(Updates with comment from California grid operator in third paragraph)

©2022 Bloomberg L.P.