The advisory is in effect from 11 a.m. Sunday to 9 p.m. Monday as temperatures could reach as high as the low 100s in some areas.
Weather officials said lows will mostly be in the 60s.
Prolonged exposure to high temperatures increases the risk of getting a heat-related illness.
Pets and livestock may require extra care. Also, the heat increases the risk of human-sourced wildfires, according to weather officials.
A warning is in effect until 9 p.m. Monday for the Santa Lucia Mountains, the East Bay Hills and Diablo Range, and interior Monterey County, including Pinnacles National Park and the southern Salinas Valley.
Weather officials said high temperatures of 98 to 112 degrees are expected Sunday and Monday. Lows will mostly be in the 60s and 70s but may not get below the low 80s in the hills.
Another warning goes into effect at 11 a.m. Sunday and ends at 9 p.m. Monday for the North Bay Mountains and the Santa Cruz Mountains.
Highs will reach 95 to 105 degrees both days. Lows will mostly be in the 60s in the valleys and 70s to lower 80s in the hills.
A third warning has been issued for the inland valleys of the East Bay. At 5:05 p.m. today, weather officials upgraded a heat advisory for this area to an excessive heat warning, which goes into effect at 11 a.m. Sunday and ends at 9 p.m. Monday.
Weather officials said highs of 98 to 108 degrees are expected Sunday and Monday. Lows will mostly be in the 60s.
An excessive heat warning means heat illnesses are likely because of the hot temperatures.
Weather officials suggest residents and visitors drink plenty of fluids, stay in air conditioning, stay out of the sun and check in on relatives and neighbors.
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