Excessive heat plagued Southern California on Sunday as triple-digit temperatures threatened parts of the region, according to the National Weather Service.

The hottest temperatures are expected to be on Sunday and Monday as a hot air mass settles into place, the weather service reported, and high pressure builds. Officials said temperatures in the triple digits will be common in the valleys, foothills and desert.

The heat wave comes as historic temperatures hit the Pacific Northwest, breaking all-time records in several cities.

The California Independent System Operator expects to issue a Flex Alert for Monday because of the high heat, according to a news release. Flex Alerts call for consumers to conserve power to keep the grid stable.

When the California ISO calls for a Flex Alert it means all Californians can jump in to help prevent power interruptions. Here’s what you can do.

Southern California beaches in the greater Los Angeles region are expected to see temperatures between 68 and 75 degrees, while inland valleys and mountains could reach 95 to 105 degrees. The Antelope Valley is forecasted to have temperatures as high as 112 degrees. An excessive heat warning is in effect until 9 p.m. Monday.

The high temperatures are expected to last into the week. A weather pattern with humidity and showers might develop between Tuesday and Thursday, according to the weather service, as monsoonal moisture moves over the area.

But forecasters are concerned about winds over southern Santa Barbara County and have issued a wind advisory.

Elevated fire conditions are expected to be in place at least through Friday, and fire-starting lightning is possible Tuesday through Friday, the weather service reported. Fast fire growth could occur if a blaze starts.