The managers of California's power grid warned there could be rolling black outs if customers don't conserve energy. The California Independent System Operator, which manages the power grid, warned as many as 3 million customers could lose power as heavy use of air conditioners amid the extreme heat continues to put a strain on the state's power supply.
Cal ISO announced that a "Flex Alert" -- a call for voluntary conservation -- will be in effect from 3 to 9 p.m. through Monday. Urging voluntary conservation is an effort to stave off too much strain on the state's electrical system, possibly leading to rolling power outages, like those that occurred during high heat last month.
Southern California Edison and the Los Angeles Department of Water and Power reported scores of scattered outages that left thousands of people without electricity all day Sunday. Power remained out for 45,000 DWP customers as of 6 a.m. Monday. It was unclear when power would be restored for all customers due to the number of outages.
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The extreme heat prompted the Cal ISO to declare a Stage 2 Emergency around 6 p.m. on Sunday. Cal ISO was urging residents to conserve energy to prevent the power grid from entering Stage 3, which would mean possible rotating outages. The Stage 2 Emergency declaration was lifted around 8:30 p.m. and no outages were ordered by grid operators, according to Cal ISO.
An excessive heat warning is in effect until 8 p.m. Monday for L.A. and Orange counties.
L.A. County set an all-time record temperature record, with a high of 121 degrees at Pierce College in Woodland Hills.
RELATED: Woodland Hills reaches 121 degrees -- L.A. County's highest temperature on record
"121, it almost feels definitely around that area. I've been sweating. We have so many animals at my house too, they've been dying, have to keep them inside," said Kyla Warren.
The Sunday reading was the highest temperature ever observed at an official recording station in the county, according to the National Weather Service.
Record or near-record temperatures were felt for a second straight day across the Southland Sunday as the blistering holiday weekend heat wave maintained its stranglehold,
Downtown Los Angeles reached 111 degrees, just two degrees short of the all-time record set in 2010. Other cities were also approaching all-time highs, with Van Nuys and Santa Clarita each reaching 116 and Burbank 114.
WATCH: Several SoCal cities break temp records as heat wave bakes region
Orange County wasn't being spared, with Anaheim and Fullerton both reaching or exceeding 110 degrees by late Sunday.
"This kind of heat can be life-threatening and people are urged to use common sense, keep hydrated and stay out of the heat and in air-conditioned locations as much as possible,'' the National Weather Service advised.
Cooling centers were expected to be open in both counties and listings can be found at ready.lacounty.gov/heat/ and 211oc.org/resource-centers/extreme-heat-cooling-centers.html. Officials cautioned that capacity is limited due to physical distancing requirements.
The hot weather is being attributed to a high pressure system, which is forecast to settle into the Great Basin area.
What is causing the heat wave in California?
In anticipating an increased fire danger, forecasters said they were mainly focusing on the dryness that will take hold in the region, partly because of the absence of monsoonal moisture. Forecasters said humidity levels would fall to single digits in interior areas of L.A. County, and a red-flag warning for increased wildfire danger was in effect through 10 p.m. Monday for the L.A. County mountains.
Several brush fires broke out Sunday, including a 1,800-acre wildfire burning near Cogswell Dam in the Angeles National Forest. Structures were being threatened, and drivers were urged to avoid Highway 39 so it can be used exclusively for emergency vehicles.
Smoke from that fire was seen for miles, and was contributing to unhealthy air quality in the region.
RELATED: 1,800-acre brush fire in Angeles National Forest could explode in size Monday, officials say
All residents were urged to avoid strenuous activity and stay indoors as much as possible.
Authorities advised that children, seniors and pets must never be left unattended in vehicles under any circumstances since temperatures can quickly turn lethal in the current conditions.
The heat was expected to ease by about 10-12 degrees on Monday, but still remain above 100 degrees in many areas. More significant easing was expected later in the week, with temperatures mostly in the mid-to-upper 90s in all but the warmest areas of the valleys.
The Associated Press and City News Service contributed to this report.