CAISO issued a grid warning Thursday after generators tripped offline during a record-breaking heat wave across the Southwest and much of California. 

“CAISO is forecasting a resources deficiency with all available resources in use or forecasted to be in use for the specified time period,” the ISO said in its warning notice. 

The warning was for 7 to 9 p.m., after solar dropped offline but demand from air conditioning remained high.

COO Mark Rothleder said “a couple of resources” totaling about 1,100 MW experienced forced outages earlier in the day but that 600 MW of resources unexpectedly came online, for a net loss of 500 MW.

The warning meant the ISO might have had to dip into its planning reserves, which the California Public Utilities Commission increased from 15% to 17.5% this year in anticipation of strained grid conditions. It allowed the grid operator to activate demand response programs to lower consumption and to call on neighboring balancing authority areas, including the Los Angeles Department of Water and Power, for emergency assistance with energy supply, Rothleder said in a call with reporters.

“At this point everything is looking good with those additional tools … and so we are expecting that we will be able to make it through the evening peak and net-peak hours without having to resort to further emergency efforts or any kind of power outages,” he said.

That’s assuming nothing changes to alter supply and demand, he added. 

The next step after a warning is for the ISO to declare a Stage 1 energy emergency. Neither step has been taken since last September’s energy emergencies under similar weather and system conditions.

CAISO forecast peak demand at more than 42 GW on Thursday. It said it had more than 52 GW of available capacity.

The additional capacity would help, CAISO said in its system conditions bulletin for Thursday. The current heat wave is more limited than those in August and September, which also affected the Pacific Northwest. Temperatures in Seattle and Portland, Ore., have remained relatively moderate this week.

The early-season heat wave arrived sooner than expected.

After rolling blackouts last August and close calls in September, CAISO and the CPUC spent much of the past year instituting measures to avoid shortfalls this summer, including adopting market rule changes. (See CAISO Summer Measures Get FERC Approval.)

Hundreds of megawatts of additional battery storage are slated to come online this summer, but much of it has not been connected yet.

To prepare for the current heat wave, the ISO restricted grid maintenance this week and issued flex alerts calling for customers to conserve energy.

Temperatures hit 117 degrees Fahrenheit in Phoenix and 120 in Palm Springs, Calif., on Thursday, breaking records for the date, the National Weather Service said. Other cities that set triple-digit temperature records this week included Tucson, Ariz., Billings, Mont., and Albuquerque, N.M. 

“The hottest day of the week is expected today, with excessive heat warnings and heat advisories in effect throughout the state, and record-breaking temperatures forecast in parts of the state and the Southwest U.S.,” CAISO said in its bulletin. “While the power grid operator is not anticipating rotating power outages, it has issued a Flex Alert for 5 to 10 p.m. today, Thursday, June 17, due to high heat increasing stress on the grid in the late afternoon.”