By Scott DiSavino July 3 (Reuters) - The California power grid operator passed its first heat wave test of 2013 without the San Onofre nuclear plant and another reactor at the Diablo Canyon nuclear plant without much trouble. PG&E Corp's 1,122-megawatt (MW) Unit 1 at the Diablo Canyon nuclear plant, the biggest unit in the Golden State, shut on June 27 before the heat wave started and returned on July 2 as the weather started to moderate. "California did manage to squeak by yesterday. They were fortunate to have Diablo come on line just prior to their peak load (Tuesday)," a power trader said. Generating resources in California were already below par before Diablo Canyon 1 shut after the permanent closure in June of the 2,150-MW San Onofre nuclear power plant. With the Diablo reactor down, the California ISO, which operates the power grid for much of California and parts of Nevada, issued a rare "flex alert" on Sunday for Monday and Tuesday, urging customers in the northern part of the state, primarily PG&E territory, to conserve power. PG&E is the biggest power company in California, serving about 5.1 million customers in the northern and central part of the state. Flex alerts notify the public of steps they can take to help avoid potential power shortfalls. This was the ISO's second flex alert in 2013. The grid operator issued two flex alerts in each of 2011 and 2012 and none in 2010. Other than the call for conservation on Monday and Tuesday and instructions to power companies not to conduct unnecessary maintenance on their generating facilities and transmission lines, the ISO did not have to take other steps to manage power usage during this heat wave. On Wednesday morning, the grid operator said conservation was still needed but ended the flex alert. Other big utilities in California are Southern California Edison, a unit of Edison International, which retired the San Onofre reactors, and San Diego Gas and Electric, a unit of Sempra Energy. TEMPERATURES MODERATE Temperatures in Los Angeles, the biggest city in California, reached 90 degrees F on Saturday, 94 on Sunday and 92 on Monday before returning to near normal levels in the low 80s, according to AccuWeather.com, which forecast the high in the city would reach 80 on Wednesday. But temperatures in San Jose, the biggest city in northern and central California where Diablo Canyon is located, reached 92 on Friday, 96 on Saturday, and 92 again on Monday and Tuesday, AccuWeather.com said. The mercury is expected to reach 88 on Wednesday. Prices for next-day power spiked for Tuesday delivery into the $100s per megawatt hour, which was at least a five-year high, but returned to normal summer levels in the $60s for Wednesday delivery. The ISO forecast demand for Wednesday would reach about 43,000 MW, which is well below the ISO's forecast on Tuesday for Wednesday of 46,300 MW. The grid hit peaks this week of almost 45,000 MW on Monday and was forecast to hit about 46,000 MW on Tuesday. That is still well below the ISO's all-time record of 50,270 MW, set in July 2006 before commercial and industrial usage was reduced during the economic crisis.