SAN FRANCISCO, (Reuters) - The California Independent System Operator (ISO), which operates most of the state"s power grid, issued another "Power Watch" for Tuesday as much of California bakes under a second day of heat. The ISO forecast peak demand to reach 42,737 megawatts (MW) Tuesday afternoon. The record peak was 45,884 MW July 12, 1999. Late Monday power demand on the California grid reached 43,336 MW, prompting the ISO to issue a "Stage One" power alert. Since May 1 and the start of warming temperatures in California, the ISO has called 10 "Power Watch" days for the state, 10 Stage One and five Stage Two emergencies. In a Stage Two, when operating reserves fall below five percent, utilities interrupt the flow of electricity to large industrial customers who pay discounted rates on the understanding they may be turned off to conserve supplies. In a less serious Stage One emergency, when reserves fall under seven percent, businesses and residents are urged to voluntarily trim their use of electricity. Fresno, in California"s Central Valley, was seen peaking at 100 degrees Fahrenheit Tuesday and 96 degrees Wednesday, while Sacramento was seen reaching a high of 96 degrees Tuesday and 91 degrees Wednesday, Weather Services Corp. (WSC) said. The heat wave continued to keep temperatures high throughout the Southwest, with Phoenix, Ariz., forecast to reach a high 113 degrees Fahrenheit Tuesday and 110 degrees Wednesday, while Las Vegas, Nev., was expected to reach 109 degrees Tuesday and 106 degrees Wednesday, WSC said.