Staring at the possibility of Labor Day blackouts, managers of California’s power grid issued an extended Flex Alert for Monday afternoon and evening but warned that voluntary conservation might not be enough to keep the lights on. The Independent System Operator, which runs the grid, also issued a Stage 1 energy emergency alert Monday morning, a signal that supplies will be very tight. The grid “is forecasting an energy deficiency, with all available resources in use or forecasted to be in use,” the ISO said. A Stage 1 alert is a call to generators and transmitters to make as much...