LOS ANGELES—In a preview of the summer ahead, temperatures began spiking June 15 in the Southland, beginning a heat wave that’s expected to bring triple-digit weather to many areas through June 18, possibly including downtown Los Angeles. The forecast is raising fears of heat-related illnesses, strain on the power grid and the possibility of brush fires that could quickly spread in the hot and dry conditions. The National Weather Service (NWS) has issued excessive heat warnings that will be in effect through 9 p.m. June 16 in the Santa Monica Mountains and San Fernando, San Gabriel and Santa Clarita Valleys, followed by a less severe excessive heat watch that will last until June 18. In the Antelope Valley and Los Angeles County mountains—excluding the Santa Monica range—and in the Santa Ana Mountains and foothills in Orange County, the excessive heat warning will last until 9 p.m. June 18. The coastal …