A fall heat wave peaking Wednesday and Thursday is expected to push temperatures in Southern California up to 15 degrees above normal for this time of year and potentially break heat records in Northern California. By Thursday, Southern California valleys are forecast to see highs in the 90s, with downtown Los Angeles in the low 90s, the beaches in the 80s and low deserts above 100. A heat advisory is in effect in the San Francisco Bay Area through Thursday night. … Climate change is transforming the character of the West’s hottest periods — making them more frequent, more persistent, more humid and more lethal.