* WHAT...High temperatures up to 102 degrees expected. Overnight lows between 70 and 75 degrees expected in the Sierra Foothills where the thermal belt is strongest * WHERE...San Joaquin Valley, Sierra Nevada foothills, and Coastal West Hills. * WHEN...Until 10 PM PDT Friday. * IMPACTS...Hot temperatures may cause heat illnesses. * ADDITIONAL DETAILS...Moderate to major risk of heat-related illnesses for much of the population, especially those without effective cooling or hydration.