The pleasant weather Thursday morning — bright and cool, with temperatures still in the upper 50s by 7 a.m. — will not last long. The National Weather Service issued an excessive heat watch beginning Saturday and extending through Monday. The forecast calls for a "dangerous combination of high heat and humidity."Afternoon highs during the period will be in the mid to upper 90s with nighttime lows in the mid to upper 70s.Peak afternoon heat indices of 100 to 105 are expected all three days, and nighttime heat indices could remain in the 90s.According to the . . .