San Francisco’s highest temperature on August 20 will be shaped by the strength of the marine layer and onshore westerly flow typical of late summer along the central California coast. Persistent low-level stratus and a compressed marine layer are limiting daytime heating, keeping afternoon highs near or slightly below the seasonal average of 67°F. Recent National Weather Service discussions note a subtle warming trend under an upper-level ridge, yet coastal locations remain capped by the shallow cool air mass and moderate west winds of 10–20 mph. Model guidance and short-term forecasts currently cluster in the mid-to-upper 60s, with minor day-to-day fluctuations possible if the marine layer thins or thickens overnight. The closely matched market probabilities around 66–69°F reflect this narrow, observationally driven uncertainty band.