Trader consensus around 66–69°F for San Francisco’s August 28 high reflects National Weather Service and model guidance favoring a persistent marine layer with onshore westerly flow at 10–15 mph, limiting afternoon warming. Typical August climatology (average high near 70°F) is offset by recent strengthening of the onshore gradient and residual low clouds, keeping peak readings in the mid- to upper-60s. The tight 36 % / 35 % split between 66–67 °F and 68–69 °F hinges on the timing and depth of any afternoon clearing; modest differences in boundary-layer mixing or wind speed could shift the daily maximum by 2–3 °F. Lower-probability bins above 70 °F would require stronger subsidence or reduced cloud cover not currently indicated in official forecasts.