**Official National Weather Service forecasts for San Francisco on August 13, 2026, call for a high near 66°F under mostly cloudy skies with moderate southwest winds of 9–14 mph and gusts to 21 mph.** This aligns closely with the market’s leading 66–67°F bin (36.5% implied probability), followed by 68–69°F (29%). Persistent marine-layer influence and onshore flow from the Pacific are the dominant drivers keeping daytime maxima in the mid-to-upper 60s rather than the seasonal August average near 68–70°F. A recent NWS-noted cooling trend, tied to strengthening southwest winds and increased cloud cover, has reinforced this outlook and explains why traders favor the lower half of the typical range. The narrow gap between the top two outcomes hinges on small differences in marine-layer depth, timing of any partial clearing, and wind-driven mixing: stronger or more persistent onshore flow and thicker low clouds suppress heating toward 66–67°F, while modest breaks in cloud cover or slightly weaker winds could allow brief warming into the upper 60s. No major forecast shifts have occurred in the past 24–48 hours, leaving the market reflecting the current NWS guidance and climatological norms for coastal San Francisco.