Persistent marine layer and moderate onshore flow from the Pacific high-pressure system are anchoring San Francisco’s August 21 high near the long-term average of 67 °F, driving trader consensus toward the 68-69 °F bin at 45 % implied probability. Official National Weather Service and AccuWeather guidance both point to a daytime peak of 68 °F under early clouds giving way to partial afternoon clearing, with west-northwest winds of 15–25 mph limiting warming. A stable weather pattern over the past week has kept daily maxima within a narrow 65–71 °F envelope, reinforcing the 66–71 °F range that now accounts for over 90 % of market probability. No heat-wave signals or model shifts have emerged in the latest runs, keeping upside outcomes below 5 % and aligning probabilities with climatological norms for late-summer coastal conditions.