Marine layer depth and onshore flow from the Pacific represent the dominant controls on San Francisco’s August 21 maximum temperature, with National Weather Service guidance showing a persistent stratus deck that typically limits afternoon warming to the upper 60s. Subtle differences between the 68–69°F and 70–71°F brackets hinge on the precise timing of any marine-layer thinning or burn-off, wind speeds through the Golden Gate, and modest warming aloft—all factors that current models resolve with only modest consensus. Long-term August averages near 67–68°F anchor trader sentiment, while the narrow spread across leading outcomes reflects genuine forecast uncertainty rather than any recent anomalous heat or cold-air outbreak. Updated model runs and morning soundings will provide the next clearest signals for resolution.