Recent National Weather Service and ensemble guidance place San Francisco’s August 21 high near 68–70 °F under a compressed marine layer and persistent westerly onshore flow that limits afternoon solar heating. This classic late-summer coastal regime, reinforced by cool sea-surface temperatures and morning stratus, keeps readings tightly clustered around the climatological normal of 67–70 °F. Traders therefore assign nearly identical probabilities to the 68–69 °F and 70–71 °F bins, reflecting modest model spread and the low likelihood of either strong offshore winds or deeper marine intrusion that would push the official maximum outside this narrow window.