Recent National Weather Service guidance and ensemble models indicate a highest temperature for San Francisco on August 21 most likely in the 68–71°F range, driven by a modestly compressed marine layer and moderate onshore westerly flow of 10–20 mph that limits afternoon warming along the coast. This setup aligns with climatological norms near the 67°F August average at official stations, where stronger sea breezes and low stratus keep readings below inland peaks. Traders assign the highest implied probabilities to 68–71°F brackets because short-range model runs show limited warming potential without a pronounced offshore shift, while slight forecast variability in marine layer depth sustains the tight spread between adjacent outcomes. Updated NWS briefings and next model cycle will refine the exact maximum.