**Marine layer dynamics and persistent onshore flow from the cool Pacific are the dominant factors anchoring trader consensus around 56–59°F for San Francisco’s official low on August 21, 2026.** National Weather Service point forecasts and recent soundings indicate a moderately deep marine layer (roughly 1,500–2,000 feet) that promotes overnight radiational cooling under mostly cloudy to overcast stratus, keeping minimum temperatures near the seasonal normal of ~57°F at KSFO. Onshore westerly winds and coastal upwelling maintain a tight temperature gradient between the ocean surface and land, suppressing warming and favoring the 58–59°F bracket (currently 44.5% implied probability) and the adjacent 56–57°F outcome (34%). Recent model runs and observations show the layer neither compressing sharply nor deepening dramatically, producing stable overnight lows clustered in the mid-to-upper 50s rather than the rarer sub-54°F or warmer 60°F+ scenarios. This aligns with the broader August pattern of consistent marine influence without major synoptic shifts, limiting volatility in the market-implied odds.