The tight spread of trader-implied probabilities across 70–75°F for San Francisco’s August 18 high temperature stems from the strong moderating effect of the Pacific marine layer and coastal upwelling, which typically cap daytime maxima near long-term August averages of 68–72°F. Recent model runs from the National Weather Service show consistent onshore flow and possible fog or low stratus limiting solar heating, with only modest warming possible if the inversion lifts and sea breezes weaken. Higher bins above 76°F carry low odds because historical data and current sea-surface temperatures indicate limited potential for significant heat advection without an offshore wind event, while cooler outcomes remain plausible if the marine layer persists through the afternoon. Updated forecast guidance expected tomorrow will likely refine these thresholds ahead of market resolution.