Official National Weather Service forecasts for San Francisco project an overnight low in the mid-50s on September 26, shaped by a persistent marine layer, partly to mostly cloudy skies, and light west-southwest winds that limit radiative cooling. Ensemble model spreads introduce modest uncertainty in exact minimum timing and cloud clearance, differentiating the closely matched 54–57°F bins that dominate market-implied odds. Historical September 26 averages near 55.6°F and recent stable conditions without significant frontal passages further anchor trader consensus around these thresholds ahead of final observational data.