Marine layer depth and onshore flow patterns primarily drove the tight clustering of trader probabilities around 54–59°F for San Francisco’s August 28 low. Persistent marine stratus and light westerly winds limited nocturnal radiational cooling near the coast, while variable fog and haze noted in National Weather Service observations further moderated temperature drops. These conditions aligned with above-normal minimums near the 56°F climatological baseline, producing the observed 59°F reading at the official airport station. Subtle differences in model guidance on marine layer thickness and wind speeds created the narrow spreads separating the leading 56–57°F, 58–59°F, and 54–55°F outcomes, as traders weighed how minor shifts in these factors would affect the final hourly low before resolution.