Current National Weather Service guidance and ensemble model consensus point to an overnight low of 57–59°F for San Francisco on September 18, driven by persistent marine-layer influence, light westerly onshore flow, and partly cloudy skies that limit radiative cooling. This aligns with climatological norms for mid-September, when average lows near 56°F but are often tempered by the Pacific high and coastal stratus. Recent observations show mild conditions with minimal synoptic forcing, keeping probabilities clustered in the 56–61°F bins while leaving room for small forecast shifts from cloud timing or wind variations before the minimum is recorded.