**Marine layer persistence and typical summer onshore flow are anchoring trader expectations for San Francisco's August 21 low near the long-term average.** Official forecasts from the National Weather Service and private sources project overnight lows in the upper 50s (around 56–57°F), consistent with the 1991–2020 climatological normal of 55.9°F for the date. The cold California Current, combined with a marine layer near 1,500 feet and steady west-northwest winds of 10–20 mph, promotes efficient radiational cooling under cloudy or foggy conditions without allowing significant inland warming. Recent August 2026 observations show similar patterns, with daily lows predominantly in the mid- to upper 50s and no anomalous heat or suppressed marine influence emerging in model guidance. This keeps probabilities concentrated on 54–59°F outcomes, reflecting the low likelihood of records (historical minimum 50°F) or warmer deviations given the entrenched stratus and onshore regime. Traders appear to weigh the narrow forecast spread and climatological reliability heavily, with minimal movement expected absent a sudden shift in the upper-level trough or wind patterns.