Persistent marine layer and onshore flow from the eastern Pacific are the dominant factors holding San Francisco’s August 23 high near normal August values of 68–71 °F, with National Weather Service guidance showing a modest 1000–1500 ft layer that limits afternoon warming. Recent model runs indicate only minor day-to-day variability under a stable ridge–trough pattern, keeping the daily maximum within a narrow 3–4 °F envelope and producing closely matched market-implied probabilities across the 68–77 °F bins. Any deeper marine layer or delayed clearing could cap the high at 67 °F or below, while stronger offshore flow or earlier sunshine would push readings into the low 70s; traders are therefore weighting the balance between these competing boundary-layer processes rather than a single decisive forecast shift.