Persistent marine layer conditions, fueled by the cool California Current and a strong temperature inversion capping the boundary layer, are driving San Francisco's August 4 high toward the low-to-mid 70s and underpinning the market's leading implied probabilities for 74-75°F and 72-73°F. This setup, reinforced by onshore flow and typical summer upwelling, historically limits coastal maxima well below inland values, with National Weather Service guidance pointing to upper 60s through low 70s amid ongoing stratus. Recent model consensus shows no major offshore shift or heat surge that would push readings into the upper 70s or beyond, though variable fog clearance could allow modest afternoon warming. Traders are thus pricing modest upside from the seasonal baseline while respecting the strong climatological constraint on extreme heat.