The narrow gap between the 64-65°F (34.5%) and 66-67°F (33.0%) brackets reflects ensemble model runs from the National Weather Service placing the official KSFO high near 65°F on July 18, with modest uncertainty in marine-layer depth and afternoon clearing timing. Persistent onshore flow from the cool California Current sustains a shallow marine inversion that caps daytime heating, while any temporary weakening of the layer or brief offshore wind component could allow modest warming into the upper 60s. Historical July climatology at the airport shows average highs near 66°F, and recent NWS guidance through mid-July has favored similar mild conditions absent a strong thermal trough or wind shift.