San Francisco’s coastal marine layer and prevailing onshore flow from the Pacific are the dominant factors capping daytime highs near 67–70°F on July 9, 2026, which explains why the 68–69°F and 70–71°F brackets together command over half the market-implied probability. National Weather Service guidance and ensemble model runs show modest warming under a building thermal trough, with any inland penetration of warmer air or delayed marine-layer clearance able to push the official KSFO high into the low 70s, while deeper fog or stronger northwest winds would favor the upper 60s. Historical July climatology at the airport places the mean daily maximum at 67–68°F with a standard deviation of roughly 3–4°F, so small shifts in boundary-layer moisture or wind direction create the tight clustering traders observe. Updated NWS and NOAA model runs issued overnight will be the next key inputs likely to move the closely matched leading contracts.