**Marine layer and onshore southwest flow are holding San Francisco temperatures near or slightly below seasonal norms on September 21, 2026.** Official National Weather Service guidance and multiple model outputs point to a daytime high in the mid-60s, with morning drizzle giving way to partial clearing under breezy southwest winds of 10–15 mph. This setup produces the market’s heaviest concentration of probability on the 66–67 °F bin, followed closely by 64–65 °F and 68–69 °F ranges. Persistent coastal stratus and the absence of strong offshore or downslope warming have kept recent daily maxima in the low-to-mid 60s, well below the 1991–2020 average high of ~71 °F for the date. With no forecast shift toward clearer skies or warmer advection expected before peak heating, traders have converged on these narrow bands. Resolution will hinge on the official maximum recorded at the primary observing site; any late-day clearing or wind shift could nudge readings into the upper 60s, while continued marine influence would favor the lower side of the distribution.