Traders see tightly clustered probabilities across temperature bins for San Francisco’s lowest reading on August 23 because of uncertainty in overnight marine-layer depth and fog coverage. The city’s coastal setting makes minimum temperatures sensitive to the strength of the onshore flow, inversion height, and cloud cover that limits radiative cooling. Current forecast models show modest differences in the timing of any marine-layer thinning or strengthening of the pressure gradient, which can shift the low by a few degrees. Historical August minima near 55–58 °F serve as the climatological baseline, but day-to-day variability in sea-surface temperatures and boundary-layer moisture keeps implied odds distributed rather than concentrated on any single outcome.