Seattle’s daily high on August 11 hinges on the interplay between a moderate onshore flow, marine-layer stratus, and afternoon solar heating under weak high pressure. National Weather Service model guidance shows temperatures peaking in the mid- to upper-70s, with ensemble spreads reflecting modest uncertainty in boundary-layer mixing and cloud timing. Recent coastal sea-surface temperatures near 58–60 °F have reinforced the marine influence, limiting extreme warmth, while light westerly winds and variable cloud cover determine whether the city reaches 78–79 °F or stays at 76–77 °F. Forecasters note that any earlier clearing or slightly stronger downslope warming could push readings into the low 80s, keeping the two leading bins closely contested until late-day observations are finalized.