**Marine air from the Pacific and cloud cover are the dominant drivers keeping Seattle's August 21 overnight low centered in the upper 50s to near 60°F.** The region sits under the influence of the North Pacific High, which maintains steady onshore flow of cool marine air (typically 57–61°F near the surface). This caps radiational cooling under mostly cloudy skies, with forecasts from NWS and private models consistently calling for lows near 59–60°F. Light winds and increasing cloudiness overnight further limit temperature drops by trapping heat and reducing clear-sky radiation loss. Historical August averages for the date hover around 56°F, but recent summer patterns show modestly elevated baselines due to warmer ocean temperatures and urban effects. Trader consensus in the 56–61°F bins (collectively ~97% implied probability) reflects the narrow uncertainty range around official guidance, with 58–59°F edging ahead as the modal outcome. Minor shifts could occur if marine stratus thickens or thins faster than modeled, but current conditions point to limited downside risk below 56°F.