Current National Weather Service forecasts and model runs position overnight lows for Seattle near 57–60°F on August 21, aligning with the market’s leading bins at 56–61°F. Marine air from Puget Sound and increasing cloud cover limit radiative cooling after a clearer preceding night, while light winds reduce mixing that could otherwise elevate minima. Historical climatology shows an average low of 56.4°F for the date, with recent years often recording 53–58°F under similar late-summer patterns. Traders appear to weigh these official guidance thresholds and the modest chance of showers against typical variability in urban heat-island effects, keeping probabilities concentrated in the narrow 56–61°F range as the day’s minimum is realized.