Forecast models from the National Weather Service and private sources indicate Seattle's overnight low on September 21 will likely fall in the lower to mid-50s, aligning with climatological norms near 52°F and recent stable high-pressure patterns. Light northerly winds, patchy morning fog, and partly cloudy conditions limit radiative cooling or warming, keeping temperatures moderate without strong advection from Pacific systems. Trader consensus splits between the 45°F-or-below and 64°F-or-higher bins because ensemble spreads and local microclimate effects—such as urban heat islands versus Sound-influenced sites—create genuine uncertainty around the precise minimum. Updated model runs and official observations on the 21st will resolve the market as resolution hinges on the verified daily low.