**Cooler air mass behind an early-September cold front is the dominant driver, with official National Weather Service forecasts and multiple models converging on overnight lows in the low-to-mid 50s for Seattle on September 3.** An upper-level low and associated front are pushing through the region on September 2–3, replacing warmer late-summer air with a cooler, unstable maritime flow. This produces partly sunny skies with morning showers and a slight afternoon thunderstorm risk, limiting radiational cooling but keeping daytime highs in the mid-60s. The resulting minimum temperature is expected in the 52–55 °F range, consistent with the climatological average low of 55 °F and recent model runs showing 52–53 °F or 54–55 °F as the most probable bins. Trader consensus (market-implied odds clustered tightly around 50–55 °F) reflects this narrow forecast spread, with modest probability on 50–51 °F if clearing allows stronger nighttime cooling or on 56–57 °F if marine stratus or residual warmth lingers. Key uncertainties include exact timing of any clearing and localized urban heat-island effects, but current NWS guidance and ensemble consensus favor the low-to-mid 50s outcome.