**Recent weather patterns and forecast model consensus are driving closely matched probabilities around the 45–51°F range for Seattle’s September 30 low temperature.** A Pacific weather system bringing showers and southwesterly flow on September 29 is expected to yield a cooler, more stable air mass by Wednesday. Official National Weather Service guidance and ensemble outputs currently favor overnight lows near 50–52°F under partly cloudy skies and light northerly winds, consistent with late-September climatology (historical average low around 50°F at Sea-Tac). Key scientific differentiators include radiational cooling potential under clearing skies versus retained heat from lingering clouds or higher dew points; wind speeds that either mix warmer air or allow surface decoupling; and the precise trajectory of the post-frontal air mass, which models show modest spread in 850 hPa temperatures and boundary-layer moisture. Trader sentiment reflects this narrow uncertainty band, with the 49% and 44% probabilities on the sub-46°F and 50–51°F bins capturing the range of plausible model outcomes ahead of final forecast updates.