**Forecast consensus from official and ensemble models points to a minimum temperature of 11–12°C in London on September 18, 2026, driving the market's heavy weighting toward those outcomes (12°C at 54%, 11°C at 28.5%).** Mid-September climatology for southern England features nocturnal radiative cooling under weakening Atlantic influence, with typical overnight lows near 11–13°C when skies are partly cloudy and winds remain moderate from the southwest. Current GFS and Met Office guidance show a clear to partly cloudy night ahead with dew points around 10°C and light-to-moderate westerly flow, limiting mixing and supporting surface temperatures dropping to the low-to-mid 50s °F (roughly 11–12°C). Ensemble spreads remain narrow, with limited upside risk from warmer advection and only modest downside if skies clear more than expected. Traders have priced the 13°C bucket modestly (16%) while assigning negligible probability to extremes below 10°C or above 13°C, consistent with the absence of strong high-pressure blocking or incoming frontal systems that could shift the range. The next model runs and any updated Met Office briefing will provide the primary near-term catalyst for price movement.