**Persistent rain, thick cloud cover, and moderate southwesterly winds are suppressing maximum temperatures in London on August 29, 2026, keeping trader-implied odds tightly clustered around 20–21 °C (combined ~77 %).** Official guidance from the Met Office and multiple model runs point to a wet, overcast day with daytime highs forecast between 18–21 °C, depending on the exact timing and intensity of showers. Heavier rain periods reduce incoming solar radiation and enhance evaporative cooling, while the southwesterly flow advects relatively mild but not warm maritime air. Model consensus shows afternoon peaks most likely near 19–20 °C at the London City Airport observing site used for resolution, with brief spells possibly reaching 21 °C if breaks in cloud occur. Historical context for late August under similar unsettled regimes supports maxima in this narrow band rather than the higher values seen during the summer’s earlier heat episodes. New model runs and updated Met Office briefings expected overnight will provide the final data points traders are watching before the market resolves.