Recent Met Office and AccuWeather guidance indicates London maximum temperatures on August 21 will likely fall in the 20–23 °C range, consistent with the market’s heaviest concentration of probability at 20–22 °C. A transition to cooler, more unsettled Atlantic air is expected to keep daytime heating modest after an unusually warm summer, with limited insolation from increased cloud cover and light-to-moderate westerly flow suppressing strong boundary-layer warming. Ensemble model spreads remain modest two days ahead, but small shifts in frontal timing or thickness could still nudge the peak by 1–2 °C. Historical August climatology (mean high near 22 °C) reinforces the current implied odds, while the absence of blocking highs or strong subsidence explains why probabilities for 24 °C or higher remain below 4 %. Updated short-range runs and surface observations on August 20 will provide the next key signal for any revision in trader positioning.