Official meteorological observations from the UK Met Office confirm a daily maximum temperature of 20°C across London monitoring stations on May 20, anchoring the market-implied probability at 100% for that exact outcome. This reading stems from stable high-pressure conditions and light southerly flow that limited daytime warming to near the long-term May average of 18–20°C for the region. Forecast models issued 24–48 hours earlier had converged on similar values, reducing uncertainty ahead of the observation window. While post-event quality checks at stations like Heathrow or St James’s Park could theoretically adjust the figure by a fraction of a degree, shifts large enough to alter resolution thresholds remain statistically rare.