Trader consensus around 40-60 mm for London precipitation in September reflects climatological averages of 50-55 mm driven by seasonal increases in Atlantic low-pressure systems. Key differentiators include the North Atlantic Oscillation phase, which favors wetter outcomes with a positive index and stronger jet stream, versus drier conditions under high-pressure blocking. Sea-surface temperature anomalies in the North Atlantic and evolving ENSO patterns add uncertainty, as do short-term model runs from the Met Office and ECMWF showing variable frontal activity. Historical variability means totals can easily shift across bins with updated September forecasts, keeping probabilities closely matched.