London's September precipitation averages around 50 mm historically, with substantial interannual variability driven by the frequency and intensity of Atlantic frontal systems. As the month opens in 2026, ensemble forecasts indicate recurring but moderate rain episodes separated by drier intervals, alongside above-average temperatures that may suppress some convective activity. Long-range guidance from the Met Office points to rainfall totals more likely below than above average overall, though model spread remains wide due to uncertain high-pressure positioning and steering patterns. This scientific uncertainty—typical for monthly precipitation outlooks—explains the tightly clustered market-implied odds across the 30–70 mm range, as traders weigh climatological norms against evolving signals for Atlantic moisture transport.