Trader consensus clusters around the 40–70 mm range because London's September climatological mean totals near 50 mm, with substantial year-to-year variability driven by the position and strength of Atlantic low-pressure systems. Early-month observations show changeable, occasionally windy conditions with light rain episodes, consistent with typical autumnal frontal passages. Differentiation among closely matched bins hinges on forecast uncertainty in jet-stream steering and the frequency of depressions through the remainder of the month; neutral NAO phases and modest model spread in extended-range guidance support outcomes near the historical average rather than extremes. New Met Office updates and ensemble runs later this week will likely refine probabilities as initial conditions evolve.