Recent Atlantic low-pressure systems and a transitioning jet stream position are the main drivers behind the closely matched Polymarket bins for London September precipitation, centered near the 50 mm climatological average. Positive NAO phases favor frequent westerly fronts and higher totals, while any Azores high extension or blocking could suppress rainfall below 40 mm. Current European drought signals and a record-warm summer may modestly reduce convective activity early in the month, yet model spread on steering patterns keeps 30–60 mm outcomes competitive. Traders are weighting near-term Met Office guidance on warm, mostly dry spells against the rising autumnal likelihood of organized Atlantic depressions.