Dry early-September conditions under persistent high pressure have driven the market's leading <30 mm outcome to 30% implied probability, with limited rainfall so far and short-term forecasts showing mostly trace amounts through mid-month. London’s long-term September average of roughly 50 mm provides context, yet model consensus points to below-normal accumulation initially due to subsidence and stable anticyclonic flow. Higher-probability buckets such as 60–70 mm (24.5%) and 40–50 mm (23.5%) reflect trader uncertainty over potential Atlantic frontal systems or low-pressure incursions later in the month that could boost totals. Key resolution factors include cumulative gauge measurements at official stations and any late-month shift in steering patterns.