Current dry conditions and limited early-month rainfall, with Met Office forecasts showing mostly fine weather and low precipitation chances through mid-September, underpin trader focus on the closely matched 40-50 mm and 60-70 mm bins. London Heathrow normals indicate roughly 50 mm average September totals, but interannual variability is high due to Atlantic jet stream positioning and the frequency of slow-moving fronts or showers. Official outlooks note potential for increasing instability later in the month without strong consensus on totals, while model runs diverge on the timing and intensity of any Atlantic systems. This genuine uncertainty in remaining forecast windows supports balanced market-implied odds across central ranges rather than extremes.