**Historical averages place typical September precipitation in London near 50 mm** (Heathrow 1991–2020 normals), with the 40–60 mm range capturing most years and explaining why the leading market bins cluster tightly around that level. Recent summer dryness, including the driest July on record and ongoing drought conditions across much of England, combined with Met Office outlooks favoring above-average temperatures and a higher likelihood of high-pressure influence early in the month, tilts some trader sentiment toward the drier side of the distribution. However, recurring Atlantic frontal systems and showers are still expected to deliver measurable rain throughout the period, preventing extreme dryness from dominating expectations. Variability in jet-stream position, the timing and track of low-pressure systems, and the balance between settled high-pressure spells versus unsettled interludes create the spread across 30–80 mm bins. Model ensembles show no single dominant pattern, with rainfall totals sensitive to whether early-month high pressure persists or yields more frequently to Atlantic moisture. This uncertainty around the climatological mean, reinforced by real-money positioning, produces the closely matched probabilities near 40–60 mm while allowing for both drier and wetter tails.