Trader consensus clusters near the 50-70 mm range for London’s September total, aligning with the 1991-2020 Heathrow climatology of roughly 50 mm amid the autumn transition. Differentiating outcomes hinge on Atlantic jet-stream latitude and the frequency of frontal lows: a persistently southerly jet favors repeated rain-bearing systems and higher totals (70-80 mm+), while high-pressure blocking or a more northerly flow suppresses activity and supports drier bins (under 40 mm). Early-month model guidance shows initially dry conditions, but the inherent spread in seasonal forecasts—driven by evolving Rossby-wave patterns and North Atlantic sea-surface temperatures—keeps probabilities tightly balanced across central categories as traders weigh historical variability against the month’s remaining uncertainty.