London's September precipitation totals show high uncertainty at the start of the month, with market-implied odds clustered tightly around the long-term average of roughly 50 mm from Met Office 1991–2020 normals. Early September 2026 begins mostly dry under variable high pressure, but traders weigh the potential for increased Atlantic fronts or jet-stream shifts that could boost totals into the 60–80 mm range versus drier persistence favoring sub-50 mm outcomes. Monthly aggregates depend on the frequency and intensity of low-pressure systems, with model skill declining sharply after 10–14 days.