Will Seoul have between 75-100mm of precipitation in September?

The market resolves on whether Seoul receives between 75 and 100 mm of precipitation in September. That is a narrow band within the monthly distribution, and the current 1.8% price implies the scenario is almost impossible. Yet September in Seoul is a transition month with residual monsoon moisture and typhoon activity, so landing in the middle of the range is not a rare event.

Our 18% estimate is based on climatology: the September average in Seoul is roughly 130-150 mm, but the distribution is heavily skewed. A 75-100 mm total is about 0.6-0.8 of normal, a typical dry-but-not-parched September. Historically such values occur in 15-20% of years, which anchors our estimate. The market likely underprices this because traders anchor on extreme typhoon scenarios.

For the outcome to win, total September precipitation must fall exactly within the 75-100 mm corridor: neither a drought month nor a single powerful typhoon should push the sum outside the bounds. That requires no major tropical systems and a fairly even spread of rainfall.

The main risks are a single typhoon dumping 100+ mm in a day and knocking the outcome out of range, or unusually dry weather doing the same on the low side. An active typhoon forecast or a persistent anticyclone over Korea would invalidate the idea.