Trader consensus on Polymarket has locked in a 100% implied probability for Seattle's highest temperature on May 13 at 56-57°F, backed by official observational data from the National Weather Service at Seattle-Tacoma International Airport (KSEA), the standard station for such resolutions. This outcome stems from a sharp cool-down after early May record highs of 81°F on May 4, driven by a weakening upper-level ridge that enabled persistent onshore flow, cool Pacific air advection, and low-level marine stratus clouds capping daytime heating. Forecast models from NOAA aligned with this pattern, showing limited solar insolation and temperatures stuck in the mid-50s amid typical May climatology (average high ~64°F). Scenarios challenging this near-certainty include rare post hoc data revisions from quality control audits or disputes over measurement site compliance, though none are currently indicated.