Forecast uncertainty across global numerical weather prediction ensembles drives the tight market split between 39°C and 40°C-or-higher outcomes for Jeddah’s August 18 maximum. Coastal Red Sea dynamics, including variable sea-breeze strength and humidity advection, create small but decisive differences in peak daytime heating, with recent model runs showing 38–41°C spreads depending on cloud cover and low-level wind shifts. Saudi National Center for Meteorology guidance and climatological August baselines near 37–38°C anchor trader assessments, while the narrow 1°C gap reflects genuine model divergence rather than any single forcing. Updated short-range runs over the next 48 hours will likely narrow resolution criteria tied to official station readings.