Traders assign near-certainty to an 18°C high in Wellington on September 29 because official MetService forecasts and multiple model runs project clear skies and light northerly winds allowing daytime temperatures to peak at that level under stable high pressure. This aligns with observed midday readings near 14°C and the typical spring warming trajectory, placing the outcome comfortably above the September climatological average high of roughly 14°C. Model consensus shows minimal disagreement on the peak, with no significant frontal systems or increased cloud cover expected to suppress insolation before sunset. A realistic challenge would require rapid development of unexpected low cloud or a stronger onshore breeze that limits surface heating below the 18°C threshold.