Traders see nearly equal implied probabilities for Wellington’s lowest temperature on September 11 falling at 2 °C or below versus 12 °C or higher, underscoring forecast uncertainty during early Southern Hemisphere spring. The key driver is the timing and intensity of an approaching southerly change that could advect cooler maritime air from the Southern Ocean versus continued mild northerly flow. Ensemble guidance shows divergence in frontal passage across Cook Strait, where rapid wind shifts and variable cloud cover strongly influence overnight minima. Historical September records indicate typical lows near 7–9 °C, yet day-to-day variability from these synoptic features keeps the market split. Fresh model runs and local observations on the morning of the 11th should narrow the range.