Recent ensemble forecasts from Météo-France and ECMWF models place Paris’s maximum temperature on May 18 in a tight 15–16 °C range, driving the near-even market split between those two outcomes. A cool Atlantic air mass combined with lingering morning cloud cover is expected to limit daytime warming, while light winds reduce the urban heat-island boost that typically adds 1–2 °C in central Paris. Historical climatology shows a May mean high near 18 °C, so the current pattern represents a modest negative anomaly. Small differences in the timing of any afternoon clearing or isolated showers create the narrow uncertainty band reflected in trader positioning, with new model runs and official guidance due overnight likely to resolve the final 1 °C spread.