**Forecast models indicate Toronto's overnight low on September 4 will likely fall in the 15–19°C range, driven by a post-frontal air mass with northwesterly flow and mostly clear to partly cloudy skies that allow efficient radiational cooling after daytime highs near 24–26°C.** Environment Canada projects a 15°C minimum under cloudy periods, while other guidance (including hourly runs) clusters near 17–19°C, consistent with the September climatological average low of ~15.7°C. Light winds and dew points in the mid-teens limit further drops, but small differences in cloud cover timing or boundary-layer mixing can shift the exact minimum by 2–4°C. This genuine model spread around the 15–18°C resolution thresholds explains the closely matched market-implied odds near the 13°C and 23°C extremes, with traders assigning roughly equal weight to cooler or milder outcomes pending final observational data.