The closely matched market odds favoring 20°C or 21°C stem from lingering uncertainty in official guidance for Toronto's maximum temperature tomorrow amid persistent cloud cover and a 40% chance of morning showers. Environment Canada forecasts a high of exactly 20°C under northeasterly flow, consistent with GFS and ensemble outputs showing limited daytime warming due to high humidity and reduced insolation. Minor model disagreements on cloud timing or boundary-layer mixing could allow a brief push to 21°C, while historical September climatology and current pressure patterns support values near this threshold. Updated short-range runs and real-time observations ahead of peak heating will likely refine the outcome before market resolution.