Mississippi Mills, Ontario, is in for a week of mild and mostly sunny weather, but residents should be ready for a chilly start as Environment Canada has issued a frost advisory for the area.
The advisory, effective Monday, September 21, 2026, at 12:50 PM EDT, warns that conditions are favourable for frost development overnight into early Tuesday morning. Patchy frost is likely, and temperatures are expected to dip to 3°C, making it the coldest night of the week. Environment Canada advises taking preventative measures to protect cold-sensitive plants, trees, and crops.
Despite the frosty start, the rest of the week promises comfortable late-September conditions. The forecast calls for a mix of sun and cloud on Monday with a high of 15°C, followed by mostly sunny days from Tuesday through Friday. Daytime highs will climb gradually, reaching 16°C on Tuesday, 18°C on Wednesday and Thursday, and a warm 20°C by Friday. Overnight lows will range from 3°C to 7°C, so a light jacket is recommended for evening activities.
This week’s weather is fairly typical for late September in the Ottawa Valley. The average first fall frost date for the region is around September 26, so the advisory is within historical norms. According to climate data, normal daytime highs for this period are about 18°C to 21°C, meaning the upcoming week will be slightly cooler than average but still pleasant for outdoor plans.
Mississippi Mills, with a population of about 14,740 residents, sits approximately 46 kilometres southwest of downtown Ottawa. The Mississippi River runs through the municipality, and the Mississippi Valley Conservation Authority continues to monitor water levels, though no flood concerns are currently in effect.
