Ottawa weather in September
Long-run climate averages for Ottawa, Ontario in September, plus what the forecast models say for the days ahead.
In September, Ottawa averages 21°C by day and 11°C at night. It is the wettest month of the year here.
September in Ottawa averages a daytime high of 21°C and an overnight low of 11°C, which reads as mild for this part of the world. It is also the wettest month, so build rain into any plan. Across the whole year Ottawa sees roughly 960 mm of precipitation.
Climate averages tell you what to pack. They do not tell you what next Tuesday looks like - for that, see the live Ottawa forecast with every model compared and how much they currently agree.
Ottawa weather by month
what to expect in Ottawa through the year, 1991-2020 averagesHere is Ottawa's typical weather in each month: the average daytime high and overnight low in °C, how each month feels, and the wettest month marked. Use it to plan any trip to Ottawa by season.
Bottom line: over the last 30 days the European model has been the most accurate forecast for Ottawa, scoring 91/100 on our CheckWeather Score, ahead of the American at 89. See the full CA accuracy scoreboard for exactly how every model is tested.
Common questions
How warm is Ottawa in September?
Is September a good time to visit Ottawa?
Does it rain a lot in Ottawa in September?
Climate averages from ERA5 reanalysis. Model accuracy re-scored over a rolling 30 days. Page updated .