Click on a day for an hourly weather forecast
The temperatures in Thunder Bay in September are quite cold with temperatures between 9°C and 16°C, warm clothes are a must.
You can expect about 3 to 8 days of rain in Thunder Bay during the month of September. It’s a good idea to bring along your umbrella so that you don’t get caught in poor weather.
Our weather forecast can give you a great sense of what weather to expect in Thunder Bay in September 2025.
If you’re planning to visit Thunder Bay in the near future, we highly recommend that you review the 14 day weather forecast for Thunder Bay before you arrive.
| Month | Temperatures | Rainy Days | Dry Days | Snowy Days | Rainfall | Weather | More details |
|---|---|---|---|---|---|---|---|
| January | -10° / -16° | 5 | 4 | 23 | 51 mm | Awful | Thunder Bay in January |
| February | -9° / -16° | 4 | 5 | 20 | 51 mm | Awful | Thunder Bay in February |
| March | -2° / -8° | 9 | 5 | 19 | 75 mm | Awful | Thunder Bay in March |
| April | 4° / -2° | 7 | 12 | 12 | 100 mm | Awful | Thunder Bay in April |
| May | 12° / 4° | 7 | 23 | 2 | 96 mm | Awful | Thunder Bay in May |
| June | 17° / 9° | 6 | 24 | 0 | 98 mm | Good | Thunder Bay in June |
| July | 21° / 13° | 5 | 26 | 0 | 90 mm | Good | Thunder Bay in July |
| August | 21° / 13° | 4 | 27 | 0 | 87 mm | Good | Thunder Bay in August |
| September | 16° / 9° | 5 | 25 | 0 | 84 mm | Good | Thunder Bay in September |
| October | 8° / 3° | 6 | 20 | 5 | 74 mm | Bad | Thunder Bay in October |
| November | 1° / -4° | 6 | 10 | 14 | 78 mm | Awful | Thunder Bay in November |
| December | -6° / -11° | 5 | 6 | 21 | 69 mm | Awful | Thunder Bay in December |