Weather in Thunder Bay in September

A good month. September works. It is not the peak, but nothing about it fights you. Highs average 19.6 °C, lows 9.5 °C, and rain falls on 13 days — nothing in the month is working against you.

September ranks 3 of 12 in Thunder Bay. Only August scores clearly higher, and the difference is small enough to be settled by price rather than weather.

Rain on 13 days, 2 of them heavy. Enough to need a plan B, not enough to need a plan around it. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

Averages flatten a month, and this one hides a few things: it never really gets hot: not one day in an average September reaches 30 °C. Across the twenty years measured, the hottest September day reached 32.5 °C and the coldest night fell to -0.1 °C.

September is statistically steady but not small: the year-to-year variation is modest in relative terms, yet the wettest September on record still delivered 128 mm against 15 mm in the driest. In a month this wet, even a tame spread is a lot of rain in absolute terms — the 70 mm average is a reasonable planning figure, not a promise.

Wind is light at 2.4 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 78%, high enough to notice on the warm days without dominating them.

How September sits in the year

The month is on the way down: 4.8 °C cooler than August, with October dropping another 8.8 °C. The first week and the last are not the same trip.

Daylight in September

12.7 hours of daylight, losing 1.7 hours across the month. That is between Thunder Bay's extremes of 16.1 and 8.4 hours, so days are long enough not to constrain the plan.

An inland climate

Thunder Bay has no sea to moderate it, and it shows: 42 °C separates the hottest afternoon of the year from the coldest night. That is a genuinely continental range, and it means summer and winter here are barely the same place.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Thunder Bay versus nearby

If Thunder Bay is one option among several, the nearby comparison is worth making. Minneapolis is close enough to be a coin toss — 3.5 °C warmer, 3 days less rain. Milwaukee is close enough to be a coin toss — 4 °C warmer, 3 days less rain.

So: September is effectively as good as it gets here — 4 points behind August, which is inside the margin where the weather stops being the deciding factor and price starts being it.

Every month in Thunder Bay

See all twelve months in Thunder Bay, or where else to go in September.