September 7-9, 2026: Thunder Bay Forecast — Showers and Thunderstorms as Strong Winds Build on Western Lake Superior
THUNDER BAY – WEATHER – Monday, September 7, 2026 – Thunder Bay is heading into a damp and unsettled three-day stretch, with showers Monday, heavier rain and a thunderstorm risk Tuesday, followed by somewhat brighter but still showery conditions Wednesday.
For anyone heading onto Lake Superior, the marine forecast deserves particular attention. Environment Canada has a Strong Wind Warning in effect for Western Lake Superior, with southeast winds strengthening to 20 knots tonight and southwest-to-west winds reaching as high as 25 knots Wednesday. Waves build to 1 to 1.5 metres overnight.
Today’s Weather Overview
Current Conditions
At 7:00 a.m. EDT Monday, Thunder Bay Airport was reporting mostly cloudy skies and 13.1°C.
Humidity is 100 percent, with the dew point also at 13.1°C. Barometric pressure was 102.0 kPa and falling.
Winds were calm and visibility was a good 24 kilometres.
Monday, September 7
Thunder Bay will remain cloudy, with a few showers beginning this morning and ending during the afternoon.
A 40 percent chance of showers continues late this afternoon.
The high reaches 22°C, with a humidex of 27. The UV index is 3, or moderate.
Monday Night
Tonight stays cloudy with a 40 percent chance of showers this evening and after midnight.
More persistent showers begin overnight, with a risk of a thunderstorm.
East winds near 20 km/h become light after midnight.
The overnight low is 16°C.
Tuesday, September 8 — Wettest Day of the Forecast
Tuesday will be the most unsettled of the three days.
Showers continue through much of the day, ending late in the afternoon before skies remain cloudy.
There is a risk of thunderstorms, and Environment Canada is forecasting 20 to 30 millimetres of rainfall.
South winds develop near 20 km/h during the morning.
The high reaches 22°C, feeling closer to 29 with the humidex.
Tuesday night brings cloudy periods with a 30 percent chance of showers and a low of 13°C.
Wednesday, September 9
Wednesday brings partial improvement.
Expect a mix of sun and cloud with a 60 percent chance of showers.
The high reaches 21°C.
Wednesday night becomes considerably cooler, with cloudy periods, a 40 percent shower chance and a low of just 8°C.
Western Lake Superior Marine Forecast
Strong Wind Warning in Effect
Environment Canada has issued a Strong Wind Warning for Western Lake Superior.
Strong winds in the marine forecast category are winds of 20 to 33 knots, and those conditions are expected during this forecast period.
Monday Marine Conditions
Winds begin from the southeast at 10 knots, increasing to northeast 15 knots early this morning.
They veer to east 15 knots near noon, then turn southeast at 15 knots early this evening.
By around midnight, southeast winds increase to 20 knots.
For reference:
- 10 knots ≈ 19 km/h
- 15 knots ≈ 28 km/h
- 20 knots ≈ 37 km/h
- 25 knots ≈ 46 km/h
Waves
Waves start at 0.5 metres or less, building to 0.5 to 1 metre this morning.
They increase further to 1 to 1.5 metres overnight.
That will make conditions increasingly uncomfortable for smaller recreational craft as the day progresses.
Showers and Thunderstorms
Scattered showers begin Monday morning over Western Lake Superior.
There is also a risk of thunderstorms today, creating the potential for sudden changes in wind and visibility.
Tuesday Marine Conditions
Southeast winds around 20 knots turn south 20 knots Tuesday morning.
They then veer to southwest 20 knots Tuesday evening.
Thunderstorms are again possible Tuesday afternoon and evening.
With waves already building to 1 to 1.5 metres overnight and 20-knot winds continuing, Tuesday is not an ideal small-craft day, especially on exposed sections of the lake.
Wednesday Marine Outlook
Wednesday remains windy.
Environment Canada’s extended marine forecast calls for southwest winds at 20 knots, veering to west 25 knots during the afternoon.
West winds at 25 knots are approximately 46 km/h, enough to generate rough conditions on exposed waters.
Boaters should check the updated wave forecast Wednesday morning before departure.
Boating Outlook
Monday: Conditions deteriorate as southeast winds strengthen. Waves build to 1 to 1.5 metres overnight.
Tuesday: Strong winds continue around 20 knots, with scattered showers and a thunderstorm risk. Small-craft conditions remain unfavourable.
Wednesday: Winds strengthen further to west 25 knots during the afternoon. This may be the roughest wind period of the three-day outlook.
Anyone operating a small boat should take the Strong Wind Warning seriously and monitor the latest Environment Canada marine forecast and Canadian Coast Guard weather broadcasts.
Travel and Outdoor Outlook
Monday remains workable for outdoor plans between showers, although roads and trails will be damp.
Tuesday is the least favourable outdoor day with 20 to 30 mm of rain possible and thunderstorms in the forecast.
Wednesday offers some brighter breaks, but showers remain possible.
Drivers should be prepared for reduced visibility during heavier Tuesday rainfall, especially on Highways 11/17, 61, 102 and 130.
Wardrobe Recommendations
Monday calls for a light waterproof jacket and shoes that can handle wet pavement.
Tuesday requires more substantial rain gear. An umbrella may be useful in the city, although a hooded waterproof jacket is often more practical in wind.
Wednesday remains mild during the afternoon, but the overnight low drops sharply to 8°C, so a sweater or light fall jacket will be useful by evening.
For anyone heading onto Lake Superior, a warm, wind-resistant and waterproof layer is essential.
Weather Trivia
Thunder Bay’s normal temperature for early September is approximately 19°C during the day and 6°C overnight.
Monday and Tuesday’s highs of 22°C remain above normal despite the rain, while Wednesday night’s low of 8°C provides a much more typical early-autumn feel.
The changing marine conditions also show Lake Superior’s influence on local weather. A relatively modest wind increase on land can translate into significantly rougher conditions once that wind has room to build waves across the open lake.
Sources: Environment and Climate Change Canada Thunder Bay forecast issued 6:13 a.m. EDT Monday, September 7, 2026, and Western Lake Superior marine forecast issued 3:00 a.m. EDT Monday.
Summary
Thunder Bay weather for September 7-9, 2026: Showers Monday lead to 20-30 mm of rain and thunderstorms Tuesday before partial improvement Wednesday. A Strong Wind Warning is in effect for Western Lake Superior, with waves building to 1.5 metres and winds reaching 25 knots Wednesday.
