August 30, 2026: Thunder Bay Three-Day Forecast — Cool Sunday, Stormy Monday, Warmer Tuesday
THUNDER BAY – WEATHER – Sunday, August 30, 2026 – Thunder Bay starts the final Sunday of August under mostly cloudy skies, with a cool east wind developing and a daytime high near 20°C. The weather turns more unsettled tonight and Monday as showers and thunderstorms move through the area.
Tuesday brings a significant improvement, with a mix of sun and cloud and a high of 26°C.
On Western Lake Superior, a Strong Wind Warning is in effect. Winds increase from light this morning to southeast 20 knots Monday morning, with waves building as high as 1 to 1.5 metres before easing Monday evening.
Today’s Weather Overview
Current Conditions
At 7:00 AM EDT, Thunder Bay Airport was reporting mostly cloudy skies and 16.2°C.
Humidity was 93 percent, with a dew point of 15.1°C. Barometric pressure was 101.5 kPa and falling. Winds were calm, and visibility was 13 kilometres.
Sunday, August 30
Sunday remains cloudy, with morning fog patches dissipating.
East winds develop at 20 km/h late this morning, while the high reaches 20°C. The humidex will make it feel closer to 25, and the UV index is 5, or moderate.
Sunday Night
Tonight remains cloudy, with a 30 percent chance of showers late this evening and after midnight.
More persistent showers begin after midnight, with a risk of thunderstorms overnight.
East winds continue at 20 km/h, with an overnight low of 15°C.
Monday, August 31 — Showers and Thunderstorms
Monday will be the most unsettled day of the three-day forecast.
Showers continue through the morning before ending early in the afternoon. Thunderstorms are possible during the morning and early afternoon.
After the steadier rain ends, skies remain cloudy with a 40 percent chance of additional showers.
The high reaches 23°C, but the humidex makes it feel closer to 32. The UV index remains moderate at 5.
Monday night stays cloudy with a low of 14°C.
Tuesday, September 1 — Warmer and Brighter
Tuesday brings a major improvement.
Expect a mix of sun and cloud with a high of 26°C.
Tuesday night brings cloudy periods and an overnight low of 14°C.
For outdoor plans, Tuesday currently stands out as the best city-weather day of the three-day period.
Lake Superior West Marine Forecast
Strong Wind Warning in Effect
Environment Canada has issued a Strong Wind Warning for Western Lake Superior.
The warning, updated at 6:40 AM EDT Sunday, means winds of 20 to 33 knots are occurring or expected in the marine area.
Sunday Marine Conditions
Winds are light early Sunday, becoming east 10 knots this morning.
They increase to southeast 15 knots this evening.
For reference:
- 10 knots is about 19 km/h
- 15 knots is about 28 km/h
- 20 knots is about 37 km/h
Waves
Waves remain 0.5 metres or less today, then build to 0.5 to 1 metre near midnight.
Weather and Visibility
Showers develop overnight and continue Monday, with a risk of thunderstorms.
Environment Canada is also forecasting fog patches, which could reduce visibility over parts of Western Lake Superior.
Monday Marine Conditions
The roughest period of the three-day marine outlook arrives Monday morning.
Southeast winds increase to 20 knots Monday morning, which is about 37 km/h.
Waves build to 1 to 1.5 metres near noon Monday.
Winds then diminish sharply to southwest 10 knots Monday afternoon.
Waves also subside to 0.5 metres or less Monday evening.
For smaller boats, paddlers and anglers, Monday morning will be the least favourable marine period because of the stronger southeast winds, larger waves, showers, thunderstorms and possible fog.
Tuesday Marine Conditions
Tuesday looks considerably quieter.
Environment Canada’s extended forecast calls for light winds early Tuesday, becoming southwest 15 knots during the afternoon.
That gives mariners a potentially favourable window Tuesday morning before winds increase later in the day.
The current extended forecast does not yet provide a specific Tuesday wave height, so anyone planning to head onto the lake should check the updated marine bulletin before departure.
Boating Outlook
Sunday morning and early afternoon offer the calmest wind and wave conditions before southeast winds begin strengthening.
Conditions deteriorate Sunday night into Monday morning as waves build and thunderstorms become possible.
Monday morning should be treated as the most challenging boating period, particularly for small recreational craft.
By Monday evening, conditions improve substantially as winds and waves subside.
Tuesday morning could provide another useful boating window before southwest winds increase during the afternoon.
Wardrobe Recommendations
Sunday calls for layers, particularly near Lake Superior where east winds will make the 20°C high feel cooler.
A light jacket or windbreaker will be useful, and rain protection should be kept handy for Sunday night.
Monday will feel warmer and more humid despite the rain, with a humidex near 32. Lightweight clothing combined with a waterproof shell will work well.
Tuesday returns to warmer summer conditions, making lighter clothing appropriate during the afternoon.
Anyone heading onto Lake Superior should carry an extra wind-resistant and water-resistant layer, regardless of the warmer land temperatures.
Weather Trivia
Thunder Bay’s normal temperature for August 30 is about 21°C for the daytime high and 8°C overnight.
Sunday’s forecast high of 20°C is very close to normal, but Sunday’s forecast low of 15°C is considerably milder than the seasonal average.
Tuesday’s expected 26°C will be roughly five degrees above the late-August normal, showing how quickly temperatures can rebound after a cool, cloudy day along Lake Superior.
Source: Environment and Climate Change Canada Thunder Bay public forecast issued 5:30 AM EDT Sunday, August 30, 2026, and Western Lake Superior marine forecast issued 3:00 AM EDT, with the Strong Wind Warning updated at 6:40 AM EDT.
Summary
Thunder Bay weather for August 30 to September 1, 2026: Cloudy and cool Sunday, showers and thunderstorms Monday, then warmer weather Tuesday. Western Lake Superior is under a Strong Wind Warning with waves building to 1.5 metres Monday.
