Thunder Bay weather for August 31, 2026: Showers, 10 to 15 mm of rain and thunderstorms are expected before clearing tonight

THUNDER BAY – WEATHER – Monday, August 31, 2026 – Thunder Bay closes out August with a wet and unsettled Monday morning. Showers, mist and a risk of thunderstorms will dominate the first half of the day before steadier rain ends this afternoon. Conditions improve considerably tonight and Tuesday.

This morning while it feels warm, there is a mist in the air, and the humidity can be felt.

On Western Lake Superior, Environment Canada has a Strong Wind Warning in effect. Southeast winds are forecast to reach 20 knots late this morning, while offshore waves remain in the 1 to 1.5 metre range. Showers, thunderstorms and fog also create additional concerns for mariners.

Today’s Weather Overview

Current Conditions

At 7:00 AM EDT, Thunder Bay Airport was reporting mist and 16.7°C.

Humidity was at 100 percent, with a dew point of 16.7°C. Barometric pressure was 101.2 kPa and falling.

Winds were from the east-southeast at 15 km/h, gusting to 31 km/h, while visibility was reduced to 5 kilometres.

The reduced visibility and wet roads will make the Monday morning commute less pleasant, particularly where heavier showers move through.

Monday, August 31

Environment Canada is forecasting showers ending this afternoon, followed by cloudy skies and a 40 percent chance of additional showers.

There is a risk of thunderstorms this morning and early this afternoon. Rainfall totals of 10 to 15 millimetres are expected.

East winds near 20 km/h become light late this morning.

The daytime high reaches 22°C, but the humidex will make it feel closer to 31. The UV index is 4, or moderate.

Environment Canada’s hourly forecast keeps the probability of precipitation near 80 percent through early afternoon, before dropping to 40 percent during the mid-to-late afternoon. Conditions improve more noticeably toward the supper hour.

Monday Night

Skies clear this evening, with the overnight temperature falling to 12°C.

That clearing will provide a much quieter end to August after Monday’s humid and stormy start.

Tuesday, September 1

September begins with a significant improvement.

Tuesday will be sunny before becoming a mix of sun and cloud during the afternoon.

West winds strengthen to 30 km/h gusting to 50 km/h late in the morning, so while conditions will be dry and warm, it will also become noticeably breezy.

The high reaches 26°C, with a humidex of 31 and a high UV index of 6.

Tuesday night will be clear with a low of 12°C.

Wednesday, September 2

Wednesday remains relatively quiet.

Environment Canada is forecasting a mix of sun and cloud and a high of 22°C, followed by clear skies Wednesday night and a low near 12°C.

That makes Tuesday and Wednesday considerably better for outdoor work and recreation than Monday.

Lake Superior West Marine Conditions

Strong Wind Warning in Effect

Environment Canada’s Strong Wind Warning remains in effect for Western Lake Superior.

The warning applies when winds of 20 to 33 knots are occurring or expected within the marine area. The latest marine forecast was issued at 3:00 AM EDT Monday.

Monday Winds

Winds begin from the southeast at 10 knots, increasing to 15 knots early this morning and then to 20 knots late this morning.

Twenty knots is approximately 37 km/h.

The wind then drops fairly quickly to south 10 knots this afternoon, before turning southwest at 10 knots near midnight.

For small recreational craft, the late-morning period will be the most challenging portion of Monday’s wind forecast.

Waves

Environment Canada is forecasting offshore waves of 1 to 1.5 metres today, tonight and Tuesday on Western Lake Superior.

The wave forecast refers to offshore wave heights measured from trough to crest. Nearshore conditions can differ substantially because islands, bays, headlands and shoreline orientation can either shelter or amplify waves.

Showers, Thunderstorms and Fog

Showers are expected to end this evening, but there remains a risk of thunderstorms.

Fog patches are also forecast over portions of Western Lake Superior and may persist until near noon Tuesday.

That combination of fog, thunderstorms and stronger southeast winds makes Monday a day for extra caution on the lake.

Thunderstorms can produce sudden wind shifts and locally higher waves that are not reflected by the broader marine forecast.

Tuesday Marine Outlook

Southwest winds of 10 knots early Tuesday increase to southwest 15 knots near noon, before diminishing to west 10 knots Tuesday evening.

Fog patches should dissipate around noon.

Offshore waves remain forecast in the 1 to 1.5 metre range.

Tuesday therefore offers better visibility and generally better weather than Monday, but it will not necessarily be a flat-water day.

Wednesday Marine Outlook

The extended forecast calls for light winds Wednesday on Western Lake Superior. Light winds are also forecast Thursday and Friday.

If that outlook holds, Wednesday should provide the best overall marine-weather window after Monday’s strong winds and thunderstorms.

Boating Outlook

Monday morning into midday is the least favourable boating period. Southeast winds rise to 20 knots, offshore waves are 1 to 1.5 metres, thunderstorms are possible and visibility may be reduced by fog.

Conditions improve Monday afternoon as winds diminish to 10 knots, but waves will take longer to respond.

Tuesday is better, although southwest winds increase again to 15 knots around noon.

Wednesday currently looks most favourable, with light winds forecast.

Mariners should check the newest Environment Canada marine forecast and warning status immediately before leaving shore, particularly if travelling beyond protected waters around Thunder Bay Harbour.

Wardrobe Recommendations

Monday calls for rain protection and breathable layers. Despite the rain, the humidex reaches 31, so a heavy jacket may become uncomfortable.

Waterproof footwear will also be useful during the morning.

Tuesday shifts back toward summer clothing with a high of 26°C, but a wind-resistant light layer may be useful as west winds gust to 50 km/h.

Anyone heading onto Lake Superior should carry warmer, wind-resistant clothing regardless of the temperature in the city.

Weather Trivia

Thunder Bay’s normal temperature for August 31 is approximately 20°C for the daytime high and 8°C overnight. Monday’s forecast high of 22°C is slightly above normal, while Tuesday’s 26°C will be around six degrees warmer than the seasonal maximum.

Sunrise Monday is at 7:13 AM EDT, while sunset is at 8:41 PM EDT.

The rapid shortening of daylight becomes increasingly noticeable at the end of August, even when afternoon temperatures still feel like midsummer.

Sources: Environment and Climate Change Canada Thunder Bay forecast issued 5:30 AM EDT Monday, August 31, 2026, and Western Lake Superior marine and wave forecasts issued 3:00 AM EDT Monday.

Summary

Thunder Bay weather for August 31, 2026: Showers, 10 to 15 mm of rain and thunderstorms are expected before clearing tonight. Western Lake Superior is under a Strong Wind Warning with southeast winds to 20 knots and waves of 1 to 1.5 metres.