Thunder Bay weather for August 17, 2026: Morning fog gives way to sunshine and a high of 25°C before a 60 percent chance of showers and thunderstorms

Thunder Bay is waking up to a cool and foggy Monday morning, but conditions will improve quickly. Sunshine takes over as the fog burns off, sending the temperature into the mid-20s this afternoon. The next weather change arrives tonight, when clouds, showers and the risk of a thunderstorm move into the city.

Tuesday will be breezier and slightly cooler behind that system, while Wednesday brings a small shower risk. Thursday currently looks like the brightest day in the midweek forecast.

Today’s Weather Overview

Current Conditions

At 7:00 AM EDT, Thunder Bay Airport was reporting shallow fog and 9.7°C. Humidity was at 100 percent, with a dew point also at 9.7°C. Barometric pressure was 101.6 kPa and rising, while winds were light from the west-southwest at 7 km/h. Visibility was reported at 32 kilometres.

The fog patches will dissipate this morning, leaving sunny skies for Thunder Bay. Winds become west at 20 km/h late this morning. The temperature climbs to a high of 25°C, with the humidex making it feel closer to 28. The UV index will reach 6, or high, so sunscreen and sun protection are worthwhile for anyone spending an extended period outdoors.

Today’s forecast high is about 2°C warmer than the normal mid-August maximum of 23°C for Thunder Bay.

Tonight — Showers and Thunderstorm Risk

Conditions change after sunset. Environment Canada forecasts mainly cloudy skies with a 60 percent chance of showers and a risk of a thunderstorm.

Southwest winds at 20 km/h will become light this evening. Fog patches may develop again overnight as the temperature drops to 15°C.

People planning evening outdoor activities should keep rain gear nearby and watch for changing conditions as the shower and thunderstorm risk increases.

Tomorrow’s Forecast

Expected Conditions — Tuesday, August 18

Tuesday brings a mix of sun and cloud after any morning fog dissipates. Northwest winds increase to 20 km/h with gusts to 40 km/h during the morning.

The high will reach 23°C, right around the seasonal normal, with a moderate UV index of 5. Tuesday night turns clear and considerably cooler, dropping to 9°C.

The stronger northwest wind will give Tuesday a fresher feel than Monday, especially near Lake Superior and in exposed locations.

Wednesday, August 19

Wednesday will feature a mix of sun and cloud with a 30 percent chance of showers.

The daytime high reaches 22°C, followed by another clear and chilly night with a low of 9°C.

For outdoor plans, Wednesday looks generally workable, although a passing shower cannot be ruled out.

Thursday, August 20

Thursday currently offers the best weather of the next few days.

Environment Canada is calling for sunny skies and a high of 24°C. Clouds return Thursday night, with an overnight low of 14°C.

Looking farther ahead, Friday becomes cloudier with a 40 percent chance of showers and a high near 23°C.

Wardrobe Recommendations

Monday starts cool enough for a light jacket, hoodie or sweater, but you’ll likely want lighter clothing by afternoon as temperatures climb to 25°C. Sunglasses, sunscreen and a hat are good additions with the UV index reaching the high category.

For Monday evening, carry a light rain jacket or umbrella because showers become increasingly possible and thunderstorms are in the forecast.

Tuesday’s gusty northwest wind will make a wind-resistant jacket useful, especially in the morning. Anyone heading out early Wednesday should also plan for temperatures around the single digits following overnight lows near 9°C.

The temperature swing between warm afternoons and cool nights is becoming increasingly noticeable as August moves toward its final two weeks.

Weather Trivia

Thunder Bay’s normal temperature for this point in August is approximately 23°C for the daytime maximum and 10°C for the overnight minimum. Monday’s forecast high of 25°C is therefore a little warmer than normal, while Tuesday and Wednesday night’s lows near 9°C are close to seasonal values.

Thunder Bay will also have plenty of daylight today: Environment Canada lists sunrise at 6:53 AM EDT and sunset at 9:08 PM EDT.

Source: Environment and Climate Change Canada observations from Thunder Bay Airport and the forecast issued at 5:30 AM EDT on Monday, August 17, 2026.