A Wild Weather Week Ahead — Heat Peaks, Then a Cooldown Creeps In
THUNDER BAY – WEATHER – If you’re the type who likes a little bit of everything in your weather, you’re in for a treat. This week in Thunder Bay is shaping up to be a classic September roller coaster—starting with muggy mornings, a heat surge mid-week, and eventually winding down into cooler, cloudier days. You’ll want sunglasses, an umbrella, and maybe a weather app on standby.
Monday Morning: Damp, Foggy, and Full of Potential
As of 5:00 AM EDT, conditions at Thunder Bay Airport are mostly cloudy with a warm and humid 14.6°C, paired with 100% humidity—so yes, it’s a bit of a damp, glass-fogging kind of morning. The dew point is matching the temperature, signaling saturated air. Visibility is at 19 km, and a gentle east wind at 10 km/h is barely stirring the mist. The barometric pressure is 102.1 kPa and rising, indicating some improving weather through the day.
Fog patches will lift later this morning, revealing a mix of sun and cloud by the afternoon, and temperatures will rise to a high of 21°C. With the added moisture in the air, the humidex will make it feel more like 26°C. The UV index is moderate at 5, so you might still catch a sunburn between the clouds.
Tonight: Calm Skies, Fog Returns
Tonight, expect partly cloudy skies, but with the warm daytime moisture lingering, fog patches are likely to redevelop after midnight. The low will dip to 11°C, still mild enough for sleeping with the windows open—if you’re not fogged in.
Tuesday: Hello, Heatwave!
Tuesday, September 16, is shaping up to be a steamy outlier in the forecast. After morning fog burns off, skies will clear, and temperatures will soar to 28°C. Factor in the humidity, and you’re looking at a humidex of 35°C—a level usually reserved for mid-July, not mid-September.
By Tuesday night, some instability returns with cloudy periods and a 30% chance of showers, and a low of 13°C.
Midweek: Storms Brew, Then Temps Tumble
Wednesday, September 17, brings a mix of sun and cloud and another 30% chance of showers, with a slightly cooler—but still above-average—high of 25°C. Wednesday night stays cloudy with a 40% chance of showers, and a low of 14°C.
Thursday, September 18, sees a more noticeable cooldown, with cloudy skies and a 40% chance of showers, and the high dropping to a crisp 17°C. Thursday night stays mostly cloudy with a 30% chance of showers, and a much chillier low of 8°C.
What to Wear?
Start with layers today—it’s damp in the morning but will warm up quickly. Tomorrow calls for full-on summer gear: shorts, breathable fabrics, sunscreen, and maybe a wide-brim hat. Rain jackets and umbrellas make a return appearance later in the week as temperatures fall and showers become more likely.
Weather Trivia: A September Heatwave in Thunder Bay?
Yes, it happens! While rare, Thunder Bay has experienced temperatures above 28°C as late as the third week of September. Thanks to warm southern air masses sweeping up across Lake Superior, the city can catch a few surprise summer days before the fall chill locks in. The 35°C humidex expected Tuesday would be more typical of late July—so soak it in while you can!
