September Slows Down with Subtle Shifts and a Touch of Rain
THUNDER BAY – WEATHER – It’s a mostly cloudy start in Thunder Bay this morning, and while autumn hasn’t arrived with a dramatic bang, it’s definitely gently tapping on the window. With cooler highs, calm winds, and the occasional mention of showers, the weather over the next few days is classic mid-September—layer-friendly, and just unpredictable enough to keep you guessing.
Thursday: A Grey Start with a Brighter Finish
As of 7:00 AM EDT, Thunder Bay Airport is reporting a temperature of 14.1°C under mostly cloudy skies. The humidity is sitting at 86%, making the morning air feel a bit thick. A light northeast breeze at 5 km/h adds a slight chill to the early commute, and visibility is excellent at 24 km. The barometric pressure is 102.2 kPa and rising, suggesting some short-term stability in the forecast.
By this afternoon, the cloud cover will begin to break into a mix of sun and cloud, and the wind will shift to northeast at 20 km/h, before easing again later. The high for today is 20°C, which is comfortable for most outdoor plans, and the UV index is a moderate 4—not dangerous, but still enough to earn a squint if you’re caught without sunglasses.
Tonight: Mostly Clear and Cool
Tonight, skies will clear out further, leaving behind just a few clouds. The low dips to 9°C, bringing crisp sleeping weather perfect for open windows and cozy blankets. No need for frost warnings yet, but you’ll want a jacket if you’re headed out late.
Friday: Mostly Cloudy with Hints of Chill
Friday, September 19, will bring mainly cloudy skies with a high of 18°C. It’s one of those “don’t forget your hoodie” kind of days. The air will feel cooler and a bit heavier without the sun to warm things up, but overall it’s calm and dry. Friday night stays relatively dry too, with cloudy periods and a low of 8°C.
The Weekend: Mixed Skies and Spotty Showers
Saturday, September 20, introduces a 30 percent chance of showers under a mix of sun and cloud. The daytime high drops to 16°C, making it a chillier, moodier day overall—great for a hike or some early fall yardwork, as long as you don’t mind the odd sprinkle.
Saturday night will keep the pattern going with cloudy periods and a 30 percent chance of showers, with an overnight low of 10°C.
Sunday, September 21, stays in line with Saturday: a mix of sun and cloud, a continued 30 percent chance of showers, and a high of 17°C. Overnight into Sunday night, there’s once again a 30 percent chance of rain, and a cooler low of 7°C.
Looking ahead to Monday, September 22, we’ll return to a mix of sun and cloud, with a pleasant high of 19°C, and cloudy periods continuing into the night with—you guessed it—another 30 percent chance of showers and a low of 8°C.
What to Wear?
This is prime layering weather: think long sleeves with a jacket in the morning and evening. Waterproof gear might come in handy Saturday and Sunday if you’re planning on spending time outside. And yes, it’s time to pack away the shorts (unless you’re the die-hard, shorts-in-November type).
Weather Trivia: Thunder Bay’s Secret Rain Pattern
Thunder Bay tends to experience its most unpredictable precipitation in mid-to-late September, thanks to shifting jet streams and the influence of nearby Lake Superior, which often fuels low-pressure systems with extra moisture. Translation? Even a 30% chance of showers can turn into a quick drizzle—or nothing at all.
