Thunder Bay Weather Forecast – Bright Skies, Breezy Days, and a Chilly Night Ahead
Sunny Skies Take Centre Stage, but Winds Keep Things Brisk
Thunder Bay, ON – Wednesday, May 21, 2025: The Lakehead is waking up under mostly cloudy skies this morning, but that won’t last long. At 6:00 AM, the temperature sits at 7.4°C, and a steady east-northeast wind at 23 km/h is adding a bit of bite to the air. With a dew point of -1.3°C and humidity at 54%, the atmosphere feels crisp and fresh — like spring trying on its summer shoes. The barometric pressure is at 101.9 kPa and rising, a sure sign that clearer skies are on the way.
By mid-morning, the sun will break through and dominate the day. Expect sunny skies and a high of 17°C, which should warm things up nicely, even if those east winds pick up to 20 km/h and gust to 40 km/h. You might need to hold onto your hat, but at least you’ll be basking in the sun while doing it. The UV index is a high 7, so don’t forget your sunscreen — the lake breeze might be cool, but the rays are still strong.
Cloud Cover Creeps In Tonight with a Frosty Possibility
Tonight, expect increasing cloudiness, though it’ll stay dry for now. Winds will shift northward at 20 km/h, becoming light later in the evening. Temperatures will drop to +3°C, and with that, there’s a risk of frost, so gardeners — you might want to tuck those seedlings in with a blanket or bring your planters inside.
Cloudier Thursday, Then Sunshine Returns for the Weekend
Thursday brings cloudy skies, and winds will stiffen again — this time from the north at 20 km/h, gusting to 40. Temperatures stay fairly steady with a high of 16°C, slightly cooler thanks to the cloud cover. The UV index dips to 4, so some protection is still needed if you’re out for long.
Thursday night features cloudy periods with a 30% chance of showers, and a low of +3°C, keeping things on the chilly side.
By Friday, the sun is back in charge. Expect a beautiful day with sunny skies and a high of 16°C, followed by a clear, cool night at +2°C.
Marine Forecast: Choppy Waters on Superior
For anyone heading onto the water, take note — a strong wind warning is in effect on western Lake Superior. Winds today are from the northeast at 25 knots, easing slightly this afternoon, before increasing again overnight. Waves of 1.5 to 2 metres are expected, so if you’re boating, make sure your gear — and your stomach — are up to the challenge.
What to Wear?
It’s a day to dress smart: think windproof layers and sunglasses. A light jacket will keep you comfortable in the morning breeze, and sunscreen is a must with that high UV index. Evening plans? Add a sweater to the mix — it’s going to get chilly quickly after sunset.
Historic Temperatures for May 21
Thunder Bay’s weather records remind us just how wide the temperature swings can be. The record high for May 21 is 30.6°C, and the record low is a shivering -4.4°C. Today sits comfortably in the middle — not a record-setter, but a fine spring day nonetheless.
Lakehead Weather Trivia
Did you know Thunder Bay is one of Canada’s windiest cities thanks to its proximity to Lake Superior? Those gusty northeast winds you’re feeling today are a signature move of the region — and they help explain why your patio umbrella just tried to take flight.
