THUNDER BAY — WEATHER — Thunder Bay is heading into a generally comfortable late-September weekend, with temperatures near or slightly above seasonal values Saturday before cooler air settles in Sunday and Monday.
The bigger weather concern will be on Lake Superior, where Environment Canada has a strong wind warning in effect. Offshore winds are forecast to increase from light conditions Saturday to northeast 20 knots Sunday morning.
For residents staying on land, the three-day outlook is relatively quiet. Boaters should pay closer attention, particularly Sunday when open-lake conditions will become rougher.
Thunder Bay Three-Day Weather Forecast
Saturday, Sept. 19 — Mild With a Late Shower Risk
Saturday will be the warmest day of the three-day period.
Environment Canada’s latest published city forecast calls for mainly cloudy conditions with a 30 per cent chance of showers late in the afternoon and a high near 20 C. The UV index is expected to reach 5, or moderate.
Saturday night remains mainly cloudy with a 40 per cent chance of showers and an overnight low near 7 C.
For outdoor plans, most of Saturday should remain usable, but carrying a light rain jacket would be sensible later in the day.
Sunday, Sept. 20 — Sunshine Returns
Sunday should offer the brightest weather of the weekend.
Environment Canada is forecasting sunny skies and a high near 16 C.
That is close to Thunder Bay’s normal daytime temperature for this point in September.
Cloudy periods develop Sunday night with a low near 5 C.
While conditions in the city look pleasant, Sunday is expected to be a more challenging day on Lake Superior because of strengthening northeast winds.
Monday, Sept. 21 — Mix of Sun and Cloud
Monday continues the cooler pattern with a mix of sun and cloud and a high near 16 C.
Monday night is forecast to clear, with temperatures falling to around 1 C.
That will put rural, inland and low-lying areas close to frost territory Monday night.
Gardeners with sensitive plants should keep an eye on updated forecasts.
Western Lake Superior — Strong Wind Warning in Effect
Conditions on Lake Superior will change considerably through the weekend.
Environment Canada’s 3 a.m. Saturday marine bulletin has a strong wind warning in effect for Lake Superior.
Saturday begins with southeast winds near 10 knots, becoming light early Saturday evening.
Winds then increase from the north to approximately 15 knots overnight.
The bigger change arrives Sunday morning.
Environment Canada expects winds to increase to northeast 20 knots, before easing to northeast 15 knots Sunday evening.
Environment Canada defines strong winds on the Great Lakes as sustained winds between 20 and 33 knots.
Nearshore conditions can differ significantly from offshore forecasts because islands, headlands and the shoreline can alter wind speed and direction.
Sunday Could Be the Toughest Boating Day
The northeast wind Sunday deserves particular attention from recreational boaters.
A 20-knot open-lake wind can produce uncomfortable and potentially hazardous conditions for smaller vessels, especially where the wind has a long stretch of open water over which to build waves.
Conditions inside Thunder Bay Harbour can look dramatically calmer than conditions beyond the breakwall or along exposed portions of the Lake Superior shoreline.
Boaters should not use harbour conditions alone to judge whether the open lake is safe.
The latest marine bulletin should be checked again immediately before departure because Environment Canada is scheduled to issue regular updated forecasts through the weekend.
Lake Superior Safety Remains Important in September
September sunshine can make the lake look inviting, but Lake Superior remains cold.
Anyone operating a boat, kayak, canoe or personal watercraft should wear an approved personal flotation device and carry reliable communications equipment.
Boaters should also:
- check the latest marine warning before departure;
- tell someone their route and expected return time;
- carry appropriate cold-water clothing;
- watch wind direction as well as wind speed; and
- avoid assuming conditions near shore represent conditions offshore.
The strong wind warning is especially important for smaller craft.
Three-Day Forecast at a Glance
| Day | Thunder Bay | High | Night |
|---|---|---|---|
| Saturday, Sept. 19 | Mainly cloudy; 30% late shower risk | 20 C | 40% chance of showers; low 7 C |
| Sunday, Sept. 20 | Sunny | 16 C | Cloudy periods; low 5 C |
| Monday, Sept. 21 | Mix of sun and cloud | 16 C | Clear; low near 1 C |
Weather Desk Bottom Line
Saturday offers the warmest conditions, although showers could develop later in the afternoon and evening.
Sunday turns cooler but brighter, while Monday stays near seasonal temperatures before another chilly night.
For boaters, the story is different.
Western Lake Superior conditions deteriorate Sunday as northeast winds increase to around 20 knots, putting the lake into strong-wind-warning territory.
Anyone planning time on the water should refresh the Environment Canada marine forecast before leaving shore.
Forecast information was checked Saturday morning, Sept. 19, 2026. Weather and marine conditions can change quickly.
Summary: Thunder Bay stays mild Saturday before cooler weather and stronger Lake Superior winds Sunday.
