Thunder Bay Weather Update for Sunday May 17 2026
Thunder Bay, ON – After a brighter Saturday and a decent start to Sunday, Northwestern Ontario is heading back into a cool and soggy weather pattern for the early part of the week. While spring tried to make an appearance, Lake Superior’s cold influence and a slow-moving weather system are keeping temperatures below seasonal norms.
At 10:00 AM EDT at Thunder Bay Airport, the temperature was 5.5°C under mostly cloudy skies. Humidity sat at 58 percent with easterly winds at 21 km/h gusting to 31 km/h. Visibility remained excellent at 32 kilometres, and barometric pressure was 102.1 kPa and rising.
Today’s Weather Overview
Current Conditions
Sunday is offering Thunder Bay residents a mix of sun and cloud, though the cool easterly wind off Lake Superior is making it feel brisk despite the improving sky conditions. The high today is expected to reach just 9°C.
The UV index is 7, considered high, so despite the chill, sun protection is still recommended during brighter breaks in the cloud cover.
Tonight: Rain Moves In
Cloud cover thickens this evening as a developing low-pressure system pushes moisture into the Lake Superior basin. Rain is expected to begin tonight with total rainfall amounts between 10 and 20 mm possible by Monday morning.
Overnight temperatures will remain relatively mild at 6°C thanks to the cloud cover and incoming moisture.
Monday Through Midweek Forecast
Monday, May 18
Rain will continue into Monday morning before tapering off near noon. However, the skies will stay mostly cloudy with a 30 percent chance of lingering showers during the afternoon.
- High: 11°C
- Rainfall amounts: 5 to 10 mm
- Winds easing through the day
Monday night remains unsettled with periods of rain returning and temperatures dropping to 2°C.
Tuesday, May 19
Tuesday looks downright raw for mid-May with periods of rain and a daytime high of only 6°C. By Tuesday night, colder air pushes back into the region, creating a 40 percent chance of rain showers mixing with flurries overnight as temperatures dip to the freezing mark.
Wednesday and Beyond
Wednesday stays cool with a 40 percent chance of showers and a high of 8°C before skies finally begin to improve.
By Thursday and Friday, Thunder Bay finally starts seeing a more spring-like pattern return:
- Thursday: Sunny, high 16°C
- Friday: Sunny, high 17°C
The warmer temperatures and sunshine should help jump-start gardens and green spaces that have been lagging behind due to the prolonged cool spring.
What to Wear
Today calls for layered spring clothing. A warm jacket remains essential this morning thanks to the strong easterly wind. Waterproof outerwear and proper footwear will be needed tonight through Tuesday as rain becomes widespread.
Motorists should also prepare for reduced visibility and slick roads during periods of heavier rainfall Monday and Tuesday.
Lake Superior Marine Outlook
Marine conditions on Lake Superior will remain unsettled through Monday.
- Easterly winds increasing to 20–30 knots today
- Building wave action along the north shore
- Reduced visibility in rain tonight into Monday
- Small craft operators should exercise caution
Conditions gradually improve by Wednesday as the system exits eastward.
Weather Trivia
Did you know?
Thunder Bay’s average high temperature for May 17 is approximately 15°C. Today’s expected high of 9°C is well below normal, continuing a trend of cooler-than-average conditions across Northwestern Ontario this spring.
Lake Superior plays a major role in that chill. The lake surface temperature remains only a few degrees above freezing in mid-May, often keeping shoreline communities dramatically cooler than inland regions.
Overview
Thunder Bay Weather Update for Sunday May 17 2026. Cool temperatures and rain return to Northwestern Ontario with up to 20 mm expected. Sunshine and warmer weather finally arrive later this week.
