September 23, 2025: Thunder Bay’s Detailed Weather ForecastTHUNDER BAY – WEATHER – It’s a fog-filled morning in Thunder Bay, with near-zero visibility in many areas and 100% humidity wrapping the region in moisture. Once the fog lifts, a beautiful stretch of sunny, late-September warmth takes hold, lasting through Thursday. Showers could return by Friday night, but until then, it’s a golden opportunity to enjoy fall at its finest. Night: Clear skies, with a low of 9°C . Night: Clear skies, and a chillier low of 7°C .