Montérégie weather is expected to turn brighter Sunday after a cold overnight period brought frost to parts of the region.
Environment Canada’s forecast for Saint-Hyacinthe calls for a sunny day with a high near 12 C on Sunday, October 18, followed by a clear night and a low near 5 C.
Forecast conditions around Longueuil and Salaberry-de-Valleyfield are also expected to remain mainly sunny, with afternoon highs ranging from about 15 C to 16 C.
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Montérégie Weather: Frost gives way to sunshine
The coldest conditions were expected before Sunday morning, particularly away from the St. Lawrence River and in rural areas where skies cleared and winds weakened.
Saint-Hyacinthe’s forecast included a low near minus 1 C overnight Saturday, with frost possible before temperatures climbed during the day.
The sharp temperature change is typical of mid-October in southern Quebec, where clear skies can produce chilly mornings even when afternoons become comfortable.
Warmer Monday before another cool stretch
Temperatures are expected to rise on Monday, October 19, with a high near 16 C in Saint-Hyacinthe and similar readings across parts of western Montérégie.
Tuesday is forecast to remain dry, with afternoon temperatures near 15 C in Saint-Hyacinthe before daytime readings ease later in the week.
The broader outlook calls for mostly sunny or clear conditions through much of the week, although overnight temperatures are expected to remain close to the freezing mark in some communities.
What residents should watch
Frost may affect exposed plants, gardens and vehicles during the coldest early-morning hours.
Drivers should be alert for reduced visibility from low-level fog in areas near waterways and fields.
Outdoor conditions should improve quickly after sunrise as temperatures move into double digits.
No active severe-weather alert was identified for Montérégie in the latest forecast information reviewed for Sunday.
Residents should still check the latest local forecast and review Québec’s emergency preparedness guidance before travelling, especially because Environment Canada updates conditions as the weather pattern changes.
Frequently Asked Questions
What is the Montérégie weather forecast for Sunday, October 18, 2026?
Sunday is expected to be mainly sunny, with afternoon highs generally ranging from about 12 C to 16 C depending on the community.
Was there frost in Montérégie?
Frost was possible overnight Saturday and early Sunday, particularly in inland and rural areas where temperatures dipped close to or below freezing.
Will Montérégie be warmer on Monday?
Yes. Monday, October 19, is expected to be warmer, with temperatures near 16 C in parts of the region.
What should residents protect from the cold?
Exposed plants, outdoor water lines and vehicles may need attention during the coldest early-morning hours.
Where can residents check official weather alerts?
Environment Canada’s weather-alert page provides the latest warnings and statements for communities across Montérégie and Quebec.
Fact-Checked: Key forecast details were checked against Environment Canada’s official regional forecast information, including the Sunday temperature outlook and overnight frost risk. ([beta.weather.gc.ca](https://beta.weather.gc.ca/en/location/index.html?coords=45.626%2C-72.946&utm_source=openai))
Disclaimer: Weather conditions can change quickly, so residents should consult the latest official forecast before making travel or outdoor plans.