Surge of heat and humidity for Montreal

A perfect retreat during the warm summer weekend, along the St. Lawrence River in LaSalle overlooking the Lachine Rapids. Just a few fair weather clouds dotted the sky the entire weekend, with comfortable daytime highs near 28C (83F), and lows near 16C (60F), along with low humidity. It was an ideal weekend for anything outdoors in Montreal. (Valley Weather Photo)

Widespread heat and humidity warnings are in effect from North Dakota to eastern Ontario and New England and will likely be issued for parts of Quebec for Tuesday.

Some of the warmest temperatures so far this summer are set to surge into southern Quebec and Ontario this week. How warm? Dozens of record highs were established in Manitoba over the weekend, as highs topped out in the middle 30s. Even overnight temperatures remained in the upper 20s. Humidex reading were as high as 45C. That heat is set to arrive Monday and spread east.

For Montreal, Monday will feature hazy sunshine, with a high close to 30C (86F). A few clouds and perhaps an isolated shower or thunderstorm are possible late in the day, but the risk is low. Warm southwest winds will develop Monday and increase on Tuesday, gusting 20-50km/h at times.

That breeze will usher in very high humidity, along with temperatures as warm as 35C (85F) across some southern Québec locations on Tuesday. Morning lows on Tuesday and Wednesday will be very warm, in the middle to upper 20s for most locations. Some smoke from northern Quebec forest fires is possible, especially in the Ottawa Valley.

On Sunday, parts of northwest Ontario, including Sioux Lookout, flirted with the 100F (37.8C) mark. There is a chance that some locations from Windsor, Ontario east towards western New York may reach 100F Tuesday. I think Montreal will remain below our all-time record high of 37.6C (99.6F) set on August 1, 1975. We should easily challenge the daily record high of 33.3 (91.9F) set in 1921.

Along with the heat will be high humidity levels, pushing humidex values into the middle 40s for most locations. A cold front late Tuesday is expected to produce some very strong if not severe thunderstorms along and south of the St. Lawrence Valley. There is an isolated risk of some supercell storms, that could produce strong winds, hail, heavy rain and even an isolated tornado. The threat would be between 6pm and midnight for southern Quebec.

Warm but less-humid weather will continue for the rest of the week, with seasonably warm highs in the upper 20s.

The fire situation remains serious across central, western and northern Quebec. Any type of outdoor fire remains prohibited across all of Quebec north of the St. Lawrence River, and is highly discouraged in other regions. To date, 295 fires have occurred in Quebec this year, consuming more than 102,000 hectares of forest. Of those fires, 227 were caused by human activity, with 56 blamed on lightning according to SOPFEU.