Strong thunderstorms possible Thursday - heatwave next week

Strong thunderstorms producing heavy rainfall are possible Thursday afternoon across southern Quebec. Flash flooding will be possible with the strongest storms. (Valley Weather Photo)

Strong thunderstorms possible Thursday afternoon. 

Summer arrives at 10:42PM on Friday, and along with that will be the warmest air of the year to date. First off we have an active weather day across eastern Ontario and southern Quebec. The region has been blanketed by a sultry air mass, with a warm high of 30C (86F) on Wednesday and overnight lows remaining in the low 20s (70F). Along with elevated humidity levels, temperatures felt more like 35C (95F) Wednesday afternoon.

On Thursday, strong low pressure over northern Ontario, will move into central Quebec. Heavy rain will fall north of Montreal, with warnings already posted. A warm front has lifted north of of the city on Thursday morning, setting the stage for strong afternoon thunderstorms. Depending on how much clearing we receive, the storms could be severe locally.

The biggest threat for severe weather Thursday, including strong winds and hail, will be southeast of metro Montreal. (ECCC)

Most of the severe weather should occur south and east of Montreal. The main threat for the city west to the Ontario border, will be torrential rainfall, with the possibility of up to 50mm locally. That amount of rain over a short period of time could produce some flash flooding. Pay attention today to the latest forecast and any potential weather warnings.

After several flash flood events last summer in Montreal, we know which areas are prone to flooding. Try to avoid parking there today. Isolated showers will persist into Friday morning, before skies clear, with slightly cooler, less-humid air.

Over the weekend, the heat and humidity will begin to build back in, with more showers and thunderstorms late saturday and Sunday. Temperatures will start out in the mid 20s Friday, rising into the middle 30s by Tuesday. Dangerous heat, with humidex levels approaching 40C (104F) are possible in metro areas across southern Quebec, Monday and Tuesday.

Heat and humidity increase this week across southern Quebec

The weather was perfect all weekend for the Canadian Grand Prix at Circuit Gilles Villeneuve.

Light winds, low humidity and ideal temperatures greeted race fans in Montreal. More importantly after rain created significant problems for organizers,  fans and drivers in 2024, the weather remained dry for the entire 2025 three-day event. (F1.com photo)

High pressure anchored over southern Quebec managed to keep the clouds and rain to our south over New England and New York all weekend long.

As we move towards the summer solstice this coming Friday, the heat and humidity will start to build this week. The weather looks rather unsettled for the upcoming week, but not a washout by any means. June has been rather dry when compared to the rest of the spring in Montreal, so we could use some rainfall. To date, only 12mm of rain has fallen at Trudeau Airport in June.

The weekend was spectacular, with ideal conditions for the Canadian Grand Prix or any other outdoor activities. So as we start a new week, high pressure will give us one more ideal day, with sunshine and very warm temperatures. The high in Montreal will be near 28C (83F). 

Clouds and humidity will begin increasing tonight into Tuesday, along with higher dew points, making the airmass feel quite muggy. These conditions will prevail for most of the work week. We may see some isolated showers on Tuesday, with the chances for precipitation and even thunderstorms increasing into Thursday. Temperatures will remain warm, around 27C (80F) for highs and remaining warm overnight, in the upper teens to near 20C (68F). Slightly cooler air may arrive by next Friday or Saturday, but it will be short-lived as the heat will increase again by the Fête Saint-Jean here in Québec.

Montreal may have a rain-free weekend for the Canadian Grand Prix

A quick moving thunderstorm dumped torrential rain on Montreal on Tuesday afternoon. Thankfully the storm only lasted a few minutes, with 10mm falling at Trudeau Airport. (Valley Weather)

After a dismal spring that has had rain on just about every weekend, we may be looking at a break, just in time for the Formula F-1 Canadian Grand Prix in Montreal.

Weak high pressure is moving across southern Quebec on Wednesday, to be followed by a quick moving cold front this evening. That front will generate gusty southwest winds in the St. Lawrence Valley, possibly exceeding 50km/h through the overnight hours. There may be a spot shower or two, but no much precipitation is expected.

High pressure builds back in for Thursday and Friday, with brilliant sunshine and cooler, less-humid conditions. The high Wednesday will be close to 27C (81F0, dropping into the low 20s through the weekend. In terms of precipitation, dry weather is expected through late Friday. In the pre-dawn hours of Saturday, a warm front will lift north across New England. A few showers or isolated thunderstorms are likely along it. The question will be how far north will it move before heading back south.

The thinking at this time is that any showers will remain south of Montreal. Sunday at the moment looks ideal for the race, light winds, low humidity, sunshine, and a high of 24C (76F). The hope is that the venue will avoid the major problems they had last year, when over 30mm of rain fell over the course of the event, causing significant water accumulation on Ile Notre Dame.

Environment Canada is predicting a very hot and humid summer for Montreal and southern Quebec. Hot and dry weather expected across the Prairies, may exasperate the western wildfire situation, leading to more hazy and smokey days as well for our region. (ECCC)

HOT SUMMER ON TAP

Environment and Climate Change Canada has released the national forecast for summer 2025. It is simple, above normal temperatures for nearly all of us from June through August and likely persisting into the fall. Here in Montreal, we are expecting a hot and humid summer, with frequent thunderstorm activity. Northing new to see here, as this is quickly becoming the norm for southwestern Quebec of late.

One final note, Tuesday's quick moving thunderstorms dropped around 10mm on Montreal, brining the monthly total to 11.6mm. The same storms produced a small tornado in upstate New York, near Beekmantown, just north of Plattsburgh. The late afternoon EF-0 storm with winds of up to 120km/h, produced only minor damage. 

Air quality slowly improving in southern Quebec

Smoke from wildfires burning across northern Saskatchewan and Manitoba, produced some of the worst air quality on the planet in Montreal on both Friday and Saturday. Conditions began to improve on Sunday, and that trend should continue early in the week, with southwest winds and showers expected to"scrub" the air.

On Friday, southern Quebec and Ontario had some of the worst air quality among major cities on the planet. The hazy, smokey skies were created by high altitude smoke from the western Canadian wildfire, being driven closer to the surface on north winds. The result was a thick haze filtering out the sun and making it difficult on the eyes and respiratory systems. Air quality advisories were in effect for all our regions from Friday into Sunday morning before being lifted.

By no means is the air perfect Monday morning, with readings in Toronto and Montreal just under the unhealthy level on the air quality index at 94 and 93 respectively. That ranks the two cities 14th and 15th in the world Monday morning. Conditions should further improve late Monday as a warm front lifts north into the St. Lawrence Valley, accompanied by showers and southwest winds.

On Tuesday a cold front will generate showers and thunderstorms, with up to 10mm of rain possible. We need the moisture. To date, Montreal has only received 1.2mm of rain in June. The dry conditions have increased the risk of wildfires in the province, with SOPFEU reporting our region as extreme for fire risk. Outdoor burning should not be done under any circumstance in southwestern Quebec, until we receive some moisture. Conditions are expected to improve this week with the forecasted rainfall.

Temperatures this week will be seasonable, with daytime highs in the low to mid 20s and overnight lows in the middle teens. Unsettled weather, with showers and thunderstorms are likely from Monday afternoon into late Tuesday. By mid week, high pressure is expected to build into southern Quebec with plenty of sunshine to end the work week.

Western wildfire smoke drifts into southern Quebec

Filtered sunshine in Montreal due to western Canadian wildfire smoke has resulted in air quality advisories for southern Quebec. (Valley Weather)

A special air quality statement has been issued by Environment Canada regarding the western Canadian wildfire smoke that has drifted into eastern Ontario and southern Quebec on Friday morning. The smoke was present at higher altitudes earlier in the week, but has worked its way down to the surface currently, creating moderately poor air quality. 

Visibility is under 5km in Montreal at 8am, with widespread smoke and haze. There is definitely a smell of fire in the air Friday morning. The smoke and haze should remain through most of the day on light north and northeast winds, as the Montreal region lies to the north of a warm front over southern New England. The day will be hazy and warm, with a high of 25C (77F).

Environment Canada is advising residents to limit or reschedule outdoor activities until the air quality improves. 

Overnight, humidity levels will be on the rise as the warm front nears our region. Overnight lows will be very mild in the upper teens. On Saturday, we can expect another warm, and humid day, with haze and perhaps an isolated afternoon shower or thunderstorm, with a high of 27C (80F).

High pressure will nudge into the region on Sunday, with clearing skies and a warm high of 24C (76F). It should be one of the best weekend weather days we have had so far this year.

Over 60 Quebec firefighters and support staff left earlier this week to help the province of Manitoba battle widespread wildfires. The smoke form those fires arrived in southern Quebec Friday morning. (SOPFEU Photo)

The fires in Manitoba have forced the evacuations of thousands of residents form the central and northern part of that province, including the community of Flin Flon. There have been 111 fires in Manitoba so far in 2025, above the long-term average of 89. Thousands of hectares have burned, along with many structures. sadly there have been to fatalities.

Help as arrived from other Canadian provinces and the US, including 60 firefighters and personal from Quebec. A state of emergency is also in effect in Saskatchewan. The spring has been dry and warm in many locations, and while some rain and cooler temperatures are possible in the short-term, forecasters are expecting a hot dry summer for a large part of western Canada.

Summer weather finally returning to southern Quebec

That was one miserable weather weekend in Montreal, with a cold rain, gusty northwest winds and high temperatures in the low teens. Thankfully high pressure will build into southern Quebec for the upcoming week, with clearing skies and rapidly warming temperatures. By Wednesday, we may see our first 32C (90F) high temperature of the year in Montreal.

After an unseasonable cold and windy weekend, summer will quickly return this week to Montreal. Sunday was downright cold. The high in Montreal was only 13.1C (55F), well-below the normal high of 22C (72F), and just 0.8 degrees warmer than the all-time record low maximum for the date of 12.2C (54F) set back in 2003. Add in a gusty northwest wind, at times close to 60km/h and occasional showers and the day was just miserable, often feeling like the middle single digits.

Nobody could blame you for thinking that spring is getting off to a poor start. The data backs it up. In May, rain fell on 22 of 31 days. That amounted to 118.2mm, above the long-term average of 85.6mm for Montreal. Some form of precipitation has fallen on 12 of the last 14 weekends in Montreal. dating back to March 1st. Believe it or not, the May average temperature was actually very close to normal at 19C (66F). We can thank a few mild nights for shifting of the data.

SUMMER WARMTH

Now for the good news. High pressure is building into the region Monday morning and should prevail into early Thursday. Skies are already clearing out and we can expect sunshine through midweek. The result will be dry air finally, along with rapidly warming temperatures. Monday will see a high of 20C (68F), followed by a very summery 28C (83F) for Tuesday, and hot by Wednesday, with a near-record high of 32C (90F) possible. Overnight lows will start off cool, but warm as well to a muggy 21C (70F) by Wednesday. The only glitch in the weather will be the arrival of some high altitude smoke from the western Canadian wildfires. This will reduce the air quality a touch and provide hazy sunshine by Wednesday over much of southern Quebec and Ontario.

By Thursday, humidity levels will be on the rise ahead of a cold front arriving from the northwest. Showers and thunderstorms will be possible, continuing into Friday. Unfortunately, low pressure travelling along the front may prolong those showers into Saturday once again. The hope is skies should clear out late in the day, providing us with a sunny and seasonably warm Sunday.

Another dreary Saturday on tap for Montreal

A pop-up thunderstorm moves into the Montreal burrough of Saint-Laurent on the afternoon of Monday, May 26. The storm produced torrential rain, hail and lighting. Fortunately the cell was fast moving and as a result the impacts were minimal. Monday was the 11th consecutive day with measurable precipitation at Trudeau Airport. (Valley Weather)

Wednesday may have been the nicest day in May. Widespread sunshine allowed the temperature to rise to 27.4C (81F) at Trudeau Airport in Montreal. It was a rare moment in what has become a dull and wet spring. In May to date, 111.2mm of rain has fallen, that included precipitation on 11 consecutive days up to and including this past Monday. It has rained on 19 of the 28 days so far this month.

On Monday, some decent isolated, pop up thunderstorms put down heavy rain, lots of lightning and even some small hail during the afternoon hours. All the moisture has had at least a positive impact on the fire season in southern Quebec. There are only three active fires in the province at this time. Such is not the case elsewhere in Canada where massive wildfires have resulted in evacuations across parts of Manitoba and Saskatchewan.

The trend of unsettled weekend weather across southern Quebec will continue as we head into June. A weak warm front will lift north on Thursday, introducing some humid air. Spotty showers are possible into Friday. On Saturday, another unseasonably strong Nor'Easter will move north into New England, with abundant Atlantic moisture. At this time, it appears that Montreal and the Ottawa Valley will remain on the western edge of the storm. While heavy rain is expected in Vermont and the Eastern Townships on Saturday, Montreal should only have scattered showers and gusty northeast winds. Sunday should be partly cloudy, with perhaps a shower or two and milder temperatures.

Highs from Thursday into Sunday will drop into the upper teens from Wednesday's summer warmth. Lows will be in the lower teens. Briefly looking ahead to next week, we can expect warmer, but also a more humid airmass to arrive. As a result showers and thunderstorms will remain a real possibility in the St. Lawrence Valley.