Sunday's record-setting storm could repeat on Thursday

Montreal Gazette

3 days ago

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The storm that flooded basements and inundated highways in Montreal Sunday set a record for the amount of rainfall recorded in a 24-hour period, meteorologists reported Monday.
With weather conditions for the next two days mirroring what was seen just prior to Sunday's deluge — extreme heat and high humidity followed by a cold front — Montreal could be swamped with a similar downpour on Thursday.
Environment Canada measured 81.6 millimetres of rain at its recording station at Montreal—Pierre Elliott Trudeau International Airport on Sunday. That surpassed the previous record set last summer on July 10, when the remnants of Hurricane Beryl dropped 79.2 millimetres of rain on Montreal.
Rainfalls of anywhere from 70 to 100 millimetres hit different parts of the island Sunday, Environment Canada said. But only the readings taken at specific data sites are used for official tallies.
'It was intense, climatologically,' said Environment Canada meteorologist Julien Pellerin. 'It was one of the most intense days ever.'
The first wave of the storm at around 3:30 p.m. dropped 58 millimetres of rain on Montreal in an hour, followed by less severe thundershowers afterward. Boucherville on the South Shore received 50 millimetres in 45 minutes, and Longueuil was soaked with 43 millimetres in an hour. Blainville and Rosemère on the North Shore saw 100 millimetres of rain in 90 minutes.
The storm also brought gusting winds that felled trees in Montreal and hydro poles in Ste-Martine, south of the city.
Unlike last year's deluge, which came at the end of a hurricane, Sunday's rain was caused by the moisture that collected in the atmosphere because of the high heat and humidity on the weekend. Then a cold front that was generated over Hudson Bay in northern Quebec swept through the southern part of the province, propelling the moisture pooling near the surface upward, which caused rapid condensation followed by precipitation.
A strong thunderstorm line formed west of Montreal at about noon and moved eastward throughout the day. Montreal and its surrounding regions were hit with successive waves of thunderstorms.
Downpours of such intensity are common in July, which is the most active month of the year for thunderstorms, Pellerin said.
The heat is expected to return Tuesday and Wednesday, with maximum temperatures between 31 and 33 C both days. High humidity will result in humidex values approaching 40 C. Another cold front is expected to sweep through on Thursday, which could bring similar conditions to Sunday.
The intensity of the storm depends on when it hits. If it strikes at night or in the early morning, when the amount of moisture in the air is lower, it won't be as powerful, Pellerin said. But if it strikes in the afternoon, when the heat of the day is at its maximum and there's a significant amount of moisture in the air, 'I would say if you have something planned for Thursday, you might want to reschedule,' Pellerin said.
Adding to the discomfort, Environment Canada issued a 'special air quality statement' Monday at noon advising residents that smoke from forest fires in the Prairies was causing poor air quality and reduced visibility. Conditions were expected to gradually improve by Tuesday.
The weather service noted that as smoke levels increase, health risks increase. It advised citizens to limit time outdoors and to reduce or reschedule outdoor sports, activities and events if possible.
Increased smoke could cause eye, nose and throat irritation, headaches or a mild cough. More serious but less common symptoms include wheezing, chest pains or severe cough. Residents should keep doors and windows closed, but if there is a heat warning at the same time, keeping cool is the priority, Environment Canada said.
Signs of excessive heat exposure may include headache, nausea, dizziness, thirst, dark urine and intense fatigue. Stop your activity and drink water if these symptoms occur.