Strong thunderstorms for the first day of summer

Lightning illuminates the sky over Ile Perrot during recent thunderstorms. There have not been to many storms this season to date, but a frontal boundary is poised to generate severe thunderstorms Monday afternoon across southern Quebec. Be alert to rapidly changing weather conditions if you have any outdoor activities planned for Monday. (Valley Weather Photo)

Elevated risk for severe thunderstorms Monday afternoon in Montreal

Summer officially arrives at 11:32 PM Sunday evening, and right on cue, a warm and humid airmass will arrive across southwestern Quebec. Sunday was a sunny and pleasant day, with temperatures topping out in the high 20s, 29C (85F) here on Ile Perrot, along with low humidity. The evening will remain dry and mild, with clouds on the increase after midnight. Lows will remain warm, in the upper teens.

A warm front will lift across the region during the pre-dawn hours Monday, accompanied by scattered showers and perhaps a rumble of thunder. Skies will then partially clear Monday morning, with strong, gusty southwest winds briefly transporting tropical air into the St. Lawrence Valley. Temperatures will rise quickly into the upper 20s and low 30s across the region. Humidity levels will rise as well, making for a muggy day. The airmass will become quite unstable, with plenty of energy to fuel developing storms.

A frontal boundary is expected to move into the region late in the day, with strong to severe thunderstorms developing in advance of the trough. The storms are expected to form across eastern Ontario by 2pm and move east-northeast along the boundary into southern Quebec. At this time, there is an elevated risk of severe weather. The main threat will be strong gusty winds up to 100km/h, but large hail, torrential downpours and dangerous lightning are possible as well. A severe thunderstorm watch will likely be issued by Environment Canada some time Monday morning.

If you have any outdoor plans on Monday, keep and eye to the sky. This is especially true if you will be out on the open water. The threat for severe weather will diminish into the evening hours Monday. Skies will partially clear on Tuesday, along with cooler, less humid conditions. High temperatures will be in the low 20s for Montreal.