Heat, humidity and thunderstorms - July in Montreal

The calm after the storm as a vibrant rainbow graces the Montreal skyline. The weather has been very active and highly changeable this month in Montreal. (Photo Norman Leblanc)

A dome of high pressure is forecast to strengthen across the midwestern United States this weekend and into next week, providing a large portion of eastern North America with increasing heat and humidity. The warmest temperatures, approaching 38C (100F) in some locations, will be found across the Ohio Valley, Midwest and Middle Atlantic states.

Montreal, southwestern Quebec and eastern Ontario will be on the northeast edge of the heat dome, with warm and humid weather and occasional thunderstorms starting Monday. We call those systems that produce then thunderstorms ridge runners or ridge rollers, disturbances the move along the periphery of the nearly stationary ridge of high pressure.

Typically these can produce strong, very moist thunderstorms, just what Montreal does not need. The best chance for those storms would be next Wednesday and Thursday, however isolated cells can pop up anytime in the warm and humid airmass.

June in Montreal can be summarized in this one photo, as thunderstorms and sunshine have battled all month. The result has been highly changeable weather along with plenty of rainy weekends. (Valley Weather Photo)

Before that, we have a cold front arriving Friday, with scattered showers and thunderstorms. It does not look like a repeat of last Saturday, with most storms rapidly moving from west to east across the region. My experience in Montreal in both the recent flash flooding event all the way back to the infamous 1987 Decarie Expressway flood, is that it is the north to south or south to north moving storms are the ones that cause us the most trouble.

It has been a wet June, already closing in on 150mm (6 inches) of rain at Trudeau Airport, with some locations in the city easily surpassing 250mm (10 inches), so anymore rain will only add the saturated ground and full rivers and lakes.

As far as temperatures are concerned, we will reach 22C (72F)  Friday, before the heat starts to build through the weekend and into next week. High temperatures will rise into the upper 20s and low 30s by the middle of next week. High humidity will push humidex values into the upper 30s to near 40C across large parts of southern Quebec and eastern Ontario.