Heat and humidity build into eastern Canada this week

A surge of hot and humid air will move into southern Ontario and Quebec this week, with forecast high temperatures over 32C (90F) in Montreal, Ottawa and Toronto.

After a dismal weekend, with solid cloud cover and occasional rain, the region is looking at clearing skies this morning. Around 20mm of rain fell in southern Quebec on Saturday and early Sunday, as low pressure skirted down the St. Lawrence Valley and into New Brunswick. Skies are clearing this morning, with patchy fog and warming temperatures. The clouds and rain kept temperatures cool, near 20C (68F) all weekend, well below the normal high of 26C (79F).

Heatwave?
In Montreal, a heatwave is usually declared when high temperatures reach or exceed 30C (86F) over three consecutive days. However, the official Environment Canada definition is three days over 32C (90F). A heat advisory is posted when the humidex value, the combination of heat and humidity, is expected to be near 40C or higher. It's all relative. However you want to look at it, it will be a very hot and sticky week. High pressure will build into Quebec today, cresting over us tonight and sliding to our east on Tuesday. As the center moves east, a rather potent surge of heat and humidity will move into the Great Lakes Basin and St. Lawrence Valley. High temperatures will be on the rise in Montreal, from 26C (79F) today, up to the low 30s (near 90F) by Wednesday. The humidity will build as well, with humidex readings approaching 40C (104F) in southern Ontario and Quebec by Wednesday afternoon. Environment Canada has posted a special weather statement for Montreal, Ottawa and Toronto regarding the potential heatwave this week. Heat warnings or advisories may be needed for several metropolitan areas by mid-week. Overnight low temperatures will become rather uncomfortable as well, remaining in the low 20s (low 70s). The heat will also provide the fuel for pop-up thunderstorms each afternoon from Wednesday to Friday. Some storms may be on the strong to severe side, especially Friday. Relief will come from the heat in the form of a cold front by late in the week. Next weekend at this time looks ideal, with low humidity, sunshine and warm temperatures.