A large, complex storm system over the U.S. midsection will continue to ride northeastward toward central Ontario then northwest Quebec over the next two days. East of the storm, near-record warmth will surge up toward the St. Lawrence Valley by Friday followed by thunderstorms. At the same time, a snowstorm will be ongoing across northwestern Ontario.
Thunderstorms likely tonight (Wed night)
A warm front now across the Ohio Valley will advance northeastward tonight, and this will likely be a trigger for some thunderstorms later tonight into tomorrow morning across parts of Ontario.
I do not think this will be a severe thunderstorm situation as the storms will be elevated in nature, but we could see some very heavy downpours and frequent lightning tonight starting in the Windsor area then spreading up to the GTA and perhaps Barrie and over to Peterborough late.
Major warmth Friday
The warm sector will surge up through eastern Canada Thursday and Thursday night, and many areas from eastern Ontario through southern Quebec will see rising nighttime temperatures.
The timing of the front suggests that parts of southern Quebec could see record warmth Friday afternoon as there will be a southerly downslope wind and some sunshine, which could push temperatures above 25 C (77 F) degrees. Farther west, clouds and eventually precipitation will keep temperatures lower.
More thunderstorms Friday
Behind the warm front will be the usual cold front, which will march eastward across Ontario on Friday. Based on the timing of the front with daytime heating I would say that the greatest chance for severe thunderstorms (mainly straight-line damaging wind gusts) would be from the Niagara region up through the GTA and Barrie then east-northeast toward Ottawa during the late morning through the afternoon.
Sunday certainly looks like the pick of the weekend with sunshine and a mild afternoon across much of eastern Canada.
Significant warm-up begins to take shape across BC during the middle of next week then spreads eastward into the southern Prairies during late next week.
Wildfire smoke causing significant problems across Saskatchewan.
Spring-like storm to soak parts of eastern and Atlantic Canada this weekend as the West cooks.
Heat wave for western Canada by the weekend as the East cools down a bit.
Severe weather threat Monday evening/night for Ontario. Building heat wave late this week into next week out West.
My latest interpretation of the weekly ECMWF forecast model output.
Potential for heavy rain and strong thunderstorms for parts of eastern Canada Friday while cooler air spreads into the West.