Taking a look at the GFS, Euro and NAM high forecasts for different days this week:
The NAM has areas north of the lake between 27 and 33 and areas on the lake cooler. I think the big factor for Toronto will be how much cloud cover will there be on tuesday and thursday. Wednesday is pretty much in the bag for being above 32 C in my opinion. If we get an E or NE breeze off the lake then Toronto won't get up to 32C but areas in southwestern Ontario will. I don't think this is likely and we will probably experience a story SW flow due to the strong warmup. This should help keep temps rising in the city of toronto. The NAM is probably not picking up on this and overdoing the cooling influence from the lake. If the sun can stay out for most of the day then i think we will be able to heat up fast enough considering how dry it is (Ottawa included). Windsor should reach heart wave status in my opinion.
Wednesday looks very hot and humid for most of Southern and Eastern Ontario. Should exceed 33C in many areas as long as that sun can come out. Euro shows 33 or even higher of rOttawa, Toronto, London and all the way SW to windsor. On wednesday skies will stay very clear for the most part with humidity levels pushing the 45 degree mark.
I think the heat will continue to surge northward very quickly in front of a clod front that will move through MID DAY thursday which will be the key to getting enough daytime heating and hopefully a very strong severe weather outbreak. Skies will be clear on thursday until about 2pm allowing temps to surge well into the 30's hopefully. Timing of this front will be key. If it can hold off a bit longer it will make a huge difference.
flow coming strongly out of the SW in advance of the front.
I think areas from windsor to London probably get an official heat wave as long a there are no sudden changes in the next few days. The only thing stopping them will be if the from on thursday comes before temperatures have chance to go up enough. Same goes for the GTA, I think tuesday will be hot enough as well as wednesday but thursday is a question mark. Ottawa will have problems tuesday getting hot enough but if Ottawa can reach 32 on tuesday then i think it will have a good chance at a heat wave because the front will move through there a bit later thursday. Either way, we can be expecting very hot conditions across ontario this week as well as oppressive humidity levels whether we get an official heat wave or not. Please plan accordingly and enjoy!