It will be a mostly cloudy day today in Winnipeg, before things begin to clear mid/late afternoon. There may be a few showers, with perhaps a rumble of thunder, this afternoon and evening. Highs will be generally ''only'' around 27 or 28°C.
The oven really turns on tomorrow. We will be under a thermal ridge, which will bring us some very hot and humid weather. 850 mb temperatures are expected to rise into the low to mid twenties throughout southern Manitoba. *The reason why we use 850 mb temperatures is that it can be used to estimate the temperature at the surface. In the RRV, you can add about 10 to 13°C to the 850 mb temperature to get the surface temperature on a sunny, light wind summer day.* Therefore, with 850 mb temps in the low to mid twenties, you can estimate highs in the mid to high thirties for southern Manitoba tomorrow.
A westerly wind in the afternoon will also give a favourable downslope effect to the RRV, which will help maximise the daytime high.
However, there is a limiting factor to the heat tomorrow; and that will be humidity. Dewpoints are only expected to drop into the high teens in the afternoon. The problem with too much humidity is that humid air does not heat up as well as drier air. Thus, if we stay humid during the afternoon, we will likely only see highs between 33 and 36°C across southern Manitoba. If we manage to get dewpoints lower, into the low to mid teens, then highs between 36 and 39°C would be possible.
Currently, my best guess is for a high of 34°C in Winnipeg tomorrow, with 35 to 36°C highs in the western and southern RRV (Carman, Portage, Morden, Emerson). What are your guesses? You can write your guesses in the comment section of this post.