It appears that we will have one big day of natural fireworks on Monday, as insufficient forcing and moisture will be present on Sunday for more than isolated storms.
On Monday, a warm/quasi-stationary front will be draped over E ON and S QC, where a good combination of surface heating, moisture, bulk shear, and forcing will likely produce severe convection.
A few storms will likely occur in ON, but the best parameters and storm coverage are expected near and north of Montreal.
This 06Z NAM sounding is about 50-75 km north of Montreal, with very favourable parameters for supercells and tornadoes near the warm front. Farther south, winds are expected to back to the southwest resulting in a more unidirectional wind profile and consequently a conversion to squall lines which may produce damaging wind gusts due to some low/mid-level dry air entrainment and mid-level winds over 50 kt.
Areas downwind of lake Ontario (which may include the entire Seaway, sadly) are expected to have surface-based instability virtually disappear later in the afternoon due to the onshore flow. However, elevated storms may still occur due to the presence of moisture near and above 850 mb.