Looks like three areas of concern for today, and those region are circled above green the weakest, yellow stronger storms, and red the area where the worst of the storms will be today. Starting in SW'rn ON, there is un upper level vorticity maxima currently near the Sault Ste. Marie region tracking southward. This upper disturbance along with the frontal boundary will make for a few thunderstorms this afternoon. CAPE values look high enough for some storms to reach sever limits with soundings supporting some strong straight line winds and LCL's look to be a little elevated.
Area from Kingston up to Quebec city looks pretty good with the main upper jet passing through the warm sector which should provide some extra kick to the system. Shear vectors are oriented as such that isolated updrafts will be supported and low level shear looks to be supportive of a few supercells. Hodographs are indicating though mostly splitting modes and thus not likely no real organized supercells will get going. CAPES are high enough though to provide a good hail risk and chance for some strong winds and likely see some severe storms through the Ottawa, Cornwall areas and down into Kingston.
The greatest risk for today is in NW'rn ON from Fort Francis across to the Lake Superior shore line. Here strong upper level jet and low level flow and moisture will provide enough fuel for strong thunderstorms. Some of which a on going through the MB area. All parameters here looks to support strong thunderstorms with the risk of some large hail, winds, and even an isolated tornado is possible. The greatest tornado threat is in the MN but regions right along the U.S. border (as outlined above) cannot rule out the risk of a brief tornado.