Looking at the system affecting much of central to southern ON and into Atlantic Canada will start with 500 mb vorticity. As the system develops we can see there are two bits of energy to watch, 1 coming up from the Gulf tracking up the eastern U.S. and the second from the upper low dropping south from the Canadian Prairies. Looking at the ECMWF, GLB, and GFS for Sunday morning (when I suspect the worst will be) can see that they have pretty good agreement with how they handle the dynamic energy. The GLB/GFS are a little faster pushing the secondary low little further east but overall nothing to different. The uncertainty will be how the two will interact but for the most part the left overs of the initial low from impulse 1 and how that will interact with impulse 2. But at this point the northeastern U.S. and Atlantic Canada will see the worst as impulse 1 will strengthen as it tacks northeast and absorb impulse 2.