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.

Looking at the surface for overnight Saturday into Sunday morning the ECMWF/GFS show the decaying low tracking through the Niagara region with the new low redeveloping along the triple point (intersect of occlusion, cold front and warm front) along the east coast. With the GFS being the more aggressive model with the decaying low and the ECMWF keeping little further south. The GLB at this point has dissolved the initial low into a surface trough but this is quite drastic from the previous run which was quite similar the 12Z GFS shown below, thus making me a little suspect of the GLB at this point. The other stand out feature is the ECMWF shows a low over Lake Superior as a result from the second vorticity center as pointed out before. The GLB shows an extended but pretty strong trough with closed surface low in the northern thumb of Michigan and the GFS showing an elongated trough with the wind field (not shown) indicating a cyclonic circulation. These features will ultimately have great impact on how this system will play out. The issue now becomes where the decaying low will tracking. The GFS will have the greatest potential for off lake enhancement off Lake Ontario for the GTA, Golden Horseshoe, and along the QEW corridor to St. Catharines with winds down the axis of Lake Ontario. The ECMWF does show a brief period of some lake enhancement but not as strong as the GFS and the GLB has none with on a trough pushing through. To add insult to injury the GLB is now beginning to hint at a dry slot scenario for parts of southern Ontario, mainly for the Lake Erie and Lake Ontario regions which would result is very little snow. As the secondary impulse swings around the secondary low spins up a little more which will result in a little more snow for eastern Ontario and along the 401 east of Kingston for the day on Sunday.

In terms of snowfall ECMWF/GLB still have some agreement but that is because of the lower QPF amounts shown by the ECMWF. Across much of southern Ontario general 5-15 cm is possible from Saturday evening through to Sunday midday. Below I show the GFS 12 hour snow forecast which is on the extreme end showing 15-30 cm across southern Ontario and 5-10 cm for central Ontario. Obviously the GFS will be higher given the trend it shows above. Thus for now a blend between the ECMWF/GFS looks reasonable and my current thoughts are as follows:

Along and south of 401 from London through to Ottawa 10-20 cm are possible with Golden Horseshoe into the higher end of the range.

Remainder of province into the 5-15 cm range depending on how the second low form northern Ontario pans out.

This is just my initial guess and likely adjust some as the forecast evolves. There is the risk of a dry slot scenario as well which if pans out heavy accumulations will be pushes northwest.