Here is a brief overview of the very unusual case of cloud moving northwestward over southern Ontario Friday night July 12.
First, we had an upper level (500 hPa level) low south of Ontario. It's highlighted in the chart below which shows the 500 hPa pattern at 00 UTC Saturday (or 20:00 EDT Friday evening). How did it get there? It was the south end of a large trough that moved eastward across Ontario several days ago and got 'cut off' and left behind as the northern trough continued moving eastward across eastern Canada, while the huge ridge over the southern-central US began spreading northeastward over the Great Lakes region (to link up with the Atlantic ridge or the 'Bermuda High'). The cutoff low was detached from the upper level jet stream circulation that would normally move the upper low eastward, and the circulation around the upper ridge causes the stranded low to move westward. Cyclonic or counterclockwise circulation around the closed low, and around an associated weaker low south of Lake Ontario, caused moisture (from the Atlantic coast) to circulate westward on the north side of the low, directly toward Toronto. The satellite image below for Friday evening shows the cloudiness circulating cyclonically toward the Toronto area.
Above, the 500 hPa pattern shows the closed upper low south of Lake Erie, a weaker trough south of Lake Ontario, and the upper ridge spreading northeastward over the Great Lakes region.
Above, satellite image shows the cloud circulating cyclonically around both the main upper low and the associated weaker low south of Lake Ontario.
Next update (hopefully this afternoon): how much cloudiness will remain over southern Ontario Saturday night?
