A perfect chain of events the past couple days has led to a rare sighting here in Western Wisconsin…lake effect snow. It happened on Sunday near Lake Pepin. As you can see by the radar image below a long, but narrow band of snow developed on the east-southeast side of the lake. Lake Pepin is a relatively small lake, so the snow band was only 3 miles in width. The peak in its intensity came at 6:16 A.M., and the snow came down pretty good. Visibilities were as low as a quarter mile. Pretty impressive! All these images are provided by the La Crosse National Weather Service Office.
So how did this happen? You may have thought that the lake and river would be frozen over. Well it was as early as last Friday. You can see by the satellite picture below that the lake was totally frozen over on the 18th. However, temperatures on Friday were well above normal. Highs topped out in the 40s, it stayed warm overnight, and topped out in the 40s once again on Saturday before that Arctic cold front came through.
That likely weakened the ice over the lake. Then those strong winds gusting between 40 to 60 mph kicked in behind the front. This combination led to the ice breaking up over the lake. You need open water to get lake effect snow. You can see the open water and the difference by the image below taken on Sunday.
So with this open water, the weather on Sunday was ideal for a lake effect snow band to form. How? Well, the air over the lake was quite cold. Comparatively speaking, the water temperature of the lake is far warmer. This caused lift over the lake. The winds then carried the snow that formed down-wind creating that very long and narrow snow band you saw on the radar. The diagram below shows this process really well. This is the same process that happens on the Great Lakes and can dump feet of snow. It’s just on a much larger scale cause they are much bigger lakes.
Because winds have stayed blustery, it’s made it hard for ice to reform on the lake. That means once again here on Monday night we could see another round of lake effect snow form and impact the same areas. It’ll depend what happens with the winds overnight. Pretty cool, huh?