Lake Effect Snow

Since our weather here in the Bay Area remains benign, let's look at some interesting weather occurring further east.  After a mild December, very cold air is finally making its way into the Midwest and East.  For many locations, strong winds with temperatures in the teens and 20s are expected, but only light snow.  The major exception will be locations downwind of the Great Lakes.

This time of year, the water temps of the Great Lakes are usually in the 30s and 40s.  When an Arctic air mass drops south over the lakes, with air temps in the teens and 20s, a large vertical temperature gradient is created, from the relatively warmer lake waters to the very cold air above.  This results in instability.  As the cold air moves over the lakes, it is heated and moistened from below and rises.  As this lake-modified air rises, it cools and releases its moisture in the form of clouds and precipitation, impacting areas downwind of the lake.  It's a relatively simple atmospheric process but the devil is in the details.  Forecasting lake effect snow with precision is difficult.  Lake snow tends to set up in narrow bands of heavy snow - in one location, whiteout conditions may be occurring, while a few miles away, the sun is shining.  Wind direction is the major determiner in what locations receive the most snow.  Usually for the Midwest and East, cold air comes from the west, northwest, or north.  Thus, the heaviest lake effect snows are usually found to the south, southeast, or east of the Great Lakes.  Cities like Marquette and Grand Rapids in Michigan, Cleveland, and Buffalo are all on the east or southeast sides of the Great Lakes, and typically receive much more snow than cities on the west sides of the lakes, such as Detroit, Chicago, and Milwaukee.

As I said earlier, a cold air mass is currently moving southeast over the lakes, triggering heavy lake snows along the Lake Superior and Michigan shores, and will develop along the eastern Great Lakes shortly.  The radar image shows the narrow bands of heavy snow (darker green).  This event is expected to produce about a foot of snow some spots, along with 40-50 mph wind gusts.  In the worst historical lake snow events, some locations receive several feet of snow - this tends to occur most along Lake Superior and downwind of Lake Ontario.

Still no major rains coming to California anytime soon.  If this dry pattern continues through January, we will risk a serious drought situation heading into the spring and summer.