Ice cover on the Great Lakes has rapidly increased during the last few days as the widespread extreme cold has caused water temperatures to plunge after what had been a milder than average winter thus far. The image above shows the ice cover on Lake Erie, which is nearly covered. Note on the top right portion of the image that western Lake Ontario is almost entirely ice-free. Has it been warmer over Lake Ontario? No, temperatures haven't been warmer across Lake Ontario, but Lake Ontario itself is warmer than Lake Erie. While Lake Erie is the southernmost Great Lake, it routinely freezes over during the winter while the other lakes only freeze over in the coldest winters, and even then not entirely.
To understand why it is critical to understand the density of water at different temperatures. The plot below shows the density of water as a function of temperature. Note that the highest density occurs at temperatures just above freezing with the density of water decreasing at both higher and lower temperatures.
The net result is that as water in a lake cools it sinks and warmer water flows to the surface. Only when the entire lake has cooled to near freezing can the surface finally begin to freeze. Since Lake Erie is the shallowest lake, it freezes first. Lake Ontario is about four times deeper than Lake Erie and so takes much longer to cool and freeze over.
Is the level of ice on the Great Lakes unusual? Not at all thus far. The plot below shows the maximum ice coverage in percent by year. Ice cover likely hasn’t peaked yet, but the current 48% coverage is still well below the 80-90% coverage seen in many frigid winters and still well below last year’s peak.
The bottom line is that while the Great Lakes are icing over, they still have a long way to go to reach the levels seen in winters like 2013-14.