Blame the lakes! (Not the Weather Guy)

For the city of Peterborough the best thing about today is that yesterday is behind us. Over all we saw around 22mm of precipitation fall. The first 10-15mm of that fell as snow (resulting in over 13cm of snow) and the rest of it fell as rain…which helped to melt the snow. Today may have been windy, it may have been cold but at least we didn’t see all that much snow…that is unless you live north of the city.

For folks in the Bancroft area and specifically Haliburton, snow was a big issue today. Westerly winds are tracking large bands of lake effect snow right across the north resulting in squalls that cause whiteout conditions and leave behind a huge amount of accumulation.

So what is lake effect snow? Looking at the Environment Canada King City radar image I captured earlier today, you can see those massive bands, or “streamers,” of snow. This is what happens when cold, dry air is whipped across the relatively warm lake water. Lake Huron and Georgian Bay (and to a lesser extent Lake Simcoe) are large bodies of water and they hold their heat longer than land. They have a long “thermal memory” so to speak. To the west of us is a very cold air mass. Cold air holds less water vapour than warm air, it has a smaller vapour capacity. (This is why the tropics are so humid and the arctic is so dry). As this air is carried east over the warm lake water it is warmed up. Since it is still very dry it allows for rapid evaporation of the lake water and, much like a sponge, it soaks up a whole lot of moisture. The air is then forced aloft as it hits the shoreline (orographic lifting) and cools adiabatically (which means by expansion) where it is forced to shed its water vapour resulting in huge bands of snow on the leeward side of the lake.

This means that if you live to the east or southeast of a large body of water you are most likely to experience higher snowfall totals at the start of the winter season than you normally would. At least until the lakes freeze over. Since wind occurs most often from the west, folks living on the eastern shores of the Great Lakes are all too familiar with lake effect snow.