Lake-effect snow risk looms for Ontario
Meteorologist
Friday, November 11, 2016, 8:05 PM - The present warmer-than-average surface temperatures of the Great Lakes will make it much easier for lake-effect snow to develop in southern Ontario once an Arctic airmass moves over the region.
This behaviour was addressed last week in a separate article I wrote and can be found here.
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If you’ve taken a look at your 7-day forecast recently, you may have noticed that overnight temperatures by the end of this week will be at or near the freezing mark for much of southern Ontario. Given that the lakes are still 12 to 13 C, are those overnight low temperatures cold enough to allow for lake-effect snow to develop?
In theory, yes. But will it be a high-impact event? Probably not.
I claim this because although having a large temperature difference between the air over the land and the lake is the key that initially turns on the lake-effect engine, it is not by itself the driver that shifts snowfall accumulations into high gear. In order for that to happen, a few more crucial parts would need to be added and work smoothly together, which I’ll endeavour to cruise through with you here.
So buckle up, put on some good tunes, and enjoy the ride as we navigate our way through the complete science behind lake-effect snow and leave this rather extended introduction and metaphor in the proverbial dust!
The complete science behind lake-effect snow:
For the sake of completeness and accuracy, it is first necessary to clarify my previously mentioned statement regarding the empirically tested 13˚C temperature difference required for lake-effect snow to develop. This is not the temperature difference between the air over the surface of the water and the air over the surface of the land, but rather the difference in temperature between the air over the surface of the water and the air roughly 1.5 km above the surface of the land.
While it may seem like a minute (or rather arbitrary) detail, it is clarified here to highlight that there can be HUGE differences in air temperature between the surface and 1.5 km above it.
Take the image below as an example, courtesy of WeatherBell, that shows the American (GFS) model’s forecast temperature for Friday overnight into Saturday 1.5 km above the surface of southern Ontario:
Yes, you’re reading that right. Temperatures here are forecast to range between -8 C and -11 C (that overnight low of 0 C isn’t looking so bad now is it?).
So with Lake Ontario’s surface temperature averaging 11 C as of Nov. 7, this would generate a temperature difference of over 20 degrees! But as I hinted at earlier, there are a few other contributing factors that need to be considered when addressing the likelihood of lake-effect snow formation.
The important role of winds
Even with a temperature difference exceeding 20 degrees, lake-snow will not develop without properly aligned winds.
In the simplest terms, lake-effect snow forms when the wind blows cold air over a relatively warmer large body of water. As it does, moisture from the surface of the lake evaporates, rises, then condenses, forming clouds and ultimately snow – all in that order.
In order for this process to generate significant lake-effect snow, the winds must blow roughly in the same direction at the surface as they do 1.5 to 2 km above it (give or take 30 degrees). This creates a unidirectional wind profile in this layer of the atmosphere, which maximizes the amount of moisture that can be evaporated and transported by the wind.
Moreover, empirical evidence shows us that these unidirectional winds must also be able to blow unimpeded across at least 100 km of open water. This occurs most easily when the winds are aligned parallel to the longest fetch of a Great Lake (i.e. when northwesterly winds blow across Lake Huron/Georgian Bay, southwesterly across Lake Erie, and either easterly or westerly across Lake Ontario).
But that’s not all!
Even IF the winds are favourably aligned, and the temperature difference is sufficient, there is one final piece of the puzzle that needs to fit into place before we get significant lake-effect snow: there needs to be sufficient moisture in the lower and mid-level regions of the atmosphere to saturate the Dendritic Growth Zone (DGZ).
(If that sounded Greek to you, rest assured you’re not alone!)
A simpler translation is as follows: there needs to be enough water vapour available in the very specific region of the atmosphere where the biggest, fluffiest snowflakes preferentially form.
The DGZ is a region where dendrites, a specific type of snowflake, form when temperatures are between -12oC and -18oC. However, if minimal moisture is available where the atmosphere has that temperature, these specific snowflakes will not form, leading to significantly less snowfall accumulation at the surface.
While it is certainly possible (and indeed likely) for a few centimeters of lake-effect snow to develop even if this particular condition is not met, you can be assured that the most impactful lake-effect snow events occur when it is.
The end of the week
Although we’ll have a significant temperature difference between the lakes and the air 1.5 km above Southern Ontario, it is very unlikely at this point in time that we’ll get enough moisture into the DGZ to warrant a high-impact lake-effect snow event.
What we do know with a higher degree of confidence is that the winds will be blowing from the north-northwest Friday into Saturday. So in addition to a significant temperature drop, anyone adjacent to the shores of Lake Huron or Georgian Bay should be on the look-out for a few centimeters of lake-effect snow.
This caveat applies to those downwind of these bodies of water as well, including cities further south like Orangeville and Markdale as examples.
For those of you feeling left out along those other bodies of water, don’t worry, you should get your turn before the end of the month!
