Snow squall warning in effect for parts of Cape Breton
Buffalo, New York has been in the news over the past few days and with good reason. It's not often that a year's worth of snow falls in just a few days. The mechanism that has buried Buffalo in the white stuff is referred to as lake effect snow.
Believe it or not we here in the Maritimes can see a very similar weather event and it is in fact occurring both today and likely for tomorrow (Though nowhere near as intense as what was experienced in and around Buffalo; I should be clear about that).
Instead of lake effect snow in the Maritimes we typically see sea effect snow.
So how and why does this happen? It has to do with convection, or the movement of air vertically due to differences in buoyancy.
From time to time a very cold mass of air will break down out of the Arctic. As this cold air mass moves east it moves over top of the bodies of water around the Maritimes (Bay of Fundy, Northumberland Strait, Gulf of St. Lawrence etc.) The temperature of these bodies of water is much warmer than the cold air above them. This means that the layer of air above the ocean waters is also warmer and contains more moisture than the cold and dry air moving above.
Moist, warm air is less dense and therefor more buoyant than cold, dry air. The warm, moist air begins to rise up through the colder air where it cools until it reaches its saturation point, condensing into clouds and fuelling the development of snow. This snow (in the form of flurries or more intense snow squalls) is then moved over land by the prevailing winds.
What is the likelihood?
There is actually a checklist of sorts available for analyzing the likelihood of lake or sea effect snow including, but not necessarily restricted to:
1) First the air at a pressure level of 850 mb in the atmosphere (about 1.5 km up) must be at least 13 degrees colder than the sea or lake surface water temperature. The greater the difference in temperatures the more intense the snowfall is likely to be.
2) You need an appropriate amount of wind shear. Wind shear is the change in direction of the winds as you move vertically upwards in the atmosphere. For a sea/lake effect snow event this shear is typically analyzed between the surface and 700 mb (about 3 km up). A wind shear of 30 degrees or less is typically associated with more intense snowfall.
3) You need the right amount of fetch. What is fetch? That is simply the distance the colder air is being blown across the warmer waters below. Too short of a fetch and there is insufficient time for enough moisture to be brought up into the atmosphere to produce a heavy snow event. Fetch played a very important role in what happened in Buffalo as the cold winds traversed Lake Erie nearly perfectly from southwest to northeast, maximizing the fetch and amount of moisture lifted.
With cold air in place for both today and tomorrow, sea effect snow in the form of flurries and snow squalls is being (and will) be seen in Nova Scotia and P.E.I. The greatest potential for more intense snow squalls for both today and Saturday is in Cape Breton.
In particular both Inverness and Victoria county will be vulnerable to this type of weather and Environment Canada has issued a snow squall warning for Inverness County and a watch for Victoria County (part of the reason for this is the Highlands, topography plays a big role — especially with something like the Highlands that adds extra lift to the whole situation).
The bands of snow squalls are often very thin, but very dangerous as you can go from blue skies to white out conditions with the space of just a few kilometres.
Care should be taken on the Cabot Trail today and tonight. The potential for sea effect snow should gradually ease through the day on Saturday.
The attached image feature shows the general direction of the flurries/squalls as they move in as well as the "check list" for sea effect snow.
