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Science Behind The Weather: Winter storm warning

The Know-It-All guide to winter weather terms


Dayna Vettese
Meteorologist

Sunday, January 25, 2015, 12:03 PM - By this point, every Canadian has had a little (or a big) taste of winter. There are many weather terms that start to roll off our tongues this time of year, and you may or may not know what they mean. Here’s a quick primer.

Arctic Outflow

During the wintry months, cold arctic air more than often makes itself known across Canada and northern British Columbia. On occasion, this chilly air will make its way to southern British Columbia and the coast. Some of the coldest temperatures we feel in the winter are associated with high pressure systems. For arctic outflow to occur, a high pressure system starts to drift south from the arctic and situate itself over northern or central British Columbia. Air rotates clockwise around a high pressure system so the cold arctic air swings south and west into British Columbia funneling through the mountains, valleys, and fjords. An Arctic Outflow Warning is issued by Environment Canada when a combination of temperatures and the wind create wind chills of -20ºC or colder for at least six hours.  



Alberta Clipper

Alberta Clipper is the name given to a low pressure system that develops off the Rocky Mountains in Alberta. It is given the name “clipper” because it is a fast moving system, like the clipper ships used in the 19th century, that race across the country. Because clippers don’t have much moisture available to them, they generally only drop light amounts of snow, maybe 5-10 cm on average. Due to its origins, cold air often follows in behind the Clipper system.


Blizzard

We’ve all said it before, “It’s a blizzard out there!” But a lot of us don’t realize we’re actually misusing the term… to a certain degree. We all know what a blizzard is: blinding snow, howling winds. There is a specific criteria, though, that a storm must meet or be anticipated to meet before it’s deemed a blizzard or given a Blizzard Warning. The criteria is that winds must be sustained at 40 km/h or more for at least four hours combining with falling or blowing snow to cause visibilities to be reduced to 400 m or less. Whew, that’s a mouthful but we remember it here by calling it the 4-4-4 rule. There is an exception: areas north of the treeline line in Canada have to have the same conditions but it must last at least six hours to be deemed a blizzard.



Chinook

A chinook is a phenomenon that occurs in Alberta. It is a type of wind and is given the name “Chinook” which has been said to be a Native word for “snow eater” but also the name of the people in the region where the term was used. It’s given this name because it is a warm wind that tends to melt snow. Essentially, upper level flow in the atmosphere forces air to go up and over the Rocky Mountains. As the air descends on the east side of the Rockies, it warms rapidly. The faster the wind speeds, the stronger the Chinook, the greater the temperature jump. Wind gusts during these events can exceed 100 km/h. Observations have shown the temperature in regions affected by Chinook winds can jump from -20ºC to +10ºC in hours. Areas of southwestern Alberta are the most prone to Chinooks but can affect regions as far north as Red Deer and as far east as Medicine Hat and southern Saskatchewan. In 1962, it was recorded that a Chinook caused the temperature in Pincher Creek to rise 41ºC (from -19ºC to +22ºC) in just one hour.



Colorado Low

A Colorado low is a low pressure system that originates in about the Colorado state area. Colorado lows typically track toward the Great Lakes region and Quebec and can bring substantial weather to those areas. As the low moves toward the Great Lakes region, it gathers moisture from the Gulf of Mexico to the south. Colorado Lows are our more moderate to strong low pressure systems in the winter time. Because of its involvement with moisture from the Gulf of Mexico, we can often see varying types of precipitation within the storm where some areas receive rain, others freezing rain and others snow. They can often be fairly windy as well and tend to bring our more moderate to heavy snowfalls for the Great Lakes and Quebec.



Lake/Sea Effect Snow & Snow Squalls 

Not every Canadian experiences sea or lake effect snow but these events can create some of the most extreme snowfall totals and snowfall rates of any winter weather scenario. Lake (or sea) effect snow forms when a cold air mass moves over relatively warm bodies of water. The temperature difference between the body of water and the air mass aloft must be at least 13ºC. The layer of air just above the body of water is warmer relative to the air mass and because warm air is less dense than cold air, it rises and cools. This process causes the formation of clouds and thus falling snow. When these clouds are forced toward land, they encounter friction (since land provides more friction due to things like vegetation, buildings, terrain, than water) and the snowfall rates can then increase. Lake effect snow can form in to narrow bands (snow squalls) and can either be a single intense band or multiple bands. Within the snowsquall, sometimes snowfall rates can reach 10+ cm per hour! You may have heard us use the term “fetch” once or twice and we’re not referring to the movie “Mean Girls”… When we talk about the amount of “fetch”, we talking about the distance the cold air has over the water. When the wind direction lines up perfectly with the longest axis of the lake, let’s say, we have a very large fetch. Sometimes the wind direction is across a lake so the fetch isn’t as great. The longer the fetch, the bigger the temperature difference and the strong the winds, the most intense the snow squall can be. We have similar situations set up but with the sea rather than lakes. For example, western Newfoundland and Cape Breton, Nova Scotia often receive sea effect snow. It forms the same way but the difference is the body of water is the ocean rather than a lake.



The November lake effect snow event in Buffalo is fresh in all of our minds where some areas received about 2 metres of snow (that’s almost 7 feet)!

Nor’easter

A term Atlantic Canadians shudder at…

Nor’easters are the most intense winter storms we can receive and can produce blizzard conditions. The term “Nor’easter” is given to the storm because when it impacts the east coast of North America, the winds over the coastal areas blow from the northeast. A Nor’easter is not just a winter phenomenon but are more intense during the winter months. This, in part, is due to the fact that the temperature difference between the continent and the ocean is much greater in the winter time providing the needed ingredients to strengthen a storm system. Cold Arctic air is transported southward by the Jet stream toward the Atlantic Ocean. In the Atlantic, warm air from the Gulf of Mexico is being transported northward. This is where we get the stark temperature contrast between the ocean and the continent. Nor’easter form along the east coast of the United States and as they track toward Atlantic Canada, they intensify. Sometimes the intensification is so rapid, it’s referred to as a “weather bomb”; this is a real weather term and actually has a very specific definition.


Pineapple Express

No, not the hilarious movie with Seth Rogen and James Franco… We’re talking about an atmospheric river! An atmospheric river is the term given to narrow areas in the atmosphere that are responsible for a vast majority of the horizontal transport of water vapour (outside of the tropics). One such example is the pineapple express. This is the abundant transport of moisture from the Hawaiian tropics (hence the term pineapple) to the west coast of North America generally directly impacting British Columbia. Due to the positioning of the Jetstream, the moisture is transported resulting in warmer temperature but heavy precipitation for the B.C. coast. Alberta is indirectly impacted when this flow causes the formation of chinooks (as described above).



Gulf/Texas Low

A Gulf Low or a Texas Low is a low pressure system that originates, you got it, in and around the Texas/Gulf of Mexico region. The generally track north or northeast either toward the Great Lakes region and Quebec. They tend to be moderate to strong low pressure systems, like Colorado Lows, and bring ample moisture with them from the Gulf of Mexico. Due in part to the connection to the Gulf of Mexico, often times we will see different precipitation types across the storm.

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