How Do Winter Storms Form, and Where?

Winter Storms - One name for many forms of extreme weather.They may differ in name and form, but they are all disruptive.

The topic is a large and complex one. Some types of winter storm occurregularly each season, sometimes more than once, and have beengiven their own names - Alberta Clipper, Nor'easter, Panhandle Hook. Others, just as severe, are not so romantically named.

But rather than describe each type of storm, let's look for some common ground. When you want to find out more about the events which affect you most often, wander on over to The Weather Doctor's pages.

A Few Basic Facts and Principles

I'm sorry to do this to you, but most winter storms have a number of features in common, so let's set the scene.

But first, let's clear up the term "storm". For our purposes, a storm is any disturbed state of the atmosphere affecting the earth's surface, accompanied by unpleasant or destructive weather. In scale a storm ranges from an individual thunderstorm to hurricanes or even larger extra-tropical depressions. Low pressure, usually as an enclosed low, is associated with many of them.

Winter storms fit in towards the larger end of the scale, particularly as far as the area affected is concerned.

Not the best of definitions I guess, but the overall complexity of storms prevents a simple definition

So let's set the stage for storm formation. Here are some basic weather and climate fundamentals.

  • The atmosphere is composed of Air Masses which vary in temperature and the amount of water vapour they contain. They are usually fixed in position during a particular season, although they expand and contract, and some may disappear altogether as the seasons change.

  • In winter in North America, five air masses are present, and four of them interact to control storm development. They are
    1. The maritime tropical (mT) warm moist air mass centred over the southern Atlantic and Gulf of Mexico.
    2. A maritime polar (mP) air mass in the northern Atlantic, consisting of cool moist air.
    3. Another mP mass in the northern Pacific.
    4. And between the two maritime polar air masses, a continental polar mass (cP) - cold and dry. Sitting to the north of this air mass is an even colder continental Arctic body (cA), which has little effect on storm formation.

  • Air masses can expand in different directions, taking the place of part of the neighbouring air mass.

  • The boundaries between air masses are narrow zones where temperature and/or water vapour content may change rapidly. These contacts are called fronts. If the colder air mass is moving into the warmer it is a cold front, and a warm front is the opposite

  • In winter, the most important boundary is the Polar Front. In North America it occurs at the southern contact between the continental polar airmass and some form of the maritime tropical air mass. When conditions are stable it runs east west across most of continental North America.

  • Following this boundary are the fast, high altitude winds of the polar Jet Stream (there is also a subtropical jet stream which barely concerns us here), blowing from west to east somewhere between 7.5 and 10.5km (25,000 to 35,000 ft). By definition the winds in a jet stream blow at 57mph (92kph), but faster zones, called jet streaks, are common.

  • During winter, prolonged cooling of the polar and arctic continental air masses produces a large and growing mass of dense cold air - a high pressure zone. Eventually, as a means of equalizing temperature and pressure, this cold dry air will spill out southwards, forming a wedge, usually over the centre of the continent. Naturally the Polar Front will follow the boundary between the air masses, and so will the jet stream

    The jet stream therefore develops a large loop as it tracks the polar front south around the cold air outburst and back again.

  • Jet streaks anywhere along the path of the jet stream are likely to cause disturbances to the flow of air around them. These disruptions often take the form of closed low pressure cells (cyclones or depressions), from which winter storms form.

  • Many winter storms are propelled and guided by the jet stream, but can move in any direction, although west to east is favoured. The storm's final path will be affected by coriolis effect, which will deflect it to the right of its direction of movement in the northern hemisphere. The storm may also be slowed or blocked by other systems in front of it.

Storm formation is complex enough even within this scenario (and this is by no means the full story), and the factors listed above also apply reasonably well to eastern Europe. But now let's add in the effect of the Rocky Mountain chain.

Many of the surface disturbances develop over the northern Pacific and move east to the North American mainland. But when they hit the Rockies, they are forced up and around, losing much of their moisture content and energy as rain and snow is dumped. Many retain enough of their character to reform on the eastern side of the mountains, and start all over again as a significant storm. (Europe has no significant chains of north - south mountains to interrupt the movement of weather systems.)

So during winter we have a group of lows forming in the northern Pacific, reforming over or past the Rockies, and moving on to affect the northern and northeastern USA and Canada.

A second group of winter storms forms in the Gulf of Mexico and western Atlantic.These storms track up the eastern side of the USA, varying from inland of the Appalachians to 200-300 miles (300-500km) out to sea.

Of course these two groups can find many ways to interact, and that is often when things get really interesting.

Winter Storms with a Western Origin

Eastern Pacific storms have two sources. Many of them originate in the relatively warm maritime polar airmass, particularly in the Gulf of Alaska. High mountains around the Gulf restrict these Gulf of Alaska Lows, causing them to move off to the south to ultimately cross the coast anywhere between Canada and California. Perhaps helped along by the jetstream, they are forced to rise over the Rockies, dumping their contained moisture as they cool, as either rain or snow.

The second source is more a means of moving weather than a storm in itself. It is associated with the subtropical jetstream and because it arrives from the direction of Hawaii, is called the Pineapple Express. This fast moving stream of warm, very moist air can meet the west coast anywhere from southern Canada to California, and is capable causing very high rainfall and snow as it rises over the Rockies.

Mix the Pineapple Express with a Gulf of Alaska Low and it's time to dust off the record books.

In crossing the Rockies these storms have lost most of their moisture, but still have the capacity to reform. Although this can happen anywhere, there are two main centres for regeneration - Alberta and Colorado.

Alberta Clippers

Gulf of Alaska Lows form rapidly and follow each other quickly, and so do Alberta Clippers. The name comes from the fastest of the nineteenth century sailing ships. They form around the Alberta plains and head off along the polar front, generally passing into Montana or North Dakota before heading east towards the Great Lakes and New England and the Maritime Provinces of Canada.

Because they have lost most of their moisture and have little access to more, they are rarely responsible for heavy rain or snow - 2 to 4 inches on the northern side of their path is about the norm. They are usually followed by very cold northerly winds, and their rapid generation can result in some long spells of rather miserable weather.

Alberta Clippers can result in heavy Lake Effect snows on the lee sides of the Great Lakes, and although they are otherwise not usually responsible for much new snow, their strong winds can cause blizzards as previously fallen snow is picked up and moved around.

Colorado Lows and Panhandle Hooks

Pacific storms often reform around Colorado. When the jetstream is in its normal position and travelling east around the Canadian border, these and other winter storms forming in the southern Rockies will normally track east, pick up some moisture from the Gulf of Mexico, and cause a little rain and rarely snow.

But when the jetstream loops south, the Colorado Lows and the related Texas Panhandle Hooks head off to the north or northeast and have the potential to cause serious problems.

They follow the polar front and jetstream north, picking up plenty of moisture from the gulf maritime air to the east. Circulation around the low pressure center brings strong northerlies, including the Blue Norther of southern states, with potentially large snowfalls to the west of the storm's path. Characteristic paths for these large winter storms include the Ohio valley, and strong winds and large snowfalls for the Chicago - Great Lakes area are possible.

These storms are also likely to move on to New England and the Canadian Maritime Provinces.

East Coast Storms

A large group of winter storms affecting the east, and particularly northeast Atlantic coast of the USA and Canada are known as Nor'easters, after the direction of the winds bringing the worst of the rain, strong winds, and snow.

In keeping with the rule that nothing is simple, Nor'easters can form in several ways, have a wide range of paths ranging from inland to well offshore, and have the potential to virtually explode into devastating events ("The Perfect Storm")

One group of storms forms in the Gulf of Mexico or nearby western Atlantic. Depending on source and location of mid continent highs or jetstreams, these storms can move along the western margin of the Appalachians, along the coastal plain, or anywhere within a few hundred miles off shore.

Typical of winter storms, snow is likely to fall to the left of the storm's track, with rain more likely to the right. Storms moving north over the ocean will usually produce snow in coastal areas (to their left) as they are continually picking up moisture from the relatively warm sea surface, often including the Gulf Stream. Other storms form off Cape Hatteras and move in the same way.

Some Nor'easters, particularly those intensifying off New England or eastern Canada, may be the remnants of Alberta Clippers, Colorado Lows or Panhandle Hooks. Normally these systems will move off rapidly to the east to be seen no more, but other scenarios are possible.

If, for example, a Colorado Low runs into a Nor'easter moving north along the coast, or even an extra-tropical low moving in from further out in the Atlantic, an extremely powerful storm can grow very rapidly. Winter storms such as these are responsible for huge dumps of snow from New York northward, hurricane force winds, and huge seas accompanied by storm surges. These weather bombs or bomb cyclones can form in other ways, but their characteristic is rapid development of a very powerful storm.

Conclusion

Winter storms are a great field for observation and study by the owner of a home weather station.

For a start, no matter where you live in North America, you would expect to see several reasonable winter storms each year between the beginning of fall and the end of spring, whereas a hurricane or severe thunderstorm may be rare or nonexistent in many regions.

Secondly, most winter storms take a while to build up, pass through and decline, giving plenty of time to assemble records.

Thirdly, with the sort of weather likely in most winter storms you probably will be staying home with enough time on your hands to indulge in a bit of weather study.

And finally, depending on where you are in relation to the storm's path, the sequence of weather and trends in observed data may be quite different even though the type of storm is the same.

Additional Resources

The topic of Winter Storms covers a wide variety of weather patterns and types of threatening, if not Severe, Weather. While many websites cover warmer weather events like hurricanes and tornadoes, winter weather doesn't attract quite the same coverage.

Still, The Weather Doctor provides a large number of informative and entertaining articles on many aspects of winter weather. For an overall, concise view work your way through the weather section of USA Today. Back to the Top, or return to the Home page.


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Last update 05/28/2011