February 1, 2013
By Thomas A. Horne
Snowmageddon covered the East Coast during a 2010 storm.
It’s November 2012 as I write this, and winter weather has already made its first bold appearance in the form of Superstorm Sandy. Sandy began as a tropical depression south of Jamaica on October 22, became a tropical storm, went to hurricane status before crossing Cuba, lost some of its punch as it went through the Bahamas, then reintensified to hurricane status by October 28. By this time it was well east of North Carolina’s Outer Banks. But then the storm was drawn into the circulation of a huge upper-level trough over the eastern United States. This, along with the blocking effects of high pressure centered over Greenland, made the storm turn left, ultimately making landfall along the New Jersey shore.
What does any of this have to do with snowfall? Well, the storm’s counterclockwise circulation drew in a lot of cold air from that Greenland high. And while colder, drier air usually means the beginning of the end of a hurricane, in this case it meant that the cold air was cold enough to cause snow to fall along the Appalachians. This is a good example of wraparound precipitation mechanics. Take air that’s cold enough, let it get sucked around the north side of a low that’s full of precipitation, and presto: snow from a hurricane.
This example is interesting because it illustrates how nor’easters along the East Coast can create widespread snowfall. Wraparound moisture is the big reason why many airports east of the Appalachians can be closed by winter weather. Wraparound precipitation can be easily observed by locating high cloud tops forming in a dense comma moving around a center of low pressure. Often, those lows are off Cape Hatteras, which explains why some call them “Hatteras lows.” If you loop a satellite image, you’ll see that those clouds are moving counterclockwise, or trending that way. If your proposed flight path is in the line of fire, you’ve been warned.
Nor’easters—so called because their worst weather comes on winds out of the northeast—are but one type of major winter snowmaker, and they affect areas from North Carolina to the Canadian Maritime provinces. Here are some other areas to watch for snow trouble:
Pacific Northwest. Very often, North America’s biggest snow systems enter the United States in or near Washington state, then travel across the width of the nation. In the days before landfall, keep an eye on satellite imagery of the northern Pacific. See any large cloud bands travelling in the telltale comma shape? Looping the imagery will show the counterclockwise motion that signals any big low-pressure system.
Colorado. Another birthplace of major snow systems is to the east of the Rocky Mountains. From there, parent lows tend to take up tracks that run through the Great Lakes states, maturing along the way.
Western Canada. Alberta clippers—fast-moving snow showers ahead of fronts emanating from lows in their namesake province—are better known for wind than heavy snows, but they spell trouble just the same. The blizzards (defined as snowfalls accompanied by wind speeds of 30-plus knots and visibilities lower than one-quarter mile at the surface) that affect the northern tier of states are frequently caused by Clippers.
Gulf of Mexico. What? Snow storms from the sunny Gulf? True. They may not produce snow in the Gulf, but the low pressure centers that form down there have a habit of travelling up the spine of the Appalachians all the way to New England, spreading snow along the way.
Great Lakes. We’ve all heard of lake effect snow, and the areas downwind of the Great Lakes are home to major snows when cold air flows over the comparatively warmer lakes, picks up moisture, crystallizes it into snow, then dumps it on lee shores.
Any upsloping terrain. Any system encountering mountains will have some extra vertical energy imparted to it by winds riding upslope—enough energy to freeze moisture at altitude and have it fall as snow.
Conveyor belts. What do all these types of winter storms have in common? The same elements in any low-pressure system: a parent low; warm and cold fronts issuing from it; and an adequate source of moisture. (The Canadian snow systems are the most deprived of moisture, because no large bodies of water are nearby, which explains why they generally produce little snow.) To this mix let’s add two other important elements. Of course, one is temperatures cold enough to create snow. The other is upper-level support in the form of lifting forces generated by a core of high-speed winds embedded in jet streams.
The conveyor belt model is often used to help explain the mechanics of major snow events—especially those occurring in the northeast United States. In this model a mass of cold air enters the low pressure system to the north, paralleling the surface cold front. Out of the south, a mass of moist, warmer air flows into the warm sector of the frontal system, then up and over the cold conveyor belt’s air. Sound like a warm front to you? Me, too.
Precipitation falling into the cold conveyor belt saturates the air mass, and the low pressure’s lifting forces draw the cold air upwards and turn it into the main circulation. Now you know how the comma-shape cloud signature comes about.
Meanwhile, an intrusion of dry air enters the system from the west. It’s visible as a dry slot on water vapor imagery, and it’s found along the cold frontal boundary. But it doesn’t just remain there. It rides up over the warm conveyor belt, where its cold air aloft can create the kind of instability that causes elevated convection—thunderstorms—as the warmer air rises into the colder air mass. Cold air over warm is always a recipe for instability and convection. In the winter months, thundersnow (snowfall accompanied by lightning and thunder) is a feature of the more well-developed snowstorms.
Routinely check satellite imagery (visible, infrared, and water vapor) to locate any spots likely to have winter weather. Looping the imagery can help you determine if a system is intensifying, and can hint at future tracks. And remember: No matter how balmy they may sound, winter warm fronts have the worst flying weather. The adverse weather covers a larger area, freezing rain is likely, and any precipitation falling near the surface can reduce visibilities to low-IMC values because of evaporative cooling.
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AOPA thanks our members for their continued support in protecting the freedom to fly.