When the days begin to grow shorter and the sun sinks lower in the sky, weather forecasters turn their attention from the tropical systems of late summer to the winter storms of late fall and winter. Meteorologists call these storms "mid-latitude cyclones" because of where they form and the spinning low-pressure systems (cyclones) that form their core. Pushed along by the invigorated jet stream, they can cover many thousands of square miles and paralyze travel and commerce across dozens of states.
Watching the weather on TV, it can be interesting to compare the satellite images of winter storms with those of hurricanes. Although winter systems are larger and less symmetric, they still spin counterclockwise and in extreme cases can even have a sort of "eye" like a tropical storm. But a winter storm is a different creature. Whereas a hurricane gets its energy from the warm surface waters of the tropical oceans; a winter storm's primary source of energy is the contrast between cold and warm air masses. As the sun angle decreases and the days grow shorter, the air at northern latitudes gets colder and colder. Whenever this cold air finds itself in close proximity to warmer and more humid air, the atmosphere develops a sort of instability that can be converted into kinetic or wind energy. In fact, as you watch the weather maps, you'll notice that these low-pressure areas are usually connected to frontal boundaries- where contrasting air masses come together.
Winter storms tend to develop in favored areas. An especially frequent area to see these storms spin up is just east of the Rockies. Sometimes storms come all the way from the Pacific and are "reborn" on the high plains; sometimes they simply develop right there east of the mountains. Still other "Alberta Clipper" storms develop in western Canada and zip southeast toward the Great Lakes. Some of the most dramatic winter storms develop off the northeast coast, where cold air meets the warm Gulf Stream and can lead to such rapidly deepening cyclones that meteorologists call them "bombs".
Another frequent "birthing ground" for winter storms is in the vicinity of southeast Texas. These storms frequently swing northeast and affect the Heartland with rain, ice or snow.
The motion and intensity of winter storms is greatly influenced by the jet stream, which becomes much stronger and faster during the colder part of the year. Strong dips or "troughs" in the jet stream help the storms grow, especially where divergent winds aloft help the atmosphere to rise and low-pressure areas at the surface to deepen. Sometimes these upper-level troughs dip so far south they separate from the main jet stream flow and meander slowly across the country as a "cut-off" low. These slow moving whirlpools of cold air frequently bring heavy wet snow to the Midwest in the late winter and early spring.
One of the fascinating features of these winter storms is the tremendous variety of weather they can bring. A strong mid-latitude cyclone can be associated with severe thunderstorms and tornados to the south, and blizzard conditions to the north. In between, close-together areas of rain, freezing rain, sleet and snow can mean a real headache for forecasters. In fact, almost every year one or more of these winter storms will spin its way into the Mississippi and Ohio Valleys, bringing a variety of dangerous weather conditions to the KFVS12 viewing area. With the mild "La Nina" pattern of the last couple years this winter could prove to be our most interesting in quite a while.