Low pressure begets snow
Much of the world's weather revolves around cyclones, systems characterized by winds that rotate counterclockwise and precipitation. Such a system, seen as a big "L" with isobars (lines of equal air pressure) around it on a weather map, can also be called a "storm" or "low." In the United States, winter weather often is marked by specific type of low pressure systems named mid-latitude cyclones, which form between about 30-degrees to 50-degrees north latitude.
In the winter, a mid-latitude cyclone is usually attached to a cold front that extends to the southeast from the center, and a warm front that extends to the east. The airflow around low pressure means that cold air is pulled in behind the cold front, and warmer air is pulled northward, resulting in a warm front to the east.
The coldest air (and most snow) in such a system usually occurs to the north and northwest of the low. South of that, a mix of rain, sleet and snow is common, as temperatures shift just behind the front. Ahead of the cold front, the air is warmer and lifted by advancing colder, denser air, often producing thunderstorms. Along and ahead of the warm front, the air is typically too warm to generate snow, sleet or freezing rain.
The Pineapple Express
In North America, meteorologists use colorful names to identify common winter storm systems. The Pineapple Express, for example, is a low-pressure system that forms on the branch of the jet stream that begins near Hawaii. The jet stream flows northeastward, contacting the U.S. coast near Washington state, and bringing rain to much of the region and snow to the mountains just inland.
Near Alberta, Canada, north of Montana, a low can form and race south through the northern plains and into the Great Lakes region. Because of their speed and source, such storm systems are dubbed Alberta Clippers. These can trigger the renowned "Lake Effect" snows endemic to the Great Lakes region, as the storm picks up moisture from the large, relatively warm lakes and dumps it as snow on the leeward side.
Originating in the Pacific Northwest, "Colorado Lows" churn toward the Rockies. If they retain enough of their strength, these systems might take advantage of reduced friction and moisture from the Gulf of Mexico and spawn big snowstorms with gusty winds in places like Kansas City and St. Louis, Missouri, and Indianapolis, Indiana. The tail end of the cold front may push into North Texas, a phenomenon known as a Blue Norther.
Nor'easters often plague the eastern seaboard. In this system, a cold front and jet stream plunge southward to the Gulf of Mexico. If a low forms along the front, it could begin to move up the coast, strengthen off the Carolinas and then, replenished with Atlantic moisture, dump tons of snow on Washington, D.C., the mid-Atlantic states, New York and New England.
Calculating the direction and speed of the jet stream -- and mindful of well-known winter systems -- meteorologists try to forecast winter storms. For example, a low pressure system starting near Washington state may dip down to the Great Lakes and Ohio Valley. Places like Chicago and Saginaw, Michigan, could see both rain and snow, while Green Bay, Wisconsin, and Grand Rapids, Michigan get snow. In Indianapolis and Pittsburgh, Pennsylvania, there could be thunderstorms.
Small change, big difference
Variations arise when the jet stream doesn't act as expected. For instance, if the low digs further south -- down the Tennessee Valley and into the Carolinas -- Chicago might face heavy snow, with Indianapolis getting snow instead of thunderstorms. Nashville, Tennessee, not in the previous forecast, could get a mix of rain and snow, as Fayetteville, North Carolina goes from sunny skies to thunderstorms.