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U.S. Geographic Regions

Terms Commonly Used in Weather Forecasts


Ever consider how difficult it would be to discuss weather without mentioning where it is? Without geography, weather would be lost...in more ways than one. Not only would there be no identifiable locations for relaying storm position and track, but there would be no mountains, no ocean to interact with the air and modify weather on a local scale. (This local land-air interaction is known as mesoscale meteorology.)

Let's examine some of the United States' most mentioned geographic regions, the weather for which each is recognized, and the topography that shapes those weather conditions.

The Pacific Northwest


States: Oregon, Washington, Idaho, Canadian province of British Columbia

Often recognized for cities such as Seattle, Portland, and Vancouver, the Pacific Northwest extends inland from the Pacific Coast to the eastern Rocky Mountains. The Cascade Mountain Range divides the region into two climate regimes--one coastal and one continental.

West of the Cascades, an abundance of cool, moist air flows freely inland from the Pacific Ocean. From October to March, the jet stream is oriented directly over this corner of the U.S., ushering Pacific storms (including the flood-inducing Pineapple Express) across the region. These months are considered to be the region's "rainy season," when nearly 2/3 of their precipitation occurs.

The region east of the Cascades is referred to as the interior Pacific Northwest. Here, annual and daily temperatures are more varied, and the precipitation only a fraction of that seen on the windward side.

The Great Basin/Intermountain West


States: Oregon, California, Idaho, Nevada, Utah, Colorado, Wyoming, Montana, Arizona, New Mexico. The "Four Corners" is included.

As its name suggests, this region is between mountains--the Cascade and Sierra Nevada chains to the west, and the Rocky Mountains to the east. It includes the Great Basin region, a desert region due to the fact that it lies on the leeward side of the Sierra Nevadas and Cascades. (Pacific storms are blocked, resulting in the rain shadow effect.)

The Intermountain West's northern portions include some of the nation's highest elevations. These locations are often featured in weather forecasts for having significant snowfall, including the nation's first snowfalls of the fall/winter season.

During the summer, all areas experience hot temperatures. Storms associated with the North American Monsoon are frequent in June and July.

The Great Plains


States: Colorado, Kansas, Montana, Nebraska, New Mexico, North Dakota, South Dakota, Oklahoma, Texas, Wyoming

Known as the "heartland" of the United States, the Great Plains is situated at the nation's interior. The Rocky Mountains lay at its western border and a vast prairie landscape extends eastward to the Mississippi River. As the last mountain barrier of the west, the Rockies filter marine Pacific air that originates from the coast. By the time the air descends east of the Rockies it has crossed a succession of mountain ranges, is dry from having repeatedly precipitated its moisture, warm from having lowered (compressed), and fast moving from having rushed down the mountain slope.

Thanks to the warm moist air that streams upward into the central U.S. from the Gulf of Mexico, the Great Plains is also a severe storms "hot spot."

The Mississippi, Tennessee, and Ohio Valleys


States: Mississippi, Arkansas, Missouri, Iowa, Illinois, Indiana, Kentucky, Tennessee, Ohio

The three river valleys are somewhat of a meeting ground of air masses from other regions, including Arctic air from Canada, mild Pacific air from the West, and moist tropical systems streaming up from the Gulf of Mexico. These dueling air masses lead to frequent severe storms and tornadoes during the spring and summer months, and are also responsible for ice storms during the winter season.

During hurricane season, storm remnants routinely travel here, increasing the risk of river flooding.

The Great Lakes


States: Minnesota, Wisconsin, Illinois, Indiana, Ohio, Pennsylvania, New York

Similarly to the Valley region, the Great Lakes region is a crossroads of air masses from other regions--namely Arctic air from Canada and moist tropical air from the Gulf of Mexico. In addition, the five lakes--Erie, Huron, Michigan, Ontario, and Superior--for which the region is named, act as a direct moisture source for the region. During winter months, they cause the localized heavy snowfall events known as lake effect snow (LES).

The Appalachians


States: Kentucky, Tennessee, North Carolina, Virginia, West Virginia, Maryland

The Appalachian Mountains extend southwestward from Canada into central Alabama, however, the term "Appalachians" most commonly refers to the mountain chain's central and southern regions, which includes Tennessee, North Carolina, and The Virginias.

As with any mountain barrier, the Appalachians have varying effects depending on which side of it (winward or leeward) a location lies. For areas located on the windward, or west, (such as east Tennessee) precipitation is increased; whereas locations on the lee, or east, (such as Western North Carolina) receive lighter precipitation amounts due to being located in a rainshadow.

During winter months, the mountains contribute to unique weather events such as cold air damming and northwest (upslope) flow.

The Mid-Atlantic & New England


States: Virginia, West Virginia, D.C., Maryland, Delaware, New Jersey, New York, Pennsylvania; Connecticut, Massachusetts, New Hampshire, Rhode Island, Vermont

This region is largely influenced by the Atlantic Ocean which borders its east, and by it's northern latitude. Coastal storms, such as nor'easters and tropical cyclones, regularly impact the Northeast, and account for the region's main weather hazards--winter storms and flooding.

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