How Geography Shapes Regional Weather of the United States

The Pacific Northwest

States:

• Oregon
• Washington
• Idaho
• Canadian province of British Columbia

Often recognized for the cities of 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 two thirds 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

As its name suggests, this region is between mountains. The Cascade and Sierra Nevada chains sit to its west, and the Rocky Mountains sit to its east. It includes the Great Basin region, which is largely a desert due to the fact that it lies on the leeward side of the Sierra Nevadas and Cascades which block pacific storms from bringing moisture there.

The Intermountain West's northern portions include some of the nation's highest elevations. You'll often hear of these locations having the nation's first snowfalls of the fall and winter seasons. And during summer, hot temperatures and 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 sits at the nation's interior. The Rocky Mountains lay at its western border, and a vast prairie landscape extends eastward to the Mississippi River.

The region's reputation for dry winds that come sweeping down can be easily explained by meteorology. By the time moist pacific air from the coast crosses the Rockies and descends east of them, it is dry from having repeatedly precipitated its moisture; it is warm from having lowered (compressed), and it is fast-moving from having rushed down the mountain slope.

When this dry air clashes with warm moist air streaming upward from the Gulf of Mexico, you get another event the Great Plains is famous for; storms.

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, bringing an increased 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 are a constant source of moisture. During winter months, they cause the localized heavy snowfall events known as lake effect snow.

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 Tennessee, North Carolina, Virginia, and West Virginia parts.

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. on the contrary, locations on the lee, or east, or the mountain range (such as Western North Carolina) receive lighter precipitation amounts due to being located in a rain shadow.

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

The Mid-Atlantic and 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 its 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.