Denali Borough, Alaska Climate Due to its large size and wide range of geographic features, the United States contains examples of nearly every global climate. The climate is subtropical in the Southern United States, continental in the north, tropical in Hawaii and southern Florida, polar in Alaska, semiarid in the Great Plains west of the 100th meridian, Mediterranean in coastal California and arid in the Great Basin and the Southwest. Its comparatively favorable agricultural climate contributed (in part) to the country's rise as a world power, with infrequent severe drought in the major agricultural regions, a general lack of widespread flooding, and a mainly temperate climate that receives adequate precipitation. The main influence on U.S. weather is the polar jet stream which migrates northward into Canada in the summer months, and then southward into the US in the winter months. The jet stream brings in large low-pressure systems from the northern Pacific Ocean that enters the US mainland over the Pacific Northwest. The Cascade Range, Sierra Nevada, and Rocky Mountains pick up most of the moisture from these systems as they move eastward. Greatly diminished by the time they reach the High Plains, much of the moisture has been sapped by the orographic effect as it is forced over several mountain ranges. Once it moves over the Great Plains, uninterrupted flat land allows it to reorganize and can lead to major clashes of air masses. In addition, moisture from the Gulf of Mexico is often drawn northward. When combined with a powerful jet stream, this can lead to violent thunderstorms, especially during spring and summer. Sometimes during winter, these storms can combine with another low-pressure system as they move up the East Coast and into the Atlantic Ocean, where they intensify rapidly. These storms are known as Nor'easters and often bring widespread, heavy rain, wind, and snowfall to New England. The uninterrupted grasslands of the Great Plains also lead to some of the most extreme climate swings in the world. Temperatures can rise or drop rapidly, winds can be extreme and the flow of heat waves or Arctic air masses often advance uninterrupted through the plains. The Great Basin and Columbia Plateau (the Intermontane Plateaus) are arid or semiarid regions that lie in the rain shadow of the Cascades and Sierra Nevada. Precipitation averages less than . The Southwest is a hot desert, with temperatures exceeding for several weeks at