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Intermountain West Central Area 

The intermountain west central area includes the Great Plains region. This region is located east of the Cascade and coastal ranges, west of the Mississippi Valley, and north of the south-west desert area. The climate is generally cold and dry in the winter, and warm and dry in the summer. Most of the region is semiarid. The western mountain range, which acts as a climatic barrier, has an extreme drying effect on the air in the westerly circulation.

Maximum rainfall occurs in the spring and is due mainly to the predominance of cyclonic storm passages during this season. In midwinter a cold high is generally centered in this region which prevents the possibility of storm passages. Annual precipitation is normally light.

Figure 6-7-3.—United States weather regions.

Southwest Desert and Mountain Area

The southwest desert and mountain area in-cludes lower California and some of southeast California as well as the southern portions of Arizona, New Mexico, and Texas. It is an area almost completely surrounded by high mountains and is either very arid or actual desert. Annual rainfall seldom exceeds 5 inches. The more northerly sections have cold winters, and all parts have extremely hot summers. The chief flying hazard results from a predominance of summer and spring thunderstorms caused mainly by maritime tropical air being forced aloft at the mountains. For this reason nearly all signifi-cant peaks and ranges have thundershowers building over them in the spring and summer. The thunderstorms are generally scattered and are almost always severe; however, pilots can usually avoid them by circumnavigating them.

Central Plains Area

The central plains area includes the con-tinental climate regions of the Great Plains, Mississippi Valley, and Appalachian Plateau between the Rocky Mountains to the west, the Appalachians to the east, and the gulf states to the south. The western section is generally drier than the eastern section. The main weather hazards are caused by wintertime outbreaks and associated wave phenomena along polar fronts. Convective air-mass thunderstorms, which are prevalent over this area in summer, also pose a threat to flying. Frontal passages, both cold and warm, and associated weather are common in this area. Thunderstorms are usually of convective origin and are most violent if they have developed in maritime tropical air. This occurs often in the spring, and tornado activity becomes a climatic feature due to its frequency.

Southeast and Gulf States Area

The southeast and gulf states area includes all the states bordering on the Gulf of Mexico as well as South Carolina and Georgia. Stagnating south-bound cold fronts, rapidly moving squall lines, air-mass thunderstorms, and stratus clouds occur in various combinations to make this area an especially complex one for the forecaster. Frontal passages can be expected only in the late fall, winter, and early spring. A circulation phenomenon known as gulf stratus affects this area. In the winter, when the circulation near the surface i  southerly, the warm, moist gulf air is cooled from below to saturation. When this occurs, fog and the gulf stratus may form and may persist over the area for several days. The southerly circulation in summer causes warm, moist air to be heated from below, and convec-tive thunderstorms are common. Since the air is generally quite moist and unstable, these storms are generally severe.

North Atlantic Coastal Area

The North Atlantic coastal area is an area of storm track convergence, and cyclonic storm activity is frequent in winter. Moreover, these storms are intensified by the heating and addition of moisture to the air over the Great Lakes. The lake effect is directly accountable for the large amounts of snowfall often found over this area in the winter. Generally good weather prevails in summer due to the predominant influence of the Bermuda high.

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