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Colorado Climate Center -Climate of Colorado masses can cause a heavy fall of snow and the most severe of al! weather conditions of the high plains, the blizzard. This cold air is frequently too shallow to cross the mountains to the western portion of the State so while the plains are in the grip of a very severe storm, the weather in the mountains and western valleys may be mild. Occaslonalty, when the plains are covered with a shallow layer of cold air, strong westerly winds aloft work their way to the surface. Warmed by rapid descent from higher levels, these winds bring large and sudden temperature rises. This phenomenon is the "chinook;' of the high plains and temperature rises of 25 to 35°F within a short time are not uncommon. Chinook winds greatly moderate average winter temperatures in areas near enough to the mountains to experience them frequently. Due to these wind patterns, some locations in the eastern foothills are warmer than adjacent areas on the eastern plains on many days during the winter. Warm, moist air from the south moves into Colorado InfrequenHy, but most often in the spring, summer and early autumn. As this air is carried northward and westward to higher elevations, the heaviest and most general rainfalls (and sometimes wet snows) occur over the eastern pofions of the State from April through early September. For southern and western Colorado, the intrusions of moist air are most common from mid Juty into September associated with wind patterns sometimes called the Southwest Monsoon. Frequent showers and thunderstorms continue well into the summer. At times during the summer, winds shift to the southwest and bring hot, dry air from the desert Southwest over the State. Such hot spells are usually of short duration. CLIMATE OF THE EASTERN PLAINS The climate of the plains is comparatively uniform from place to place, with charactedstic features of low relative humidity, abundant sunshine, infrequent rains and snow, moderate to high wind movement, and a large daily and seasonal range in temperature. Summer daily maximum temperatures are often 95°F or above, and 100°F temperatures have been observed at all plain stations. Such temperatures are not infrequent at altitudes below 5,000 feet; above that elevation they are comparativey rare. The highest temperatures in Colorado occur in the Arkansas Valley and lower elevations of South Platte and Republican Rivers. The hottest temperature ever recorded in Colorado was 114°F at Las Animas in July 1, 1933 and at Sedgwick on July 11, 1954. Because of the very low relative humidity accompanying these high temperatures, hot days cause less discomfort than in more humid areas. The usual winter extremes in the plains are from zero to -10°F to -15°F but have reached extraordinarity low readings of -30 to -40°F during some of the most extreme cold waves. An important feature of the preciptaton in the plains is the seasonal cycle. A very large proportion (70 to 80 percent of the annual total) falls during the growing season from April through September. Cool season precipitation can be important for soil moisture recharge, but midwinter precipitation is tight and infrequent. More often, winter brings dry air and strong winds contributing to the aridity of the area. From early March through early June, periodic widespread storms bring soaking beneficial moisture that helps crops and grasslands. Summer precipitation over the plains comes largely from thunderstorm activity and is sometimes extremely heavy. Localized rains in excess of 4" sometimes fall in just a few hours contributing to local flooding. In late http://ccc.atmos.colostate.edu/climateofcolorado.php 5/18/2007