Falling Water Native Americans are believed to have migrated from summer ranges on the High Plains and settled in warmer and more protected settings in the lower foothills during midwinter. The seasons also control the movement of clouds and moisture. The storms that sweep across the mountains and plains—sometimes gently, and sometimes frightfully—deliver water to Colorado from distant oceans. During the winter months, large gradients in temperature between the tropics and the North Pole produce a strong mid latitude jet stream—a river of constantly moving air that circles the earth swiftly from west to east. At times, wind speeds may exceed 100 mph over Colorado’s highest peaks. These winds aloft govern the wintertime storm track over the United States and bring air masses from across the Pacific into the western U.S. The winds shift direction, from southwest to northwest and back again, as waves in the atmosphere cross the region. Air that rises up and over the mountain barrier rises and cools. If sufficient moisture is present, clouds form and precipitation may fall. Each wave in the jet stream offers the potential for Pacific moisture and upward movement of air, essential ingredients for precipitation. The mountains of California, Oregon, Washington, Idaho, Nevada, Arizona and Utah get the first pickings for Pacific moisture. But Colorado’s high elevation require air to rise, squeezing out more of the available moisture. The prevailing westerly winds during winter mean Colorado’s eastern plains and Front Range urban corridor are often shielded from Pacific storms. Winter sunshine is common east of the mountains and helps make the cold temperatures seem comfortable. December through February are typically the driest months of the year east of the mountains at the very time that Colorado’s highest mountains are having their wettest weather. On average, only about 1 inch of precipitation (the melted water content of snow) falls during midwinter east of the mountains.
Fifty to 80 percent of the average annual precipitation in Colorado’s mountains falls as snow. This decreases to less than 25 percent on the Western Slope near Grand Junction and to less than 20 percent on the eastern plains near the Kansas border. Historically, most mid winter precipitation is snow, even at lower elevations. Snowfall totals above 9,000 feet typically range between 200 and 400 inches per year, depending on location. Valleys receive considerably less. Aspen, for example, annually averages about 175 inches of fresh snow; nearby mountains get more than 300. Winter snowfall exceeds 400 inches in some of Colorado’s snowiest ranges, such as the Park Range, east of Steamboat Springs; the Elk Range in central Colorado; and the San Juans in the southwest. There are significant differences, though, in how often and how much snow falls. Occasional storms with warmer Pacific air bring rain to the lower elevations of southwest and west central Colorado. Colorado’s northern mountains receive frequent small to moderate snows from late October into May. Much of it is fluffy, low density snow—skiers’ favorite. Colorado’s southern mountains have fewer but larger storms that often bring higher density snows. Snow typically begins to accumulate in the mountains in October and continues until spring. This bright white coating visually assures Coloradans there will be water for the coming growing season. Mountain temperatures stay below freezing throughout the mid winter months, allowing each new storm to deposit a new layer. This developing snow pack is the frozen reservoir that provides water for the spring and summer. The depth and extent of snowpack varies with location, elevation and year. Melting snow provides upwards of 80 percent of Colorado’s surface water supplies. Spring and summer rains bring beneficial moisture for Colorado’s forests and grasslands, but contribute relatively little to the state’s rivers and streams. 7
Published on Nov 20, 2013