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<br />Historical Perspectives on Colorado Drought <br /> <br /> <br />{ <br /> <br />slate originates from the Pacific Ocean. This moisture is essentially transformed into <br />precipitation by the following mechanisms, either singularly or in combination: <br /> <br />traveling along the jet stream and <br /> <br />the forcing of air across the mountains barriers, which also provides the lift <br />needed to cool and condense water vapor in the air and produce precipitation. <br /> <br />In early spring, Pacitic-hased storm systems can effectively draw in low-level <br />moisture from the Gulf of Mexico and generate exceptionally high amounts of <br />prt-cipitation east orlhe Continental Divide (a fine example of this scenario is the <br />mid.March bliu.ard of2003 across the northern Front Range). <br /> <br />To assess the impacts of drought during the laic fall to early spring period. one <br />should look at the longer-term positioning of the jet stream at this time of year and <br />the factors that may influence it. The dominant cause of wintertime jet stream <br />variability over western and central North America is the El Nino/Southern <br />Oscillation (ENSO). which is essentially a shining of relatively warm and cold <br />surlace waters and subsequent wind patterns across the equatorial Pacific Ocean. <br />The general effects of EI Nino and its counterpart La Nina can be found in Figure <br />6. <br /> <br /> <br />It' fn-dk-U"n Cl'nlnf.'oiCEI"f.\\\'S <br /> <br />Figure 16 Typical January-March weather anomalies and atmospheric <br />circulation during moderate to strong EI Nino & La Nina (CPC 2001) <br /> <br />Page 19 <br /> <br />recycled content paper <br /> <br />HDR Engineering, Inc. <br />Printed on 30% post-consumer <br /> <br /> <br />storms <br /> <br />fting by individual <br /> <br />strong <br /> <br />I) <br /> <br />2) <br />