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Historical Perspectives on Colorado Drought Wind, topography and precipitation lbe sources of atmospheric moisture are depicted in Figure 14. Clearly the mountainous areas uflhe state are affected by moisture bearing winler winds from the west to northwest. The south....,cstern mountains favor wet winds from the southwest from summer into fall and wintcr. Upslope easterly winds from spring into summer bring green fields to the eastern half oflhe slate and the southern mountains. Thus weather factors that innucnce the seasonal frequency and moisture content of these winds have a major impact on Colorado's precipitation nwr I 'illmnwr I Sources of atmospheric moisture in Colorado (McKee et al. 2000) A majonty of the seasonal snowpack that accumulates across the higher mountain ranges of Colorado is produced between late fall and early spring. This lime period is of particular interest oc-cause it is estimated that up to 80% ofCoJorado 's surface slreamjloworiginates from snowpack that accumulates during this period before melting in the April to July time frame. Mountain snow pack provides A!l.innifirJlnlwAIp.r!;nllrr.p. During the summer and early fall, the jet stream becomes notably weaker, if not absent, and convective (i.e. thunderstonn) activity becomes the primary source of precipitation. The moisture for this thunderstonn acti\-ity derives largely from the pattern commonly referred to as the Southwestern Monsoon. The monsoon pattern is defined by a general area of high pressure, or ridge. in the mid levels (-7.000- 20,000 ft. above sea level) of the atmosphere develops over southern New Mexico/\\"cstern Texas (see Figure 15). The inOow of monsoon moisture is detcnnined by this Oow. The clockwise Oow of moisture around this area of high Page 17 recycled content paper HDA Engineering, Inc. Printed on 30% posH:onsumer \\hlll" 6 0111';1 I Winli'r I d' II l111wr.Fa SI Figure 14.