Spring Season Colorado Cyclones. Part II: Composites of Atmospheric Moisture and Moist Static Stability

Abstract

A composite based on 39 cases of April and May Colorado cyclogenesis is used to examine the moisture distribution and its evolution during the early stages of Colorado cyclones. Moist static stability and its changes are also examined using the Showalter index. In the composite, a narrow tongue of relatively higher surface dew points extends northward on the day preceding cyclone development. This tongue expands and shifts eastward on the following two days. Maps displaying the specific humidity at four levels between the surface and 700 mb reveal that the moisture is mainly confined to 850 mb and below. East-west cross sections of isentropes, normal wind component and specific humidity provide increased vertical resolution, which confirms the concentration of moisture at low elevations. The normal wind component (approximately from the south) shows a diurnal tendency (stronger at night) but also the influence of synoptic male forcing. The strongest southerly winds and northward moisture transport occur at 1200 GMT beneath the exit region of a 300-mb jet maximum. Maps of the Showalter index show the weakest static stability initially in the form of a tongue extending from northeastern Mexico into Oklahoma. As the 300-mb jet propagates into the area, a pattern of weaker stability develops, particularly in the exit region of the jet. The pattern of weaker static stability shifts rapidly east-northeastward as the cyclone develops.