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There are four types of storm which need to be considered. The first is the local storm while the <br />other three are general storms. The conclusions presented in the Williams Fork study were <br />reviewed for each of the storm types as they affect the Henderson Mill watershed location. <br />The largest of the historic storms that have occurred over regions similar in climate and <br />topography to the Williams Fork drainage basin have been identified. The list includes the largest <br />63 of these storms. That list was further refined to the most significant storms of each type that <br />should be considered for Williams Fork. There are 15 storms on this list which is presented in <br />Table 1. <br />Thunderstorms <br />Thunderstorm events over the Rocky Mountains are relatively small events <br />compared to the mesoscale convective complexes that occur over the midwestern states. <br />However, during the period of mid -July through August there is potential for significant rainfall <br />totals from this type of storm although the areal extent of the rainfall is limited. These storms <br />occur as a result of coincident spatial and temporal occurrence of a number of necessary <br />meteorological factors. Those factors include: <br />a) adequate atmospheric moisture in the boundary layer or through a more significant <br />depth, <br />b) atmospheric instability, <br />c) an initiating or triggering mechanism such as surface heating or orographic lift, and <br />d) a focusing mechanism such as channeling by terrain or standing waves from an upwind <br />terrain feature. <br />The Williams Fork study found that the maximized and transpositioned Opal, Wyoming, <br />storm of 1990 was the most significant. That storm was critically centered over the Williams <br />Fork watershed. The most important issues with this storm relative to the Henderson Mill <br />drainage basin is storm centering. Because the boundary layer inflow wind is southwesterly, the <br />storm cannot be centered any closer to the Henderson Mill drainage basin than over the Williams <br />Fork River because of the rainshadow effect of the Williams Fork Range. Hence the local storm <br />center provided in the Williams Fork study should be used for the Henderson Mill basin. The <br />Local Storm Worksheet for the Opal storm maximized and transpositioned to the Williams Fork <br />watershed is presented in Table 2. <br />Page 10 <br />