2011-09-08_REVISION - M1977342 (35)

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Discussion The Hydrometeorology Reports (HMRs) which apply to the Williams Fork drainage basin and adjacent regions to the east were reviewed and used as a baseline. HMR 49 and HMR 55A were reviewed extensively with the storm data base and standardized procedures given particular attention. When HMR. 49 was developed, guidance was drawn from HMRs for adjacent or neighboring regions. To the east of the Continental Divide, HMR 51 provided the closest reference. A comparison shows large PMP differences from west to east, with much greater PMP values east of the Divide at comparable latitudes. The issue of how to treat the resultant PMP gradient across the Continental Divide proved to be difficult to address. The result was that relatively high PMP values were shown west of the Divide in HMR 49 over the immediate western slope, including the location of the Williams Fork drainage. It is important to note that in HMR 49, this circumstance is acknowledged and addressed directly. Referring to HMR 49 Figure 1.1 (Figure 1 in this report), the following statement is presented in the HMR 49 text: "The shaded portion of Figure 1.1 is a zone to the west of the Continental Divide where the PMP values are considered least certain. Detailed generalized PMP estimates including seasonal variations are not available for the slopes immediately east of the Continental Divide. PMP gradients in this region can influence PMP estimates west of the Divide. A future PMP study covering the area east of the Divide is needed before there will be comparable confidence in PMP over the contiguous portion of the Southwestern States." HMRs 55 and 55A have provided the needed studies east of the Continental Divide. However, the issue of PMP gradients west of the Divide still needs to be addressed with detailed studies such as the site - specific Williams Fork PMP study. The location of the Henderson Mill drainage on the northeast slope of the Williams Fork Range is very significant. This location is in the immediate rainshadow of the Williams Fork Range and therefore storms which occur over the basin have the Williams Fork Range as an immediate upwind moisture barrier. This barrier is the second significant moisture barrier under southwesterly boundary layer wind conditions (the inflow wind direction for the largest of the general storms) in the central Rockies. The Gore Range provides a slightly higher barrier further to the southwest. A picture of the Gore Mountains is shown in Figure 3. This view to the southwest from Ute Pass at the top of the Henderson Mill drainage basin shows how significant this range is as a barrier to low level atmospheric moisture approaching the Williams Fork basin from the southwest. Page 4