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<br />-r:r .'ib,. <br /> <br />13, Page 79, Why was the CCC isohyetal analysis not used for the 1997 For Collins storm? <br /> <br />A W A response to Question13. <br /> <br />The CCC did an excellent job of collecting bucket survey rainfall observations for this storm. <br />The CCC subjectively included NEXRAD spatial data in producing their hand drawn isohyetal <br />pattern, A W A recognizes the value of using NEXRAD data to determine the spatial distribution <br />of rainfall and has incorporated its use in the Storm Precipitation Analysis System (SPAS). In <br />general, our experience has shown that using NEXRAD, the area sizes associated with storm <br />isohyetals tends to be smaller since without NEXRAD, either smoothed base maps or some <br />mathematical algorithm (e.g. minimum curvature) are used which tend to increase areas of <br />extreme rainfall. A W A made the decision to use the GIS derived isohyetal pattern primarily <br />because it showed larger areas of extreme rainfall than the CCC isohyetal analysis and hence <br />provided larger rainfall volumes. This provided an increased degree of conservatism in the <br />rainfall analysis. The rainfall associated with the 1997 Ft Collins storm using this conservative <br />approach turned out to be significantly less than other storms used in this study (see Figure 10.2, <br />page 124), The CCC isohyetal pattern could have been digitized and used in the study but would <br />not have influenced the results of the study, <br /> <br />14. Page 82. Why was the CCC isohyetal analysis not used for the 1997 Pawnee Creek storm? <br /> <br />A W A response to Question 14. <br /> <br />This response is effectively the same as for question 13. <br /> <br />The CCC did an excellent job of collecting bucket survey rainfall observations for this storm, <br />The CCC subjectively included NEXRAD spatial data in producing their hand drawn isohyetal <br />pattern. A W A recognizes the value of using NEXRAD data to determine the spatial distribution <br />of rainfall and has incorporated its use in the Storm Precipitation Analysis System (SPAS). In <br />general, our experience has shown that using NEXRAD, the area sizes associated with storm <br />isohyetals tends to be smaller since without NEXRAD, either smoothed base maps or some <br />mathematical algorithm (e.g. minimum curvature) are used which tend to increase areas of <br />extreme rainfall, A W A made the decision to use the GIS derived isohyetal pattern primarily <br />because it showed larger areas of extreme rainfall than the CCC isohyetal analysis and hence <br />provided larger rainfall volumes. This provided an increased degree of conservatism in the <br />rainfall analysis. The rainfall associated with the 1997 Pawnee Creek storm using this <br />conservative approach turned out to be significantly less than other storms used in this study (see <br />Figure 10.2, page 124). The CCC isohyetal pattern could have been digitized and used in the <br />study but would not have influenced the results of the study. <br /> <br />15, Page 100. How was the conclusion of "not physically possible" reached based on the sub- <br />cloud layer winds? Note: 1 st sentence indicates thunderstorm produced flash flood east of the <br />continental divide in Colorado have sub-cloud layer winds southerly to easterly. Page 99 <br />indicates that sub-cloud layer winds feeding the NWS SSS would be southeasterly to easterly, <br />