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<br />. <br /> <br />. <br /> <br />. <br /> <br />FJe: PMPtopics799summary.doc <br /> <br />y1=Wr <br /> <br />JuLy 21, 1999 <br /> <br />. <br />increased up to 14% because of the topography based on 100-year precipitation <br />climatology. If in fact this is an acceptable procedure, it needs to be demonstrated that <br />the 1 OO-year precipitation is based on' rainfall not snuwfaD, a significant consideration <br />in Colorado. (Tomlinson) . <br /> <br />The 100-year precipitation study was not updated because it was beyond the <br />resources of the NWS, but group consensus is the study needs to be updated. Impact is <br />unknown until the study is updated. <br /> <br />28. Use of ol'ographic among-storm factors to adjust non--<Jrographic within-storm <br />factors (a technical issue which could significantly increase the rainfall volume). <br />(Tomlinson) <br /> <br />No consensus is reached by the group. The NWS feels that any additional ;tudy <br />results will have a minimal impact on results and there will be no further <br />examination of this issue at this time. <br /> <br />29. PMP values, both generalized and site-..pecific, are derived from analyses of the <br />most significant rainfall events observed within geographically and climatologically <br />similar regions. These observed rainfall /lJnounts are adjusted using standardized <br />procedures to provide "maximized" rainfall amounts, Le. the storm rainfall is <br />theoretically increased to its "maximum value. These theoretical rainfall totals are <br />sometime increased slightly to provide continuity with other maximized rainfall totals <br />from other storms. These maximized rainfall amounts provide the basis for <br />determining the PMP values for the basin being studied. The Cherry Creek Drainage <br />has had one of these extreme rainfall events occur within the basin in 1935. The <br />maximization procedure increases the rainfall amount by 22% (HMR 51) to provide a <br />basis for estimating the PMP for the basin.. The NWS site-specific PMP study provides <br />PMP amounts which exceed the "maximized" 1935 rainfaD by over 60%, a very large <br />increase over the maximized extreme storm value. (Tomlinson) <br /> <br />The 22% and 60% values are erronuous, but the concept of extreme envelopment <br />will be examined by the NWS. <br /> <br />30. Since PMP values are the "theoretical greatest", there is no objective way to <br />evaluate if the rainfall amounts are "correct". Huwever, the PMP values are often <br />compared to the 1 OO-year pre;ipitation amounts and are generaDy expected to be larger <br />by a factor between two and three. The range in values in HMR publications in the <br />western US varies from 1.4 to 7.5. According to the NWS study, the "maximum point <br />values...from this study exceed the largest 100-year return period precipitation in the <br />Cherry Creelr-Drainage by roughly seven times." This places the rutUJ of the Cherry <br />Creek PMP values to the 1 OO-year precipitation values is in the extreme high range for <br />the western Us. (Tomlinson) <br /> <br />Other comparisions (NWS 25) show the 6 to I ratio for the PMP to IOO-year <br />rainfall in Colorado is not an unusual qccurrence. The group reached consensus on this. <br /> <br />.. -.<.. '.~,<.:.r--_""'" ". ". . <br />.n,. '. - ..JTtijRatii21J1dy1999Tedurict1lMedbrg 7 <br />;..;.c-;::-~::..~~~.:;;:j;:~:::: ......:.._ ... <br /> <br />~.-._.-:-., <br />:.. :.:-: -;;=.: ~;~:>i;~.:,:':: .:;. ~'. ~:~~:.~~~:-:i;~~-: '.:;~~~~if~:~~ -.: '_::'<~;':'~:"~~~' <br />