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!it . 4 Ii"' - .~~~! ~dii I , I FUe: PMPtopics799su1rrmary.doc , 1 j)~Ar{ July 21" 1999 increased up to' 11% because Df t~e tDpO'graphy based Dn 100-year precipitation climatDIDgy. If in fq.ct this is an acceptable prDcedure, it needs to' be demDnstratl?d th.-It the 1 OO-year precipitatiDn is based Dil rainfall nDt snowfall, a significant CDnsid~~atiDn in CDIDradD. (TO'mlirzsDn) . I I The 100-year precipitation study was not updated because it was beyond the resources of the NWS, but group consensus is the study needs to be updated. Impact i.s unknO"W'Il until the srlrdy is updated. ' I I 1 28. Use Df DrO'grap~ic amDng-stDr"f factDrs to' adjust nDn-DrO'graphic within..stDrm factDrs (a technical issue which cDuld significantly increase the rainfall vDlume). I (TDmlinsDn) 1 I I No consensuJ is reached by the group. The NWS feels that any additional ;tudy results will p.ave a minimal impact on results and there will be no fi1l1her examination 0f this issue at this time. 29. PMP values, bO'th generalized and site-specific, are derived frDm analyses of the mDst significant rai'"fall events O'bserved within geDgraphically and climatDIDgical('1 similar regiDns. T1fese Dbserved rainfall amDunts are adjusted using standanJized prDcedures to' prO'vide "maximized" rainfall amounts, Le. the stDrm rainfall is theDretically increaSed to' its "maximum value. These theDretiCal rainfall tDtals are sDmetime increased slightly to' prO'vide cDntinuity with O'ther maximized rainfall tDtalS frDm Dther stDrms.i These maximized rainfall amDunts prO'vide the basis fDr I determining the PMP values fDr the basin being studied The Cherry Creek Drainagl? has had Dne O'f thde extreme rainfall events Dccur within the basin in 1935. The maximizatiDn prO'cedure increases the rainfall amount by 22% (H}.tfR 51) to' prDlnde 'f basis fDr estinuzting ~he PMP fDr the basin. The NWS site-specific PMP study provides PMP amDunts which; exceed the "maximized" 1935 rainfall by Dver 60%, a very large increase Dver the nuiximized extreme stDrm value. (T DmJinsDn) I i The 22% and :60% values are erronuous, but the concept of extreme envelopment will be examined by the NWS. I I 3 O. Since PMP values are th,e "theDretical greatest", there is nO' Dbjective way tOt evaluate if the rainfall amounts are "cDrrect". However, the PMP values are Dften! cDmpared to' the 1 OO-'year pre.ppitatiDn amounts and are generally expected to' be larger by a factDr between twO' and three. The range in values in HMR publicatiDns in th~~ western US varies frqm 1.4 to' 7.5. AccDrding to' the NWS study, the "maximum pDint values..frDm this stUdy exceed the largest 100-year return periDd precipitatiDn in the: Cherry Creek:Drainage by rDughly seven times." This places the ratio Df the Cherry Creek PMP values to': the 1 OO-year precipitatiDn values is in the extreme high range fD1' the western Us. (TO'n#insDn) , Other comparisions (NWS 25) show the 6 to 1 rati.o for the PMP to 1 CO-year rainfall in Colorado is! not an unusual 9ccurrence. The group reached consensus on this. I ',::" , ,-, ;:.tt~~~711'!:!Y 1999 T~Medbrg ;?':';'::;E~~-:7.-~;::';a~:t;~;::=:,,~,~t:::::~=:: d",,: .'. ,,_. 7 . -. . .-.-. :., ",,...::;,,".,~:::+:-:::.. ::-;;:;.; ~':;-:~~,.;;;:;~;.~;..:;~;~;~:;:-;:~:~. ',: ,,:;',::::":2::"';:;k~.,.