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.. . rlk: PMPtopk3799summary.doc D~-AT"I July 21, 1999 23. The maximization procedure used in HWfR 55A used a storm representative dew e point value which was computed using observations taken 560 miles to the southeast Possibly re-analysis could identify observational data closer to the rainfall event which would more accurately represent the moist mass which provided the available moisture for the storm dynamics to convert to rain on the ground. (Tomlinson) NWS will examine the data and determine its likely affect on HMR. 55A and on the Cherry Creek site specific PMP. 14_ The accepted depth-area-duration values used together with the maximization factor of 163% produced maximized rainfall values which were not consistent with and significantly larger than maximized values derived from analyses of other large storm events along the Front Range. To provide a more consistent analysis, the values were undercut (decreased) by about 15% by the authors of HMR 55A. Current PMP evaluations impose an upper limit of 150% when applying maximization factors to storm D-A-D's. Possibly an updated dew point analysis as discussed in 23 above would provide an improvement for the maximization factor. (Tomlinson) NWS will examine the data and determine its likely affect on HMR 55A and on the Cherry Creek site specific PMP. 25. The Bureau of Reclamation re-examined the Cherry Creek storm in 1985 and .. produced an updated depth-area-duration analysis which contained slightly lower .. values. However, the rainfall values (see item 11 above) and nwss curves from previous studies were not evaluated for reliability and internal consistency. (Tomlinson) Group consensus was this is based on best available data. 26. The NOAA Hydrometeorology Office completed a site-specific PMP study for the Cherry Creek drainage basin for the Corps of Engineers in July, 1995. They basically extended the non-orographic procedures provided in HlI1R 52 westward to compute the design storm for the basiIL Critical centering of the design storm assumes no orographic influence of the Palmer Ridge. This assumption is questioned by Dr Jarrett of the USGS in Denver; Mr Loren Crow, a private Certified Consulting Meteorologist in Denver; and Mr Nolan Dqeskins of the Colorado Oimate Center as well as myself. Although quantification of the influence of topography on this and other Front Range storms has yet to be determined, it provides a significant open issue for PMP analyses along the Front Range. (Tomlinson) , The group agrees with the NWS that there are topographic effects on the north and south sides of the Palmer Ridge. 27. Following extensive discussions intended to justify using HMR 52 because the topography of the Cherry Creek Drainage is not significant, the PMP amounts are e }'"'-:~:. ..:~T..p;.;sto 21 J.dy 1999 Tedutical Muting ~~~l:i~~i:~~~;:~~2:l~:1~~;~~~.~.:_~~.. :.,.....~~:...:.:__... 6 . . . ::~ '--. .-. ...-.-~~~~-;'-.;-'.'~ -. :'- . _ .~_::.: .: ::7..:.:'=-~.::: T~~ ~~~f;i;~}f;~:Z;~-,...~-;;~;~: .;~;-:::,;~ .,.:.::::.: :.:~;..~;~~~~.~..._.