FLOOD08329

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Probable Maximum Precipitation Estimates, United States Between the Continental Divide and the 103rd Meridian
• Date: 6/1/1988
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

deficient because terrain influences were not evaluated. Areas as large as
20,000 mi2 and durations up to 72 hr were covered in these reports.

Additionally, estimates of PMP for individual drainages between the Continental
Divide and the 103rd meridian have been prepared by the National Weather
Service (NWS) on occasions where the prevailing generalized reports were believed
to inadequately treat orographic influences. Throughout the United States,
including the present study, the NWS has prepared generalized studies of PMP as
requested by the Corps of Engineers (COE), the U.S. Bureau of Reclamation (USBR),
the Soil Conservation Service (SCS), and the Tennessee Valley Authority (TVA).

The concept of generalized PMP studies should not connote a level of detail any
less than that for the individual basin studies. The term generalized, in the
sense of its use here, is to describe a study that covers a broad region
involving numerous drainages. The primary advantages to generalized studies are
to be found in the consistency of development and between results when determined
for one drainage versus another. One disadvantage is the time required to
complete such studies, in many instances taking up to three years.

The increasing development of the CD-I03 region has caused renewed interest in
the expansion of available water resources and in flood control. There is also
concern for the hydrologic adequacy of many existing structures. The need
existed, therefore, to review the estimates of precipitation potential for the
region between the Continental Divide and the 103rd meridian and to expand the
areas and durations covered in the previous study. The present study provides
cri teria for estimating PMP for durations from 1 to 72 hr for storm areas from
10 to 20,000 mi2 in the eastern or nonorographic portion of this region and from
1 to 5,000 mi2 in the more mountainous western portion.

In regions west of the Continental Divide, investigations have shown that PMP
for small areas and short durations are not likely to occur in a general storm.
The concept of a local storm has been used in western PMP studies to describe an
intense, small-area, short-duration isolated event. East of the 105th meridian,
previous studies have concluded that the general storm controls PMP for all
durations. Since no known local storms have exceeded general storms in the
east, it is assumed that the general storm includes sufficient convective bursts
to envelop all local storms in that region.

In ~he present study, local-storm PMP has been defined for areas of 1 to
500 mi and for durations of 15 min to 6 hr. Both local- and general-storm PMP
are provided for the entire region between the Continental Divide and the 103rd
meridian. It is incumbent upon the user to evaluate which storm type gives the
controlling PMP for a specific area, duration, and location.

1.2 Authorization

Authorization for the study was the result of agreements among the U.S. Army
Corps of Engineers, the U.S. Bureau of Reclamation, and the National Oceanic and
Atmospheric Administration's (NOAA) National Weather Service (NWS). Financing
was provided by the COE through their continuing Memorandum of Understanding with
the NWS and by the USBR under an Interagency Agreement with the NWS dated
September 11, 1980.

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