WSP03196

• File Number: 8443.600
• Description: Narrows Unit - Studies
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 6/30/1975
• Author: Dames and Moore
• Title: Flood Study Narrows Dam South Platte River
• Water Supply Pro - Doc Type: Report/Study

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considerable judgment is involved in placing and adjusting the transposed
pattern to fit the topography of the basin to which it is transposed, even
though isohyetal patterns of storms that have occurred over the basin provide
some guidance.

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Transposition of the June 7 to 8, 1964 Montana Storm

The primary consideration in storm transposition is that the

meteorological situation which produced the storm rainfall at the original

site can also prevail at the site to which the storm pattern is transposed.

The most critical meteorological situation associated with heavy general-

type rainfalls on the eastern slopes of the generally north-south oriented

mountain ranges in western Montana and Colorado is one which produces strong

easterly winds for a day or longer. This type of situation thus calls for

the passage or formation of a low-pressure center to the south of the rain

area. Such a pressure pattern would also result in a flow of warm moist air

from the Gulf of Mexico to the storm site. The critical weather situation is

described in more detail in the USBR study ~/ and in the Hydromet review. 1/

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The meteorological situation associated with the outstanding June

1974 Montana storm can produce heavy rainfall in the mountainous regions of

the western portions of the South Platte Basin above Narrows Damsite when

the low-pressure center is displaced farther south. This was the situation

in the May 4 to 8, 1969 storm, which provided the best basis for fitting the

June 1964 Montana storm rainfall pattern to the Narrows basin. Comparison

of the weather situations associated with the two storms and the similarity

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~/ "Design Storm for Narrows Damsite - Narrows Unit - South Platte Division
Pick-Sloan Missouri Basin Program", memo dated May 23, 1974, from
C. E. Everson, Meteorologist to Head, Flood and Sedimentation Section,
Division of Planning Coordination, Bureau of Reclamation Engineering and
Research Center.

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1/ "Review of Design Storm for Narrows Damsite, Colorado", memo dated
October 18, 1974, from Chief, Water Management Information Division,
Office of Hydrology, National Weather Service, to Chief, Division of
Planning Coordination, Engineering and Research Center, Bureau of
Reclamation.

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