FLOOD06061

• County: Statewide
• Community: State of Colorado
• Stream Name: Colorado River/Great Basin
• Basin: Colorado Mainstem
• Title: Probable Maximum Precipitation Estimates, Colorado River and Great Basin Drainages
• Date: 1/1/1984
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

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'(DOPTED SMOOTH RELATION
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FIGURE 3-38 IHMR #431 FOR ~
NORTHWEST CONVERGENCE PMP .....~
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100

Figure 2.21.--Adopted 6/24-hr vs. 72/24-hr convergence PMP ratios.

6/24HR RATIOS (PERCENTI

next two days. Again, most of the rain during frontal passage was caused by
thunderstorms and therefore make this case unrepresentative of a major winter
storm. As to the meteorological cause for the other data in figure 2.19, no
check was made, but it is believed they tend to support a seasonal distribu-
tion in the ratios shown.

The recorder rainfall data for stations in least-orographic areas, table
2.4, were processed to determine monthly average 6/24-hr within-storm ratios
for maximum 24-hr rainfalls. This was done by selecting the 20 highest 24-hr
rainfalls of record for each month and station and purging to reduce the in-
fluence of short term thunderstorm events. The purging was accomplished by
eliminating 6/24-hr ratios greater than 0.90. In many instances, particularly
during the summer months, fewer than 20 cases were available. From these
cases that met the purging criterion, ratios from the five highest 24-hr rain-
falls for each station were averaged to obtain mid-month subregional ratios.
Some monthly averages had less than five cases. The subregional values are
shown on a seasonal plot in figure 2.22. Although there is considerable scat-
ter this may be due to the limited sample. There is a definite trend for
higher 6/24-hr ratios in the warm season. These monthly averages are plotted
on a seasonal plot, figure 2.23, as short dashes. Four other sets of data
have been added to this figure to aid in determining the seasonal variation.
Among-storm 6/24-hr ratios (highest monthly 6-hr rainfall divided by the
highest monthly 24-hr rainfall) were averaged for 6 stations that were help-
ful in determining the initial seasonal variation of convergence PMP (fig.
2.4). These are shown by Xs in figure 2.23. A third set of 6/24-hr ratios
were monthly averages of station data in the storms listed in table 2.5 along
with the August 1951 and September 1970 storm (open circles in fig. 2.23).