FLOOD09350

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Generalized Estimates of Probable Maximum Precipitation for the United States West of the 105th Meridian
• Date: 1/1/1960
• Prepared For: USDA
• Prepared By: US Weather Bureau
• Floodplain - Doc Type: Educational/Technical/Reference Information

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0.4 0.5 0.6 0.1 0.8 Q9 1.0 1.1
MAXIMUM OBSERVED CLOCK HOUR PRECIPITATION [INCHES}

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FIGURE 2--5,-V<8.nat1on of maximum observed clock-hour precipitation with elevation tor stations on the western slope
of the Sierra Nevada in California.

the difference exhibited by figures 2-6 and 2-7.
However, it is known that orographic barriers can
effect great reductions in W. within short dis-
tances and act to reduce precipitation downwind
(pars. 2.2.5, 2.3.5, and fig. 2-3). Since storm pre-
cipitation shows such poor correlation with sta-
tion elevation (figs. 2-6 and 2-7), the only con-
.elusion readily apparent is that orographic bar-
riers between the moisture source and the pre-
cipitation area comprise the most effective factor
governing precipitation in mountainous regions.
These barriers to moisture inflow are simply called
moisture barriers.
2.3,14 The difference in elevation of the mois-
ture barriers apparently provides the only logical
explanation for the large difference in storm pre-
cipitation indicated by figures 2-6 and 2-7. Moist
air from the Pacific reaches much of the western
slope of the Sierra Nevada in California after
crossing the coastal ranges at an average height of
1,000 to 2,000 feet. On the other hand, moist air
from an even more favorable source region, i.e.,
a more southern and warmer region of the Pacific,

10

is forced to cross moisture barriers averaging no
less than 7,000 feet in order to reach western
Colorado, where much lower :t4-hr. prec1pitation
maxima are observed.
2,3.15 Major storms occurring in the western
United States as well as in other parts of the world
have inflow winds of at least 25 m.p.h. Winds of
this speed persisting for periods of 24 hours or
longer bring air into the precipitation process
from sources hundreds of miles way. This warm,
moist air moving from the oceans must pass over
orographic barriers before reaching many regions
of the western United States. Figure 2-8 depicts
these barriers to moist air inflow. The inflow di-
rection of warm, moist air in extensive storms
capable of producing probable maximum precipi-
tation (PMP) for the longer durations was con-
sidered in the construction of this map. For in-
stance, the map shows that the moist air reaching
the region of the Great Salt Lake would be lifted
to an elevation of 7,000 feet. A local storm of
small areal extent could produce maximum point
values of precipitation for short dnrations from a