BOARD01080

• Board Meeting Date: 1/27/2004
• Description: WSP Section - Native Species Trust Fund, Recommendations for FY 2004-05
• Board Meetings - Doc Type: Memo

Kolorddo Watel'" Resources Research Institute.

Figure: Sources of Alm~pheric Moisture in Colorado

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I Summer-Fall

Although Colorado is far n:mo\'ed from oceanic moisture
sources, moist air masses are carried to !be state by global
aunospheric circulations. These circulations migrate
through the Year, changing the wurces of moisture that
enter Colorado (as shown in the figure). The mechanisms
to lift and cool air to saturation (when condensation and
precipitation can occur) also change with the seasons.

Durin~ the late fall. winter, and cady spring.lhe winds
aloft are strongest over the mid latitudes of the nonhern
hemisphere resulting from the very strong lemperature
gradients ~twcen the polar region and the equalOr. The
great n\.erof air known as "the jet stream" directs air
masses and migrating SlOnn systems approximately from
west to east. During this time of year the Pacific Ocean is
the primary source for moisture reaching Colorado. This is
the time of year when s.lonn systems are very strong and
fast moving. This is also the time of year when orographic
(mountain-related) air motion!'. are most dramatic as rapidly
moving air is lifted on the upwind side of each mountain
barrier and th('n descends and warms on the leeward side
of the mountains. Because of cold temperatures. howe\.er,
total waler vapor in the atmos.phere is limited.

What this means for Colorado is that frequent snows near
and immediately west of the ('ach mountain range occur
when enough Pacific moisture is prescnt Because of
persisting cold temperatures. in the mOuntains. most of the
precipitation falls a..<; snow and remains :t!> snowpack that
accumulates for sc\"Cral months until finally melting in the
spring and summer. As !be air mo\"Cs east of the moun-
lains. it descends. bringing very little winter moi~ture and
periodic strong. dry winds. On occ3.!>ion. a vel)' strong
winter stann will draw moisture northward from the Gulf of
~Iexico and for a matter of hours drop widespread
"upslope" precipitation east oflhe Rockies. However,
subsequent wanner days and sunshine \\oill rapidly melt
and e\'aporate much of this precipitation.

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s..wh l'lllll~

Sprlne-Summer I

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.\.rkD........

...
\ I Summer
~

During the spring month.'" the westerly winds alofl begin
10 slow a bit and the air begins to warm. Stonn systems are
still present but move slower. This allows more moisture
from the Gulf of Mexico to move nonhward and westward
in advance Ofthesc stonns. As a resull. a few spring
~torms typically bring widespread precipitation along and
easl of the mountains. A single spring slOnn can deposit
as much precipitation in eastem Colorado as most winter
storms combined. This period of episodic widespread
storms lasts from early ~larch into early June. This is the
wellesltime of year for much or the Front Range and
nonheastem Colomdo. AI the same time. storms from the
west diminish and contribute ever less Pacific moisture.
June is the driest month of the year for much of western
Colorado, while the last half of June IS frequently hot.
sunny and dry o\.cr most of the state.

During lhr summer month_~ Winds aloft tend to Ix verY
light. and moist Pacific air ma'ises no longer reach Cola:
rado. The primary mechanism for lifting and cooling air to
saturation becomes convective updrafts from the healing
of air near the ground. Thunderheads (cumulonimbus
c1oud.q become the primary cloud responsible for precipita-
tion whenever sufficient moisture is present in the air.
Occasionally, humid air from the Gulf of Mexico and the
Mississippi Valley will drift into castern Colorado. A few
weak cold fronts will also drop southward from Canada to
help trigger somc local thunderstorms on the plains.
Thundemonns aha become more frequent during July
O\er the mountains and West('rn Slope of Colorado as air
from the Gulf of California anMor Gulf of ~te.'(ico drifts
slowly northward across ~k'(ico and the Southwest U.S.
This wind circulation associated with the "Southwest
~Ionsoon" is responsible for an important wet season over
ponions of sou!bern and western Colorado from Jul" into
September. July and August are the wetlest months'ofthe
year for much of southern Colorado and many mountain
valleys. \Vhen sufficient moisture is prcSt'nt. thunder-
storms will develop cvery day in and near the mounlains.
(continued on page J 2)

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