PUB00106

• Year: 2000
• Title: A History of Drought in Colorado: Lessons Learned and What Lies Ahead
• CWCB Section: Water Conservation & Drought Planning
• Author: Colorado State University
• Description: Lessons learned and what lies ahead
• Publications - Doc Type: Tech Report

4K:olorado Water Resources Research Institute.

Figure: Sources of Atmospheric Moisture in Colorado

I Winter

I Summer-Fall

Although Colorado is far removed from oceanic moisture
sources, moist air masses are carried to the state by global
atmospheric circulations. These circulations migrate
through the year, changing the sources 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.

During the late fall, winter, and early spring, the winds
aloft are strongest over the mid latitudes of the northern
hemisphere resulting from the very strong temperature
gradients between the polar region and the equator. The
great river of air known as "the jet stream" directs air
masses and migrating storm 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 storm systems are very strong and
fast moving. This is also the time of year when orographic
(mountain-related) air motions are most dramatic as rapidly
moving air is lifted on the upwind side of each mountain
barrier and then descends and warms on the leeward side
of the mountains. Because of cold temperatures, however,
total water vapor in the atmosphere is limited.

What this means for Colorado is that frequent snows near
and immediately west of the each mountain range occur
when enough Pacific moisture is present. Because of
persisting cold temperatures in the mountains, most of the
precipitation falls as snOw and remains as snowpack that
accumulates for several months until finally melting in the
spring and summer. As the air moves east of the moun-
tains, it descends, bringing very little winter moisture and
periodic strong, dry winds. On occasion, a very strong
winter stonn will draw moisture northward from the Gulf of
Mexico and for a matter of hours drop widespread
"upslope" precipitation east of the Rockies. However,
subsequent warmer days and sunshine will rapidly melt
and evaporate much of this precipitation.

South Platte

I Spring-Summer I

"2"

A.t'krm.~

I Summer

During the spring months, the westerly winds aloft begin
to slow a bit and the air begins to warm. Storm systems are
still present but move slower. This allows more moisture
from the Gulf of Mexico to move northward and westward
in advance of these storms. As a result, a few spring
storms typically bring widespread precipitation along and
east of the mountains. A single spring storm can deposit
as much precipitation in eastern Colorado as most winter
storms combined. This period of episodic widespread
storms lasts from early March into early June. This is the
wettest time of year for much of the Front Range and
northeastern Colorado. At the same time, stoans 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 over most of the state.

During the summer months, winds aloft tend to be very
light, and moist Pacific air masses no longer reach Colo-
rado. The primary mechanism for lifting and cooling air to
saturation becomes convective updrafts from the heating
of air near the ground. Thunderheads (cumulonimbus
clouds) 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 eastern Colorado. A few
weak cold fronts will also drop southward from Canada to
help trigger some local thunderstorms on the plains.
Thunderstorms also become more frequent during July
over the mountains and Western Slope of Colorado as air
from the Gulf of California and/or Gulf of Mexico drifts
slowly northward across Mexico and the Southwest U.S.
This wind circulation associated with the "Southwest
Monsoon" is responsible for an important wet season over
portions of southern and western Colorado from July into
September. July and August are the wettest months of the
year for much of southern Colorado and many mountain
valleys. When sufficient moisture is present, thunder-
storms will develop every day in and near the mountains.
(continued on page 12)

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