Colorado's Precipitation Patterns
Colorado’s mid-latitude interior-continental location and high mountainous terrain result in a diverse and complex climate. As illustrated in the figure to the right, seasonal large-scale atmospheric circulation patterns interact with the mountainous topography to produce the following three major precipitation patterns (Collins et al., 1991):
Pacific Moisture: Throughout the winter, the primary sources of precipitation are frontal systems from the Pacific Ocean. These systems typically enter Colorado from the northwest, the west, and the southwest. Because the Rocky Mountains present a formidable obstacle to these eastward moving systems, the winter precipitation patterns have a substantial orographic component – winter precipitation increases steadily with altitude west of the Continental Divide and decreases sharply east of the divide.
Gulf/Subtropical Atlantic and Land-recycled Moisture: Throughout the spring and summer, Gulf/Subtropical Atlantic and recycled moisture can generate both intense local thunderstorms and widespread rainfall events. The intense local thunderstorms typically occur from May to early September and can occur throughout the state. The widespread rainfall events typically occur between April and mid-June and are generally limited to the Great Plains and eastern foothills.
Subtropical Pacific Moisture: In the summer, Subtropical Pacific Moisture can generate both widespread rainfall events and intense local thunderstorms, primarily in the southwestern mountainous areas of the state.
Snowfall Data/Mapping
Mean Annual Snowfall Data/Mapping: A map showing the mean annual snowfall for the Front Range was published in "Climatography of the Front Range Urban Corridor and Vicinity" by Hansen et al. (1978)
Mean Annual Snowfall
Mean Annual Snowfall Data/Mapping: As indicated in the figure below, the mean annual snowfall ranges from approximately 40 to 140 inches (100-360 cm) (Hansen et al., 1978). The figure below also illustrates that the watershed for the South Fork of the South Platte River (the southern most Front Range watershed) receives substantially lower snowfall than the other Front Range watersheds.
Mean Annual Snowfall for the Front Range. Dotted line indicates location of 2300 m contour, thick dark blue line shows location of continental divide, light blue lines show main stem of rivers.
(After Map 21 in “Climatography of the Front Range Urban Corridor and Vicinity” by Hansen et al., 1978)
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Flood Hydroclimatology in the Colorado Front Range
Within the Front Range, there are three primary causes for floods: local thunderstorms, intense widespread rainfall and snowmelt (Collins et al., 1991).
For information regarding flooding and debris flows in the Front Range see Flooding in the Colorado Front Range and Debris Flows in the Colorado Front Range.
Click a link below to learn more about flood hydroclimatology:
Floods Primary Sources
Within the Front Range, there are the following three primary causes for floods:
Intense Local Thunderstorms: Intense localized thunderstorms, which typically occur from May until early September, can generate flash floods essentially anywhere in the state. However, floods due to thunderstorms in mountainous areas are most likely to occur at altitudes below 7,500 feet (or 2,300 meters), as a result of higher rainfall intensities and greater aerial extents of the thunderstorms in the catchments for streams and rivers below 7,500 feet (Jarrett and Costa, 1988 and Collins et al., 1991). See the figure below showing the mean June precipitation (1960-1990) and the 2,300 meter (or 7,500 ft.) contour for the Front Range.
Intense Widespread Rainfall: The widespread rainfall events typically occur between April and mid-June and are generally limited to the Great Plains and eastern foothills (Collins et al., 1991).
Snowmelt: Floods associated with snowmelt are most likely to occur when excessive late winter snowfalls and low temperatures maintain a deep snow-pack over a large range of elevations into late spring. Spring rainfall on snow-pack can often result in floods throughout the United States; however, this condition seldom occurs in Colorado (Collins et al., 1991).
Mean June Precipitation (1960-1990) map showing the Front Range Watersheds and the 7,500 ft. or 2,300 m contour line.
Floods: Snowmelt vs. Rainfall
Within the Front Range, floods can be caused by either rainfall or snowmelt. However, Jarrett and Costa (1988) found that floods due to snowmelt have higher discharges than floods due to rainfall, at altitudes above approximately 7,500 feet or 2,300 meters. That is, snowmelt floods dominate at altitudes above 7,500 feet (or 2,300 meters), whereas rainfall floods typically dominate at altitudes below approximately 7,500 feet or 2,300 meters. As shown in the two figures below, this observation is illustrated with the flood frequency curves for the Big Thompson River at Estes Park, located above 7,500 feet, and at the Big Thompson Canyon Mouth, located below 7,500 feet (Jarrett and Costa, 1988).
Within the Front Range, this altitude demarcation between snowmelt and rainfall dominated floods largely reflects orographic influence on the formation and extent of thunderstorms. That is, floods due to thunderstorms in mountainous areas are most likely to occur at altitudes below 7,500 feet (or 2,300 meters), because of higher rainfall intensities and greater aerial extents of the thunderstorms that contribute runoff to rivers and streams below 7,500 feet (Jarrett and Costa, 1988).
Flood Frequency Curves for the Big Thompson River at Estes Park (Fig. 3a from Jarrett and Costa (1988) on left) and at the Canyon Mouth (Fig. 5 from Jarrett and Costa (1988) on right)
Mean June Precipitation (1960-1990) map showing the Big Thompson Watershed, the 7,500 ft. (2,300 m contour) line, and the locations corresponding to the flood frequency curves shown in the figures above.
