CWCB Wintertime Weather Mod Report

• Project Type: General OWC
• Applicant: Weather Modificaton
• Project Name: Colorado Weather Modification Program
• Title: CWCB Wintertime Weather Modification Report Fall 2002 - Spring 2003
• Date: 7/1/2003
• Water Conservation - Doc Type: Final Report

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occur during a variety of weather patterns, which tend to vary in a somewhat predictable
manner according to the time of year. During the winter period (December-February),
many of these precipitation events resulted from strong zonal (west-east) flow, with mid-
and high-level Pacific moisture moving over Colorado. Satellite images during these
zonal events were often very impressive, with high cloud cover (very bright on the IR
imagery) covering large portions of the western U.S. Deep upper-level trough situations
were also responsible for some of the precipitation events. Storm systems traversing the
southwestern United States and southern Rockies favored the spring and fall seasons.
, Typically the more significant precipitation amounts occurred in southwesterly flow
ahead of the surface frontal system (sometimes well in advance), although some did
occur in northwesterly flow following frontal passages. A thick shield of high-level
cloud cover was evident in most of these significant precipitation events, suggesting a
large amount of high-level moisture. Strong upper-level winds and a jet core near
Colorado were very often associated with these precipitation events.

2.3 Frequency of Significant Storm Events

An analysis was conducted using daily precipitation records from four Natural
Resources Conservation Service SNOTEL sites in the 9600 - 10600 foot elevation raJ?ge
in the mountains northwest and northeast of Gunnison, Colorado. The four sites were
Brumley, Butte, Independence Pass, and Park Cone. Days with approximately 0.4" or
more of precipitation during the October-April season during the 'years from 1990
through 2001 were identified. It was found that 12 or 13, O.4-inch or greater storm days
occur during the October-April season on average. April had the highest frequency of
these events (2.7 per season), followed by November and February (2.1 per season).
December had the lowest frequency with approximately Istorm day per season. An
average amount of one inch or greater occurred at these SNOTEL sites on only about
10% of these storm days.