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Keen, Richard A. ¹

¹ University of Colorado

Although the Colorado Front Range can be considered a semi-arid climate zone, it is subject to occasional large snowfalls. Â鶹ÊÓƵ once every two years, on average, a single storm will leave one to two meters of new snow in the foothills. The water content of one of these storms may supply 10 to 30 percent of the annual precipitation at some locations, making these large snow storms important factors in the water budget of the Front Range.

Synoptically, these large snow storms are caused by deep cut-off lows that extend from the surface to the tropopause, bringing a deep layer of moist air into the Front Range from the Gulf of Mexico. The Lows typically move slowly eastwards from the Four Corners area.

The NWS co-op climate observation station at Coal Creek Canyon, Colorado, about 11 miles southwest of Boulder, is often at the "epicenter" of the larger snow storms, and on several occasions has reported the greatest snowfall and/or moisture content during the storm (Doesken, 2003; Paulhus, 1953; Poulos et al., 2002; Schlatter et al., 1983). Although the Coal Creek station became a co-op site in 1994, consistent snowfall observations extend back to 1978. During the 32 snow seasons since 1978, Coal Creek has received 14 snow events that exceeded one meter (40 inches).

The climatology of these snowstorms shows that 5 events occurred during the late autumn (October through December), none during mid-winter (January and February), and 9 during the spring (March, April, and May).

Although there is no trend in the frequency of large snow events (seven during the first half of the record, and seven during the latter half), there is a profound correlation with el Niño. Since 1978 there have been nine el Niño winters, and 8 of the snow events occurred during these winter seasons. Another four events occurred during the April or May preceding the el Niño, while only two events occurred outside of el Niño periods. Thus, the probability of a meter-class snow storm occurring during the "12 months of el Niño" (the April to March period spanning the peak of el Niño) is 15 times as great as during a non-Niño year.

Doesken, N., 2003, A summary and observations of the March 17-20, 2003, snowstorm - a climatologist's view: Colorado Climate, v. 4, no. 1-4, p. 13-15.

Paulhus, J.L.H., 1953, Record Snowfall of April 14-15, 1921, at Silver Lake, Colorado: Monthly Weather Review, February 1953, p. 38-43.

Poulos, G. S., D. A. Wesley, J. S. Snook and M. P. Meyers, 2002: A Rocky Mountain storm - Part I: The Blizzard - observations, dynamics and modeling. Weather and Forecasting, v. 17, p. 955-970.

Schlatter, T. W., Baker, D. V., Henz, J. F., 1983: Profiling. Colorado's Christmas Eve Blizzard. Weatherwise, 36, 60-66.