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ABSTRACT
Over the 3-day period of 24-26 October 1997, a powerful winter storm was the cause of two exceptional weather phenomena: 1) blizzard conditions from Wyoming to southern New Mexico along the Front Range of the Rocky Mountains and 2) hurricane-force winds at the surface near Steamboat Springs, Colorado, with the destruction of about 5300 ha of old-growth forest. This rare event was caused by a deep, cutoff low pressure system that provided unusually strong, deep easterly flow over the Front Range for an extended period. The event was characterized by highly variable snowfall and some very large snowfall totals; over a horizontal distance of 15 km, in some cases, snowfall varied by as much as 1.0 m, with maximum total snowfall depths near 1.5 m. Because this variability was caused, in part, by terrain effects, this work investigates the capability of a mesoscale model constructed in terrain-following coordinates (the Regional Atmospheric Modeling System: RAMS) to forecast small-scale (meso gamma), orographically forced spatial variability of the snowfall. There are few investigations of model-forecast liquid precipitation versus observations at meso-gamma-scale horizontal grid spacing. Using a limited observational dataset, mean absolute percent errors of precipitation (liquid equivalent) of 41% and 9% were obtained at horizontal grid spacings of 5.00 and 1.67 km, respectively. A detailed, high-temporalresolution (30-min intervals) comparison of modeled versus actual snowfall rates at a fully instrumented snow measurement testing site shows significant model skill. A companion paper, Part II, will use the same RAMS simulations to describe the observations and modeling of the simultaneous mountain-windstorm-induced forest blowdown event.
1. Introduction
During 24-26 October 1997, a deep cutoff low pressure system moved eastward across the southern Colorado Rocky Mountains producing blizzard conditions along the Front Range from Wyoming to southern New Mexico. In addition, this storm caused easterly hurricane-force winds at the surface near Steamboat Springs, Colorado, which destroyed about 5300 ha (1 ha = ~2.5 acres) of west-slope forest. Heavy snow also occurred in Nebraska and Kansas. The effects on the public of these phenomena were immediate and costly: widespread power outages, a crippled international airport, and other transportation-related problems (such as the closure of Interstate Highway 25 (I-25) from the northern to southern border of Colorado simultaneously), hunters stranded in the...