Relation between Upper Air Lows and Winter Precipitation in the Western Plateau States*

A comparison is made of the relative effectiveness of the cyclonic circulation pattern a t four different tropospheric levels in specifying winter precipitation over the intermountain area of the western United States. This is accomplished by developing the synoptic climatology of precipitation resulting from Lows a t the 850-, 700-, 500-, and 300-mb. levels. Twelve-hr. precipitation amounts (expressed as a percent of the 7-day normal) a t 280 stations in the Plateau States for 13 yr. are related to the positions of nearby low centers through a computer system of moving coordinates. For each level, the upper Lows are classified into three intensity categories according to the departure from normal of their central heights. Average precipitation amount, distribution, and frequency of occurrence are then calculated for the area of the grid system. The dependence of these precipitation characteristics upon the level, intensity, and location of the upper Low is described and illustrated. The geographical distribution of Lows a t the four upperlevels and a schematic model of their associated precipitation are also presented. It is concluded that the effectiveness of upper Lows in producing precipitation generally varies directly with their intensity and inversely with elevation.