Synoptic Control of Cross-Barrier Precipitation Ratios

An important question that has received relatively little examination in the literature is how the windward/leeward ratio of precipitation across a barrier is modulated by typical midlatitude synoptic evolution. A limited number of papers have examined the temporal variability of the orographic precipitation contrasts, including the origins of such variability. For example, Siler et al. (2013) described the variability of the rain-shadow effect across the Cascade Mountains of Washington State. A number of studies have described the microphysical evolution above the Cascade Mountains as wintertime fronts and cyclones pass eastward across the area (e.g., Hobbs 1975, providing some relevant insights for understanding orographic precipitation modulation over time. To enhance understanding of the synoptic modulation of orographic precipitation ratios, this paper explores the temporal variations in the strength of the precipitation gradient across the Cascade Mountains of Washington State and describes the synoptic conditions associated with periods in which precipitation is heavier on the western side, heavier on the eastern side, or nearly equal across the barrier. The physical connection between synoptic conditions and the changing cross-barrier precipitation contrasts are discussed.