Synoptic control of cross-barrier precipitation ratios for the Cascade Mountains

Abstract

This paper examines the synoptic control of precipitation contrasts across the Cascade Mountains of Washington State during the cool season. The average hourly precipitation for three stations on the western and eastern sides of the barrier is calculated, and composite synoptic conditions are determined for situations when precipitation is greater over the eastern slopes, similar precipitation falls on both sides, moderately greater precipitation falls on the western side, and substantially greater precipitation falls on the western side. Conditions prior to occluded/warm frontal passage are favorable for relatively equal precipitation amounts on both sides of the barrier, with extensive warm advection precipitation spreading over the region. Low-level winds are generally southerly during this period, resulting in less orographicmodulation of precipitation on the north-south-oriented Cascades. In contrast, postfrontal conditions are associated with westerly flow, enhanced windward precipitation, and greater drying on the leeward (eastern) slopes. Postfrontal conditions also bring drying aloft and reduced stability at low levels, both of which contribute to the enhancement of precipitation on the western slopes. The variation of the cross-mountain precipitation ratio between El Niño and La Niña years can be explained by the associated synoptic configurations. El Niño years have weaker zonal winds and thus less orographic modulation. Furthermore, the lesser vertical stability of La Niña years enhances precipitation on the windward side of the barrier where shallow convection is released.