The Role of Upstream Midtropospheric Circulations in the Sierra Nevada Enabling Leeside (Spillover) Precipitation. Part I: A Synoptic-Scale Analysis of Spillover Precipitation and Flooding in a Leeside Basin

Abstract

Pacific-originating storms that produce heavy leeside liquid precipitation in the Sierra Nevada are rare compared to those that generate windward slope rainfall. However, these leeside precipitation events have a profound effect on the flood hydrology of leeside basins in the Sierra Nevada. This study identified 12 storms that affected the Truckee River basin in northeastern Nevada. The storms produced both moderate and extreme flooding in this leeside basin. A synoptic-scale analysis of conditions leading to leeside storms was produced using a compositing procedure. Composites for multiple pressure levels and multiple parameters were produced for class 1 storms?those storms producing moderate flood flow in the Truckee River basin?and class 2 storms?those producing extreme flooding [>10 000 cubic feet per second (cfs), or 283 m3 s?1] in this basin. The analysis confirms that the two flood populations are in fact generated by Pacific-originating storms with observably different synoptic-scale circulations. The class 2 storms are moister through a great depth in the troposphere (saturated to 750 hPa), and they occur coincident with warmer conditions in the lower and midtroposphere. Class 2 events exhibited more favorable upper-level jet streak structures in the eastern Pacific and over western North America. Both classes of leeside storms were shown to differ substantially from Pacific-originating storms that exclusively affect the windward slope of the Sierra and the coastal mountain ranges of California (California storms). The leeside storms were much warmer than California storms through much of the lower and midtroposphere, and the onshore flow was predominantly from the west-southwest in leeside storms compared to southerly flow in California storms. The findings suggest the existence of a midlevel atmospheric river delivering moisture to leeside basins of the Sierra Nevada.