Multi-season Observational Study of the Thermodynamic, Kinematic and Precipitation Structures within Flooding and Typical Storms in the Oregon Cascades

Abstract

During the winter months, extra-tropical cyclones develop over the Pacific Ocean and move across the U.S. Pacific Northwest. The Coastal and Cascade mountain ranges modify the precipitation patterns of these storms leading to enhanced precipitation. Winter storms affecting the Oregon Cascades from 2003 to 2008 are analyzed. Portland, Oregon operational WSR-88D radar data is utilized to examine the spatial patterns and distributions of precipitation structures over the windward slope of the Cascade Mountains. Data from operational soundings at Salem, Oregon and a vertically pointing MicroRainRadar in Portland are used to analyze the environmental characteristics upwind of the Cascades. Extreme storm events are associated with flooding and mudslides.

The precipitation persistence and intensity were calculated for radar subsets ± 6 hours of operational sounding launches based on the observed flow characteristics. The sensitivity of precipitation patterns to downslope flow and changing flow characteristics with altitude, as well as the strength of the cross-barrier flow, freezing level altitude, and atmospheric stability is tested. The cross-barrier flow is the most important factor in determining the magnitude of precipitation persistence and intensity along the Cascade windward slope. Two precipitation ‘hotspots’ occurring within our study domain are investigated further and we find that low-level flow convergence due to flow deflection by the Cascades is associated with the precipitation enhancement at both locations.

The conditional probability of flooding is examined based upon the values of single parameters and the combination of multiple parameters. All flooding storms were associated with land-falling atmospheric rivers, deep rain layers, strong cross-barrier flow, and long durations of precipitation. Flow being multi-layered with the presence of shear is shown to have a significant impact on the flooding potential by increasing the precipitation persistence and intensity along the Cascade windward slope. The lower-level flow acts to increase the effective barrier width, by forcing the mid- and upper-level flow to up and over the lower level flow regime. These criteria, however, are not sufficient to lead to flooding. 65% of time periods with these conditions flood over the course of our 5 year dataset. The spatial patterns of the precipitation persistence and intensity provide insight into why some storms with anomalously high values of certain environmental conditions lead to flooding and others do not. Flooding storms had more persistent and intense rainfall occurring upstream of the Cascades compared to non-flooding storms.