Prof. Jim Steenburgh
University of Utah
Abstract: Compared to the Sierra Nevada and Cascade Ranges of the Pacific states, the basin-and-range topography of the Intermountain West lacks a sustained high-mountain mass, features small cross-barrier length scales, and features broad alluvial basins. The resulting fine-scale topography, diverse precipitation climatology, and low spatial coherence of regional precipitation systems serve as a challenge for numerical forecast systems. In addition, regional forecasting is also complicated by a lower correlation between precipitation rates and integrated vapor transport, the important role of shallow orographic convection, storms that frequently deviate dramatically from climatological precipitation-altitude relationships, and large variability in snow-to-liquid ratios. An evaluation of upper-elevation precipitation forecasts produced by operational numerical modeling systems and the NCAR Ensemble during the 2016/17 cool season shows lower forecast skill over the western interior compared to the Pacific states, but also larger improvements in interior forecast skill with increasing resolution. Intercomparison of forecasts produced by the NCEP Short Range Ensemble Forecast System (SREF) and the NCAR Ensemble shows that forecasts over the western U.S., particularly the western interior, would improve with a high-resolution ensemble, but also that the the optimal ensemble design (i.e., resolution, number of members, spread) is unclear and may vary regionally.