Strengths and Weaknesses of High Resolution Numerical Weather Prediction in Precipitation Forecasting for Mountain-Desert Climate Regimes

Over the past 15 years, significant improvements in Numerical Weather Prediction (NWP) horizontal grid scale resolution have proven to directly result in more accurate Quantitative Precipitation Forecasts (QPF) across the complex terrain in the Western United States. These upgrades have not been limited to gentle rising slopes such as the Sierra Nevada, but also observable in complex orographic regions of steep terrain which display abrupt upslope and downslope precipitation signatures such as terrain found in southwestern California. An accurate spatial distribution, temporal resolution and representative magnitude in QPF across these regions have major implications on the forecast runoff potential.

In the past 5 years, further advances in resolution has resulted in 1 to 3 km convection allowing NWP which enable forecasters the ability to anticipate the location of the heaviest rainfall down to a basin level. This leads to the potential to use QPF in the hydrological flash flood modeling recently developed for a semi-arid basin in Southwest California by Schaffner et. al. (2014). While these improvements are significant, at times it has been discovered that under certain orographic enhanced forcing environments the NWP may grossly over-estimate precipitation. This study will focus on particular events when apparent over production in precipitation was generated in shallow low-level boundary layers and orographic flow. It was apparent the instability present in these atmospheric conditions were a mixture of warm and cold air precipitation processes that may yield QPF error due to inherent issues of properly resolving microphysics, and the inability to effectively reduce the instability in the orographic forcing which may result in the more accurate lower precipitation amounts and duration.