Using a Local Weather Research and Forecasting Model in an Operational Setting to Predict Strong Mountain Waves in Southern California

Southern California experiences strong mountain waves which result in strong, frequently damaging Foehn winds, often locally exceeding 30 m s-1 at the surface. These severe wind conditions can occur from mountain waves associated with either strong southwest to west (onshore) flow or strong east to northeast (offshore) flow. The strongest surface winds are with hydraulic jumps a short distance downwind from the mountain crests, but trapped lee waves can also contribute to extreme winds in southern California. The mountain waves often have a small enough wavelength that a high-resolution mesoscale model is required for the waves to be well represented. The National Weather Service Forecast Office in San Diego, California runs a Weather Research and Forecasting Environmental Modeling System (WRF-EMS) using an Advanced Research WRF (ARW) core operationally at 3.7 km grid spacing for a domain over southern California. This model will resolve mountain waves and has an option for running nests at a higher resolution (up to 1.5 km grid spacing in this study) to assist forecasters with wind predictions in complex terrain.

Southern California offers a unique location for verification of winds in complex terrain due to one of the densest observational networks in the United States, with hundreds of automated weather stations, spatially positioned across diverse terrain, including mountains, desert slopes, passes and canyons. These observations were used in sensitivity studies of model parameterizations. In this study, we will examine results of the WRF numerical weather prediction during high-wind events in southern California for different synoptic weather patterns, ranging from Santa Ana winds (katabatic offshore flow) to inland surface low pressure troughs which bring intense onshore winds to the desert mountain slopes and sometimes the adjacent desert floors. Better understanding these Foehn winds will assist forecasters with predicting weather conditions which can result in large wildfires, wind damage to structures and dust storms.