Forecasting prolonged inversion episodes in the Intermountain West out to one month

During winter in the western United States, heavy rainfall events are known to be associated with the intraseasonal oscillation (ISO). In the Intermountain West, a different weather event is gaining increasing attention, i.e. persistent inversions. Deep inversions often develop in valleys and mountain basins and frequently lead to poor air quality. A close linkage between the occurrence of wintertime persistent inversions and the 20-40 day ISO was identified. The 20-40 day ISO is a pronounced mode in the midlatitude circulations but more so, its intraseasonal timescale implies that forecasting persistent inversions in the Intermountain West is beyond the ~10 day horizon of weather forecast models. Empirical and dynamical forecasts of the ISO exhibit skill at lead times beyond two weeks, including the operational Climate Forecast System (CFS) which has demonstrated credible skill in predicting the ISO. Such circumstances imply that the CFS may exhibit potential in predicting the inversion development with a long lead time. These circumstances lead us to undertake an investigation of the CFS's performance skill in predicting persistent inversions for northern Utah. A model was subsequently developed and is in operation to produce long-range forecast for prolonged inversion events.