Forecast accuracy associated with high impact weather events over the western United States downstream of extratropically transitioning tropical cyclones

High-impact weather events can occur downstream of tropical cyclones (TCs) that have undergone extratropical transition when midlatitude flow amplifies via Rossby energy dispersion. Global model hemispheric forecast skill has been shown to decrease significantly following these extratropical transition events. As a result, ensemble forecasts often have large spread and low confidence. Previous studies have identified extratropical transition cases and investigated the predictability of resulting downstream flow, but little research has focused on their impacts in the western United States.

This study identifies several high impact weather events that took place in the western United States since January 2000. These events each occurred soon after the extratropical transition of a typhoon in the western North Pacific basin. The ability of the GEFS reforecast system to accurately predict each event is examined by calculating error between forecasts and analyses. In this presentation, we will show the ensemble system's ability to predict high impact events as well as investigate the physical mechanisms associated with both accurate and inaccurate forecasts.