Weather Regime-Dependent Predictability: Antecedent Environments Conducive to the Production of High-Impact Weather Events over the United States

High-impact weather events (HWEs) during a single season, such as those that occurred during the latter half of March 2016, can contribute disproportionately to temperature and precipitation anomaly statistics for that particular season. This disproportionate contribution suggests that (1) HWEs need to be considered in describing and understanding the dynamical and thermodynamic processes that operate at the weather–climate intersection and (2) consideration of HWEs may improve operational probabilistic medium-range (8–10 day) temperature and precipitation forecasts. On the basis of these suggestions, a methodology to objectively identify HWEs over the continental United States will be presented. For this investigation, high-impact will be defined on the basis of observations and in terms of percentiles, and events will be defined on the basis of the spatial coverage and duration of high-impact temperatures and precipitation.

Following their identification, HWEs will be stratified into event types based on the state of the North Pacific jet stream 3–7 days prior to the development of an HWE. Subsequently, disturbance-based composites will be constructed for each event type to examine the governing atmospheric flow patterns that are essential to the evolution of each event type. The 8–10 day NCEP GFS operational and ensemble forecasts for each event type will also be examined to discern whether certain event types are characterized by increased or reduced medium-range forecast skill. Knowledge of both the predictive skill of medium-range forecasts and the governing atmospheric flow patterns characteristic of each event type has the potential to provide forecasters with a “first alert” to the possibility of HWEs within the 8–10 day period. Motivated by this potential, a real-time North Pacific jet phase diagram will be presented as a tool to characterize the state and evolution of the upper-tropospheric flow pattern over the North Pacific.