The Development of the North Pacific Jet Phase Diagram at NCEP-WPC as an Objective Tool to Characterize the Upper-Tropospheric Flow Pattern

Extreme temperature events (ETEs) during a single season can contribute disproportionately to temperature anomaly statistics for that particular season. This disproportionate contribution suggests that (1) ETEs need to be considered in describing and understanding the dynamical and thermodynamic processes that operate at the weather–climate intersection and (2) consideration of ETEs may improve operational probabilistic medium-range (8–10-day) temperature forecasts. Prior work suggests that considerable variability characterizes the antecedent environments over the North Pacific prior to the development of ETEs over the continental U.S. This variability motivated the development of a North Pacific Jet (NPJ) phase diagram that is constructed from the two leading EOFs of 250-hPa zonal wind during September–May 1979–2014 in the CFSR. The projection of 250-hPa zonal wind anomalies at any one or multiple times onto the NPJ phase diagram subsequently provides an objective characterization of the state or evolution of the upper-tropospheric flow pattern over the North Pacific. Furthermore, the NPJ phase diagram offers the potential to increase confidence in medium-range temperature forecasts over the continental U.S.

This presentation employs the GEFS Reforecast Version 2 dataset during September–May 1985–2014 to construct a 30-year climatology of 9-day ensemble forecasts of the NPJ in the context of the NPJ phase diagram. Once constructed, NPJ phase diagram forecasts will be classified based on the NPJ regime at both the time of forecast initialization and at the time of forecast verification in order to examine the skill of forecasts initialized or verifying within the same NPJ regime. NPJ phase diagram forecasts subsequently will be stratified based on the magnitude of both the average 8–9-day GEFS ensemble mean error and the average GEFS ensemble member error within the NPJ phase diagram to determine the top 10% best and worst NPJ phase diagram forecasts. The best and worst forecasts will be examined employing composite analyses to illuminate the types of synoptic patterns that are most frequently associated with the best and worst NPJ phase diagram forecasts, respectively. Predicated on the ability of the NPJ phase diagram to characterize the upper-tropospheric flow pattern over the North Pacific, a real time web interface for NPJ phase diagram products at NCEP-WPC has been developed and will be illustrated.