Subseasonal Prediction of the State and Evolution of the North Pacific Jet Stream

Abstract

AbstractThe state and evolution of the North Pacific jet (NPJ) stream strongly influences the character of the downstream synoptic‐scale flow pattern over North America. This study employs data from nine models within the Subseasonal‐to‐Seasonal Reforecast Database hosted by the European Centre for Medium‐Range Weather Forecasts to examine the subseasonal (2 weeks–1 month) predictability of the NPJ through the lens of an NPJ phase diagram. The NPJ phase diagram provides a visual representation of the state and evolution of the NPJ with respect to the two leading modes of NPJ variability. The first mode of NPJ variability corresponds to a zonal extension or retraction of the climatological jet‐exit region, whereas the second mode corresponds to a poleward or equatorward shift of the climatological jet‐exit region. The analysis reveals that ensemble forecasts of the prevailing NPJ regime, as determined from the NPJ phase diagram, are skillful into week 3 of the forecast period. Forecasts initialized during a jet retraction, or verifying during a jet retraction and equatorward shift, generally feature the largest errors during the forecast period. Examination of the worst‐performing 21‐day forecasts from each model demonstrates that the worst‐performing forecasts are uniformly associated with development, maintenance, and decay of upper‐tropospheric ridges over the high‐latitude North Pacific. These results demonstrate that bias‐corrected NPJ phase diagram forecasts have the potential to identify periods that may exhibit enhanced forecast skill at subseasonal lead times based on the anticipated NPJ evolution.