AbstractAbstractThe state and evolution of the North Pacific jet (NPJ) stream exert considerable influence on weather conditions across North America. The NPJ is modulated by several multiscale processes during the cold season (November–March), including the effects of organized tropical convection on synoptic to seasonal time scales. This study specifically explores the influence of El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), and atmospheric blocking on the short-to-medium-range (0–9-day) predictability of the NPJ. NPJ predictability is evaluated using an NPJ phase diagram that is constructed from the jet’s two leading patterns of variability: a zonal extension or retraction of the climatological exit region of the NPJ and a poleward or equatorward shift of the climatological exit region of the NPJ. Results demonstrate that NPJ forecasts during El Niño or MJO phases 7–8 feature enhanced forecast skill relative to the Global Ensemble Forecast System (GEFS) model climatology, particularly when those forecasts coincide with an extended or poleward-shifted NPJ at the time of forecast initialization. Reduced forecast skill occurs during any active MJO phase if the NPJ is retracted at the time of initialization and during forecasts initialized at the onset of northeast Pacific blocking events. Analyses further suggest that NPJ predictive skill may be derived from the constructive or destructive interference of the separate teleconnections induced by ENSO and the MJO and depends heavily on the incipient state of the NPJ.Significance StatementThe evolution of the North Pacific jet stream plays a considerable role in dictating downstream weather conditions across North America. Consequently, this study examines whether there are certain situations in which the North Pacific jet stream is more predictable at 0–9-day forecast lead times, with a particular focus on the influence of subseasonal-to-seasonal modes of atmospheric variability, such as El Niño, the Madden–Julian oscillation, and midlatitude blocking events. Analyses show that North Pacific jet stream predictability depends not only on the aforementioned modes of variability but also on the state of the jet stream at the time a forecast is made. Forecasts are generally improved during El Niño and particularly when the North Pacific jet stream is extended or poleward shifted. Forecasts are less accurate during periods characterized by a retracted North Pacific jet stream and at the start of northeast Pacific blocking events.
