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This study considers the development of continental U.S. extreme temperature events (ETEs) during the cool season (September–May), where extreme temperatures are defined in terms of percentiles and events are defined in terms of the spatial coverage of extreme temperatures. Following their identification, ETEs are classified into geographic clusters and stratified based on the state of the North Pacific jet (NPJ) stream prior to ETE initiation using an NPJ phase diagram. The NPJ phase diagram is developed from the two leading modes of NPJ variability during the cool season. The first mode corresponds to a zonal extension or retraction of the exit region of the climatological NPJ, while the second mode corresponds to a poleward or equatorward shift of the exit region of the climatological NPJ. The projection of 250-hPa zonal wind anomalies onto the NPJ phase diagram prior to ETEs demonstrates that the preferred state and evolution of the NPJ prior to ETEs varies considerably based on the geographic location of ETE initiation and the season. Southern plains extreme warm events are an exception, however, since extreme warm events in that location most frequently initiate following a retracted NPJ during all seasons. The NPJ phase diagram is subsequently utilized to examine a synoptic-scale flow evolution highly conducive to the initiation of southern plains extreme warm events via composite analysis. The composite analysis demonstrates that a retracted NPJ supports an amplification of the upper-tropospheric flow pattern over North America, which then induces persistent lower-tropospheric warm-air advection over the southern plains prior to ETE initiation.

Antecedent North Pacific Jet Regimes Conducive to the Development of Continental U.S. Extreme Temperature Events during the Cool Season