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Strong Positive Phase of Arctic Oscillation Propels Stronger Than Normal Jet Stream and Strong Severe Weather Events for Spring

Discussion: The January/February 2020 arctic oscillation (AO) averaged +2.9 (second strongest on record) with the February AO index (+3.417) the strongest on record (for February). Identified by the vigorous +AO regime is an intense polar vortex anchored near the North Pole and strengthening the thermal ribbon in the upper atmosphere between very cold upper air in the polar region and warmer regime in the subtropics. The contrasting temperature is most realized in the vicinity of the jet stream and inspires a much stronger than normal jet stream in the northern hemisphere. A stronger than normal upper westerly flow across the U.S. has caused the very warm winter as mild maritime air from the Pacific has dominated the air mass regime mostly blocking cold air masses from Canada. The stronger than normal jet stream also increases outflow aloft where convective clouds form causing stronger thunderstorms more capable of producing severe weather. The +AO regime is forecast to remain strong in March. The +AO regime maintains a warm climate pattern across the U.S. for March and now adds the likely tendency for unusually strong severe weather events such as the Nashville, TN tornado episode on March 3 and the persistent flash-flood rainfall events in the Southern U.S.