Even as winter has been largely on "pause" since the first half of December, unusual large-scale weather phenomena have been occurring. The SOI wound up averaging an El Niño record +9.14 in December with 45 days during which that index was positive. The MJO moved into Phase 5 and got locked in for a winter record duration while setting a preliminary December amplitude record for Phase 5. On January 1, the MJO remained in Phase 5 for the 15th consecutive day with an amplitude of 2.715 (RMM). That amplitude is somewhat higher than the 12/31-adjusted figure of 2.508. There were only 6 prior situations when the MJO remained in Phase 5 for at least 15 consecutive days. All of them occurred either during meteorological summer and/or meteorological fall. The MJO's move into Phase 6 at a high to very high amplitude is likely imminent, as some of the guidance initialized in Phase 6. During the second week of January, it will likely progress into Phase 7. As the MJO's progression has been consistently slower than modeled, there is some risk that it could make a slower progression through Phase 6 than currently modeled. As the MJO moves into Phase 6, the SOI could experience fluctuations to negative values. Already, the SOI has been negative for 3 consecutive days for the first time since November 17-19. The AO will likely remain mainly positive at the onset, but could develop negative values as the MJO approaches Phase 7. As the MJO reaches Phase 7, the SOI will likely go persistently negative and the AO will also become increasingly negative. The second half of January could feature the development of a predominant PNA+/AO- combination. The sudden stratospheric warming (SSW) event, which recently peaked, and has seen zonal winds at 1 mb and now 10 mb reverse, remains a wild card. While the polar vortex is beginning to split, considerable uncertainty exists about the overall evolution of events related to the SSW and their ultimate impact on North America. Any outcome will likely have a long-duration effect on the overall pattern following the completion of the ongoing event.
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