U.S. Winter Outlook Emphasizes Cold Risk West; Warm East as Strong -PDO/+AMO Fuel Pattern
11/15/2024, 11:34 am EST
East U.S. Cold Days 11-15: GFS/ECM Vs. AI Forecasts
11/26/2024, 8:40 am EST
Highlight: First bomb cyclone of the 2024-25 cold season. Climate correlations are not well-understood. But changing (and on-going) mid-latitude SSTA regime may be contributing.
Fig. 1: The first bomb cyclone of the 2024-25 cold season produced a 945 MB low of the Northwest U.S. Coast yesterday morning.
Discussion: The 2024-25 cold season first “bomb cyclone” either side of North America occurred Nov. 19-20 as a storm system off the Northwest U.S. Coast plummeted to 945 MB central pressure equivalent to a major hurricane (Fig. 1). Evolution of this storm is likely in-part due to a moderate-to-strong convection phase of the Madden Julian oscillation (MJO) across the Maritime Continent coupled with intense marine heat waves across the Northwest Pacific in contrast to the rapidly cooling Northeast Pacific of the past couple weeks. The key is the active MJO, well-known for causing strong mid-latitude storms during the cold season. ENSO is neutral and La Nina 2024-25 appears doubtful. Neutral ENSO should enable an active MJO pattern during the winter season producing an increased risk of bomb cyclones.
Surprisingly, the correlation between an active MJO and above normal number of bomb cyclones during 1979-2019 is only marginally indicated (Fig. 2). In fact, the correlation between strong ENSO episodes and arctic oscillation (AO) regimes is not consistent with the most active bomb cyclone years. The only climate correlation, upon preliminary review, well-correlated to an active bomb cyclone year is the emergence of Northeast Pacific marine heatwaves (MHW) and the North Atlantic warm hole (NAWH) in 2013-17.
Based on preliminary investigation, for the data available, the uptick in bomb cyclone activity during 2013-17 is related to the changing mid-latitude SSTA regime unique to the past 10 years. The pattern featuring strong MHW’s and the NAWH regime continues.
Fig. 2: Based on research by R. Fritzen, V. Lang, and V. Gensini the number of annual bomb cyclones for 1979-2019 is indicated with the annotated number a 1-5 scale activity level of the MJO (1 lowest, 5 highest) and strong El Nino or La Nina episodes.