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. 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 updated month 1-4 climate outlook for the U.S. features increasing forecast confidence based on the influence on North America climate by dramatic cool phase of the Pacific decadal oscillation and warm phase of the Atlantic multi-decadal oscillation. A weak ENSO regime has limited influence.

During the 30-day period of Oct. 12-Nov. 11, 2024, an unusually hot climate spread across most of Australia featuring widespread 30-day departures from normal of +2C to +5C. The anomalous heat is driven by a titanic upper-level high-pressure ridge centered on central continent. The catalyst to the upper ridge, in-part, is due to warming influence on the middle troposphere by the presence of an emerging large and strong marine heatwave (MHW) off the northwest coast of Australia.

The long-term cycle of the Atlantic multi-decadal oscillation (AMO) shifted to the warm phase in the mid-to-late 1990’s. The tendency for a consistent warm phase increased in the 2 decades to follow. However, the recent warming, especially in the current decade has reached “excessive warm” values.