Climate drivers in the 11-15-day extended-range forecast indicate the Madden Julian oscillation (MJO) is weak therefore tropical forcing is weak. The stratosphere is shifting much colder following the NOV/DEC warm event as indicated by the 40-day CFS V2 projection. The core of the colder stratosphere is in the same location as the NOV/DEC warming event, across northwestern North America.

The Nino3 SSTA is the coolest (of this episode) so far at -1.0C. The Nino34 SSTA cools to -0.8C. In the subsurface, the eastern equatorial Pacific Ocean is plenty cool to sustain La Nina. A warm Kelvin Wave shifting east near the Deadline appears to undercut the cool anomaly keeping cooler waters near the surface for trade winds to up-well to the surface.

The effectiveness of the “atmospheric river” storm track is enhanced by a surge in warming of the Northeast Pacific. A marine heatwave centered in the Northeast Pacific well northeast of Hawaii has strengthened and expanded to the U.S. West Coast. The warmer ocean surface temperature adds low-level atmosphere moisture entrained by the Pacific storm track and unloaded on the West U.S. with attendant latent heat release warming much of the U.S. for the next 2-3 weeks.

The 2025-26 cold season is barely underway and already two dramatic weather regimes are identified: 1.) The just-ending arctic outbreak across the East U.S. and 2.) The evolving excessive precipitation event for the West Coast lasting the remainder of December and into January.