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.

The arctic oscillation (AO) which identifies the location of the polar vortex indicates a change to positive phase the next 1-2 weeks as the recent frigid Canadian polar vortex shifts poleward. The shift toward the North Pole allows milder Pacific maritime influence to warm the U.S. pattern. The extended-range AO forecast suggests the positive phase may continue well into January. However, there are caveats.