A strong positive phase Asia-Bering Sea-North America (+ABNA) index emerges over the next 2 weeks and cools the North Pacific which leads to (NOAA) La Nina onset and intensity increase, a warming of North America and soaking atmospheric river into the North America West Coast.

Despite agreement between CFS V2 and ECMWF on evolution of a polar vortex pattern in Canada by Jan. 6-12, forecast confidence remains below average. There are questions on whether the PV pattern is driven by late December stratospheric warming over Eurasia or whether the event is in response to an eastward shift of the Madden Julian oscillation across the American longitude. Additionally, there are questions on extent of snow cover to force an effective cold and whether a ridge bridge across Alaska can tap Siberia arctic air. Stay tuned! A potential cold scenario, but not a guarantee.

On the ENSO front, nearly 30 days of positive phase southern oscillation index (+SOI) intensifying during early December has caused increased trade winds and cooling SSTA in the eastern equatorial Pacific Ocean. The Nino34 SSTA reached the La Nina threshold of -0.50C over the weekend. The +SOI regime continues through mid-December. Oceanic La Nina onset is possible by early January.

The wet pattern starting December 2024 is (mostly) across Argentina extending northward to Southern Brazil. North of the wet zone, most of Brazil is drier than normal. The daily soil moisture analysis reveals the effectiveness of spring rains at eroding both Argentina and Brazil long-term drought.