The DEC-25 U.S. gas population weight HDD is revised colder to 949, the coldest DEC since 2010 and 3rd coldest of this century. Previously, cold JAN-26 forecasts reversed warmer with mid-NOV updates and then colder with the early DEC projection. Operational models are shifting much warmer for JAN-26 therefore a warmer revision is necessary. The JAN-26 HDD forecast warms from 981 to 947 which ranks 13th coldest of 2020-25.

La Nina has recently weakened. Some regeneration is possible for later this month/early January. However, the push toward warming by a recent intense MJO event suggests the weak late 2026 La Nina has peaked, and neutral ENSO generates for Q1/2026. The warming Northeast Pacific generated in-part by the recent intense MJO event requires monitoring. Often, warming of the Northeast Pacific is a precursor to an ENSO phase change toward El Nino. ECMWF and NMME (models) forecast El Nino onset during Q2/2026.

Theme of the Monday morning extended range U.S. temperature anomaly forecast is warmer. The most dramatic warmer change is the consensus of operational models forecast for the week ending Christmas Day and AI projections for early January.

In South America, rainfall forecasts have shifted wet risk to Brazil during the past 1-2 weeks. The 7-day percent of normal observed rainfall identifies wetter Brazil weather while rains linger in Argentina. Low pressure areas spawn heavy rain the next 1-2 days Paraguay into Brazil and Northeast Argentina late this week. Additional heavy rain is projected in the 6-10-day and 11-15-day period centered east of Paraguay.