An important climate diagnostic, missing since last April, has returned this month: Multivariate ENSO index (MEI). The MEI identifies the response of the atmosphere (30S to 30N) to the Pacific equatorial sea surface temperature anomaly (SSTA) regime. Note that the conventional climate diagnostic to identify ENSO phase, the Nino34 SSTA, only recently shifted to La Nina. A La Nina regime is indicated by MEI all but APR and MAY of the past 12 months.

Upper trough maintains the Mediterranean Sea storm track in late December/early January. North of the storm track, colder east wind from Russia develops. However, dry pattern prevents any snow gain.

Wet weather continues to emerge and intensify across Brazil. The rainfall is making a difference in soil moisture as East Brazil loses their wet signature. Additional heavy to excessive rainfall is on the way!

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.