South America Weather Pattern Change In December

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Climate Impact Company Week 2-to-4 South America Outlook

Issued: Tuesday November 25, 2025

Headline: South America weather pattern changes I December.

Charts of the day: Unusual SSTA pattern contributes to DEC-25 South America climate.

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Discussion: NOV-25 was mostly hotter/drier than normal across Central/Northeast Brazil while Argentina was temperate with patchy rain zones. In DEC-25, a pattern change is forecast. The catalyst to the pattern change is the influence on the upper air pattern of a marine heat wave (MHW) west of Chile and an evolving cool (SSTA) pool off the Central East Coast of South America. Normally, as meteorological summer begins, the opposite (SSTA) regime is present during recent years either side of South America. Near the MHW, a persistent upper-level high pressure ridge is forecast for DEC-25 yielding a hotter/drier climate for Argentina. Meanwhile, an upper trough anchors across the cooler SSTA with wet influence suppressing anomalous heat across East/Northeast Brazil. The upper air forecasts by NCEP CFS V2 and the Climate Impact Company constructed analog are reasonably similar for DEC-25 therefore a confident forecast of pattern change or early summer 2025-26 across South America.

Week-2 Valid November 30-December 7, 2025: Wet weather consolidates in Brazil.

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Discussion: Wet weather across Argentina/Brazil this weekend consolidates over Brazil next week. Argentina shifts drier and hotter while anomalous heat risk is suppressed by wet weather in Southeast/East Brazil.

Week-3 Valid December 7-14, 2025: East Brazil stays wet.

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Discussion: Upper ridge dries/heats Northern Argentina while upslope flow causes thundershowers West and Central Argentina. East Brazil stays wet.

Week-4 Valid December 14-21, 2025: Wet Central/East Brazil continuing.

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Discussion: After mid-December, hot and dry weather dominates Argentina with the wet pattern in Central/East Brazil continuing.