A weak La Nina is forecast for Q4/2025 shifting to neutral phase during early 2026 and possibly to El Nino during the middle of next year. A strong ENSO episode within the next 12 months is not expected although forecast confidence in a phase change is reasonable.

Important as a review, is the influence on summertime rainfall patterns in South America by a semi-permanent upper-level low pressure trough located off the Southeast coast of Brazil for most years in this decade. The just-issued ECMWF “monthlies” are forecasting the presence of this low-pressure system for early-to-middle summer.

Surprising is overall success of the GFS ENS skill score of forecasting the upper air pattern (using 500 MB anomalies) during the 16-30-day period across both North and South America for the past 30-day, 14-day, and 7-day intervals. Traditionally, the ECM “weeklies” are used to make a “first guess” at the expected weather during this timeframe. ECM rivaled GFS ENS for North America forecasts and finished second to GFS ENS in South America.

Last month, the “warm blob” returned to the Northeast Pacific. However, during September the “warm blob” has lost intensity and an atmospheric river (AR) storm track into Southwest Canada this week is likely to cause further erosion.