Forecast Models Rainfall Projections Are Poor Across South America Past 30 Days

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Forecast models have over-forecast rainfall across the dry-to-drought portion of Brazil consistently the past 30 days for both short-range and medium-range timescales. Forecast models have a tendency to turn northwestern South America wet with a fetch of that moisture across Brazil (at times). Instead the northwest South America dry pattern (and drought) persists and central/south Brazil rains are failing.

Persistent Rainfall Forecast Error Across South America

Discussion: The past 30 days have brought heavy rains to coastal northeast South America which is typical of a La Nina climate (Fig. 1). Enhancing the coastal northeast South America climate is warmer-than-normal SSTA in the equatorial Atlantic making trade winds wetter as they push into northeast continent. Drought over northwest South America continues enhanced by a very dry 30-day period.

During the past several weeks forecast models have consistently offered beneficial rainfall for the dry-to-drought zone across central and southern Brazil. However, verification scores by all model output identity large rainfall forecast errors for central and eastern Brazil and especially northwestern South America. Interestingly, the model errors are large in both short-term and extended-range forecasts (Fig. 2-4) the past 30 days (using the GFS).

The forecast models have clearly linked wet climate over the northwest continent and increased risk for crop areas of Brazil. The problem is that connection is not developing as summertime approaches. One potential explanation is that a La Nina climate which is present and well-fortified normally biases northeast South America wetter than normal while northwest sections has a tendency to be drier than normal.

Fig. 1-4: The 30-day observed percent of normal rainfall forecast across South America and the rainfall forecast error by the GFS model for the past 30 days in the day 1-5, day 6-10 and day 11-15 periods.