Monitoring the Southeast Brazil Upper Trough to Determine Wet/Dry Zones Ahead

Highlight: Monitoring the Southeast Brazil Coastal trough.

Fig. 1-2: GFS 15-day upper air pattern and percent of normal rainfall forecast across South America.

Fig. 3-4: ECM week 3-4/week 5-6 rainfall anomaly forecast for South America.

Fig. 5-6: NOV/DEC/JAN 2021-22 upper air and rainfall anomaly observations across South America.

Discussion: The position of the upper-level trough on the Southeast Brazil coast in the 15-day outlook is far enough to the east to push the wet weather pattern into Northeast Brazil (Fig. 1-2). Previously, the trough position was farther west and centered on Southeast Brazil.

The slight offshore position is indicated in the extended range by the ECM as wet weather in both the week 3-4/week 5-6 outlook is across East Brazil and leaves a sustained dry pattern across Northeast Argentina and far Southeast Brazil (Fig. 3-4).

Last year, the upper trough was slightly north and east of this year’s short-term projection (Fig. 5). The slight location difference enabled a wider and stronger drought across Paraguay and Southeast Brazil (Fig. 6).

Conclusion: The upper trough for 2022-23 is likely a little closer to major crop areas in Brazil than last year and soils are wet across southwest and interior southeast Brazil. Consequently, a major drought affecting central and northern major crop areas in Brazil is unlikely for upcoming summer. However, drought risk to the south of this zone increases dramatically.

On the back side of the convective rains produced by the upper trough described, subsidence will lead to a dry climate pattern. The areas affected are south/southeast Paraguay, the northern half of Argentina, Uruguay and far Southeast Brazil.