South America Calling

After months of dry-season conditions, central Brazil appears poised to enter its wet season as a slow-moving front pushes northward across the region, bringing widespread showers and thunderstorms.

A front moving north from southern Brazil is slowly getting into central Brazil. Its slow movement through the region will give rise to widespread coverage of showers and thunderstorms. As long as this forecast verifies, we can reasonably assume the start of the wet season in central Brazil. However, there are some features that may give pause to the overall favorable weather.

This front moving through has already brought heavy rain through southern Brazil. Some areas of Rio Grande do Sul have recorded more than 100 millimeters (about 4 inches) of rainfall so far. The front will continue to spread rainfall northward through the end of the week. Models are mixed on whether or not the front continues to produce scattered showers over the weekend, but the point appears moot. Another front moving up from Argentina will get into central Brazil late Sunday and Monday, Oct. 4-5, and could be followed by yet another front a few days later.

The rainfall forecast from the ECMWF model this morning is printing out 30-75 millimeters (about 1.2 to 3 inches) of rainfall through Thursday, Oct. 8, and should be enough rainfall to get producers out into their fields to rapidly plant soybeans, as their window to do so is very short.

Producers count on increasing rainfall from late September into early October to moisten soils after a four-to-five month period of near-complete dryness during the dry season. The wet season can be loosely defined as the first 30 millimeters of rainfall, but this can come from multiple sources, like fronts moving north from Argentina. Describing it this way negates what really counts -- scattered pop-up showers and thunderstorms that dot the region on a daily basis. These monsoonal showers are what combat the tropical heat stress near the equator, and give life to the rapid growth of agricultural farmland in the region. The length and intensity of the monsoon rainfall allow for a double crop of soybeans and corn (called the safrinha, or second-season) in a single calendar year. But if the monsoon is augmented in any way, shape, or form, there could be significant production implications.

El Nino, which is on its way to be record-strong this year, certainly has an impact on the monsoon in central and northern Brazil. The warm waters off the coast of Peru cause lots of evaporation and upward motion of air. But if the warm ocean causes upward motion, downward motion has to occur somewhere else. This typically happens over the western Pacific but also across northern South America, leading to spottier precipitation, longer time between showers, and increased drought potential. Seasonal weather models are forecasting this growing season to follow this typical pattern, which may cause significant issues for soybean and corn production in central and northern Brazil.

Indeed, when fronts are not pushing north into central Brazil, models are tending to limit the overall rainfall. It still rains, but not at the same frequency or intensity as is typical. If the current forecast verifies, it may lead to good planting conditions, but poor growth and risks of drought and lower production this year.

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John Baranick can be reached at john.baranick@dtn.com

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