South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 1246 PM EDT Mon Mar 17 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 17 March 2025 at 18 UTC ANALYSIS AND FORECAST OF LARGE SCALE CONDITIONS MJO and Tropospheric Waves: The MJO is propagating across phase 3, which is consistent with the current large scale upper convergence established over South America. A tropospheric Kelvin wave is propagating across the Central Pacific, but it is not forecast to arrive until the weekend/early next, when it is likely to provide some minor enhancement to areas and the intensity of deep convection in tropical portions of South America. SST: Important positive anomalies continue along equatorial portions of the Eastern Pacific and now have extended into the Gulf of Panama. Note, however, that these warm anomalies are mostly superficial and could be rapidly altered by changes in surface winds. In fact, cool anomalies have emerged along the Peruvian coast in response to strong southeasterly trades in the last few days. SYNOPTIC ANALYSIS AND FORECAST FOR TROPICAL SOUTH AMERICA Analysis: The subtropical upper ridge is centering in two cells. One is located off the coast of southern Peru while the other elongates zonally over southeast Brasil. The impact of these ridges in ventilating convection is somewhat limited by the presence of a mid-level ridge extending over southeast Bolivia/northern Paraguay into Espirito Santo in Brasil. The most active convection and precipitation is occurring in association with the ITCZ and NET in equatorial portions of South America, particularly along the north coast of Brasil into far northwest Brasil, southern Venezuela and Colombia. The Pacific basin of Ecuador continues experiecing diurnal convection, but strong upper divergence in the southeast flank of the subequatorial ridge, has favored a northward extension of areas of deep convection into Colombia during the last few days. Western Tropical South America Forecast: The western cell of the subtropical upper ridge is forecast to remain off the coast of southern Peru through the forecast cycle. Low-level winds in the Amazon basin are forecast to develop a northeasterly conponent, which will transport higher values of precipitable water into southern and western portions of the Amazon by Wednesday. This, in combination with a decrease in upper divergence over Colombia, will yield to a general shift of the areas of heaviest precipitation from the northwest Amazon/Colombia/Venezuela early in the cycle to Peru/oriente Ecuatoriano, north Bolivia and west Brasil by Wednesday. Higher values of available moisture by mid-week will also favor a slight increase in accumulation in diurnal convection across the Altiplano and Andes of Peru, although seasonal amounts are expected. In the Pacific basins of Ecuador/far northwest Peru, variability in the low-level winds will continue to play a role in modulating diurnal convection. Enhanced onshore flow and higher values of precipitable water are expected on Monday and Wednesday, which would likely yield to higher accumulation during these days. Important rainfall accumulation forecasts: -Pacific basin of Ecuador: On Monday, maxima of 25 - 50mm, decreasing to maxima of 20 - 35mm on Tuesday, increasing again to maxima of 25 - 50mm on Wednesday. -Western Amazon: Increase to maxima of 25 - 50mm from Colombia into north Peru on Tuesday, while in the Peruvian Amazon/Acre expect maxima of 20 - 35mm. On Wednesday expect maxima of 25 - 50mm from southwest Colombia/oriente Ecuatoriano into central Peru/Acre, while in south Peru, north Bolivia and Rondonia expect maxima of 20 - 35mm. -Altiplano/Peruvian Andes: Gradual increase to sustain seasonal daily accumulations by Wednesday. -Central Amazon basin: Maxima of 25 - 50mm in NW Brasil on Monday and Maxima of 20 - 35mm in Para. Maxima of 20 - 35mm in Para/east Amazonas on Tuesday. Maxima of 20 - 45mm on Wednesday. Eastern and southern Tropical South America Forecast: A system of interest is a mid-upper trough propagating in extratropical South America along 70-67W and south of 25S on Monday. This trough is forecast to start amplifying across Paraguay and southeast Bolivia late on Tuesday. As it amplifies, it will aid with the ventilation of convection from southern Bolivia into Minas Gerais/Espirito Santo on Tuesday. During Wednesday it is forecast to propagate across southern and southwest Brasil, placing its divergent tier over southeast Brasil. This will favor locally heavy precipitation in eastern Rio de Janeiro, southeast Minas Gerais and Espirito Santo in combination with a weak surface front and associated southerly winds in the Serra do Mar of Sao Paulo and Rio de Janeiro. Further north, expect active convection to persist across the north coast of Brasil and interior nordeste although a mild decreasing trend in accumulation is expected. Conversely, an increase in low-level moisture in central Brasil will favor a mild increasing trend in diurnal convection, to favor seasonal accumulation. Important rainfall total forecasts: North coast of Brasil/northern Nordeste: Maxima of 25 - 50mm on Monday, decreasing to maxima of 20 - 40mm thereafter. Bolivia/Paraguay: Maxima of 20 - 45mm in central Bolivia and west Paraguay/far north Argentina on Monday. On Tuesday expect maxima of 25 - 50mm in south Bolivia and north Paraguay/Pantanal, while in centrla Paraguay and central Mato Grosso do Sul expect maxima of 20 - 40mm, decreasing thereafter. Southeast Brasil: Maxima of 20 - 35mm on Monday and Tuesday, increasing to 25 - 50mm in Rio de Janeiro, east Sao Paulo and southeast Minas Gerais on Wednesday. SYNOPTIC ANALYSIS AND FORECAST FOR EXTRATROPICAL SOUTH AMERICA: Analysis: An upper trough is crossing Patagonia on Monday exerting limited effects on regional precipitation. Another short wave upper trough is propagating eastward across the Rio de la Plata. This is sustaining weak cyclogenesis off the coast of Uruguay and the Buenos Aires Province, with limited impacts in continental precipitation. An exception is onshore flow in Rio Grande do Sul, which is stimulating the development of diurnal convection in the Lagoa dos Patos region. Of interest, another mid-upper trough is approaching from the west while propagating along 100W and south of 38S. It associates with an atmospheric river that contains total precipitable water values exceeding 25mm, and will likely have impacts in Austral Chile on Tuesday. Forecast: The primary system of interest for precipitation is the atmospheric river, forecast to enter Austral Chile on Tuesday morning, accompanied by a negatively tilted short wave upper trough. This will favor a peak in accumulation on Tuesday through early Wednesday, when expect amounts of xx - xxmm/Day and maxima of xxmm. Although the main event is forecast to fade thereafter, cold air cumulus south of the atmospheric river will favor intermittent precipitation on Austral Chile on Wednesday into early Thursday, when expect maxima in the xx - xxmm/Day range. Otherwise, expect generally dry conditions in areas south of northern Argentina and southeast Paraguay, given limited amounts of available moisture through the forecast cycle. Important rainfall total forecasts: Austral Chile: Maxima of 25 - 50mm developing on Tuesday as atmospheric river arrives. Decreases to maxima of 20 - 35mm on Wednesday. For associated QPF graphical information: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Galvez...(WPC) Ledesma...(WPC)