South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 233 PM EDT Wed Mar 19 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 18 March 2025 at 17:15 UTC ANALYSIS AND FORECAST OF LARGE SCALE CONDITIONS MJO and Tropospheric Waves: The MJO continues propagating across phase 3, consistent with large scale upper convergence established over South America. A tropospheric Kelvin wave is forecast to arrive during the weekend/early next week, to provide some minor enhancement in the coverage and intensity of deep convection in tropical regions. SST: Important positive anomalies continue along equatorial portions of the Eastern Pacific and the Gulf of Panama. These are, however, superficial and could be rapidly altered by changes in surface winds. Still, they will continue to fuel convection and associated rainfall amounts in Ecuador and west Colombia. SYNOPTIC ANALYSIS AND FORECAST FOR TROPICAL SOUTH AMERICA Analysis: A cell of the subtropical upper ridge continues strengthening off the coast of southern Peru. Speed divergence north of the ridge is venting widespread deep convection in far west Brasil/SE Colombia and NE Peru. Another cell centers over Espirito Santo but has limited impacts in precipitation given the presence of a mid-level ridge and associated dry air over east Brasil. Over the equatorial Atlantic, the subequatorial upper ridge extends along 05-07N. enhanced diffluence and divergence south of this axis is venting convection along the Atlantic ITCZ, located mostly off the north coast of Brasil and into the Amazon Delta. Western Tropical South America Forecast: The upper ridge is forecast to reamin almost stationary off the southern coast of Peru through Wednesday, moving east into south Peru on Thursday. This will sustain upper divergence in most of west Brasil, Peru and Ecuador through the forecast cycle. In the low-levels, a moist air mass of total precipitable water exceeding 55mm is propagating across the central Amazon. It is forecast to reach the andes of Ecuador, north Peru and south Colombia on Thursday, to favor an increase in rainfall amounts. In the Pacific basin, an increase in the onshore component of low-level winds is expected on Wednesday, to favor moderate rainfall amounts in Ecuador once again. An additional increase in onshore flow and in precipitable water will support higher accumulations on Thursday across the entire Ecuadorian coast and far southwest Colombia. Noteworthy quantitative precipitation forecasts: -Pacific basin of Ecuador: Isolated maxima of 20 - 35mm on Tuesday, increasing to maxima of 25 - 50mm on Wednesday and to 35 - 70mm on Thursday, with a risk of heavy thunderstorms. -West and Central Amazon: Maxima of 20 - 45mm in Amazonas-Brasil into Peru/Oriente Ecuatoriano on Tuesday. On Wednesday maxima in the 20 - 50mm/Day range. On Thursday, maxima of 25 - 50mm west Amazonas into south Colombia and north Peru. -Altiplano/Peruvian Andes: Maxima generally in the 15 - 20mm/day range or seasonal values. Eastern and southern Tropical South America Forecast: Of interest is a mid-upper trough forecast to amplify while propagating across south Brasil and Paraguay/SE Bolivia late on Tuesday. This will first yield to an increase in ventilation in the southwest Amazon into southern Minas Gerais/Sao Paulo on Tuesday. As the trough moves east on Wednesday, its divergent tier will enhance ascent in southeast Brasil to focus heavy precipitation in Rio de Janeiro/southern Minas Gerais and east Sao Paulo in combination with moist southerly flow along coastal areas. On Thursday, expect the heaviest precipitation in Espirito Santo/SE Minas Gerais. In northern/northeast Brasil, scattered convection and moderate amounts are expected on a daily basis in association with the Atlantic ITCZ and the NET. A perturbation in the easterly trades is forecast to favor an increase in rainfall totals from Piaui into east Para on Thursday, as it is expected to associate with precipitable water values approaching 60mm and enhanced trade wind convergence. Noteworthy quantitative precipitation forecasts: -Bolivia/Paraguay/MG Sul: Maxima in the 20 - 50mm/Day range on Tuesday decreasing thereafter. -Southeast Brasil: Maxima of 20 - 35mm/Day on Tuesday increasing to maxima of 40 - 80mm in Rio de Janeiro/S Minas Gerais and S Espirito Santo on Wednesday. Maxima of 30 - 60mm in N Rio de Janeiro/Espirito Santo/SE Minas Gerais on Thursday. -North coast of Brasil/northern Nordeste: Maxima of 20 - 45mm in Maranhao/Amazon Delta on Tuesday and Wednesday. Increases to maxima of 25 - 50mm on Thursday. SYNOPTIC ANALYSIS AND FORECAST FOR EXTRATROPICAL SOUTH AMERICA: Analysis: A broad trough in the Bellingshausen sea is hosting a potent negatively tilted short wave trough near 90W and south of 45S on Tuesday morning. This includes the cold exit of an upper jet, which is highlighting ascent. The mid-upper trough and jet are interacting with an atmospheric river of total precipitable water values near 30mm, and low-level winds exceeding 55kt south of the front. This system will have an important impact in precipitation in Austral Chile as it moves inland later today. Otherwise, generally fair weather and dry conditions are present from northern Argentina-Rio Grande do Sul into central Patagonia. An exception is a front propagating across the eastern Buenos Aires province, which is sustaining strong thunderstorms this morning. Forecast: The system of interest is the atmospheric river/negatively tilted upper trough, forecast to enter Austral Chile later on Tuesday. The largest accumulation is expected in interior portions of southern Aysen/far northern Magallanes on Tuesday, although remaining convective stability will sustain cold air cumulus showers through Thursday, to produce lesser amounts. Generally fair weather is expected elsewhere with the exception of isolated diurnal thunderstorms in the Argentinean central and northern Andes on a daily basis; and the Buenos Aires Province on Tuesday with the front. A short wave trough crossing the Andes will favor an increase in the coverage and intensity of diurnal convection along eastern flanks of the Andes on Thursday, especially between south Bolivia and central Argentina. This will also highlight precipitation in northeast Patagonia sustaining a risk for heavy thunderstorms on Thursday into early Friday. Noteworthy quantitative precipitation forecasts: Austral Chile: Maxima of 25 - 50mm on Tuesday from north Magallanes into central Aysen. Maxima of 20 - 35mm in the same region on Wednesday. Maxima of 15 - 25mm and mountain snow from central Aysen into west Magallanes on Thursday. Interior NW Argentina: Maxima of 20 - 35mm on Thursday. NE Patagonia: Maxima of 15 - 20mm on Thursday with a risk for heavy TS. For associated QPF graphical information: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Galvez...(WPC) Ledesma...(WPC)