South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 1252 PM EDT Mon Sep 22 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 22 September 2025 at 1830 UTC On Monday morning, upper level ridging is located in the northern and tropical region of the continent, extending into Bahia and Espirito Santo-Brasil, and into the Atlantic Ocean. This upper ridging is interacting with a mid to upper level trough axis extending from east of Patagonia-Argentina and extending northward into Bolivia and Mato Grosso-Brasil. A frontal boundary is advancing northward along the Yungas region of Bolivia, into Mato Grosso do Sul, Sao Paulo, and exiting into the Atlantic Ocean. This frontal boundary is associated with the upper level trough and is interacting with the periphery of the upper trough and ridge along north Bolivia and Southeast Brasil, where in the lower levels, moisture convergence along the frontal boundary will favor precipitation maxima ranging from 20-40mm. Localized higher amounts could be expected. On Tuesday, the upper level trough continues propagating to the east, while the upper level ridging weakens over northeast Brasil, but meanders over the northwest of the continent. In the lower levels, the frontal boundary becomes stationary over Acre-Brasil, extending along the southern Amazon Basin, and into the south-central region of Brasil, exiting through Espirito Santo into the Atlantic Ocean. The easterly flow over the Amazon Basin will favor moisture advection over region and enhanced moisture convergence from the central to the west Amazon Basin. Expect widespread precipitation maxima ranging from 20-40mm in the West-Central Amazon Basin, and maxima of 15-30mm from east Amazonas-Brasil and along the frontal boundary in central Brasil, and including south Central Brasil. On Wednesday, the upper ridge becomes more organized over northwestern South America, while the upper trough extends into Mato Grosso-Brasil. In central Brasil, expect the interaction between the two upper level systems to interact along the periphery, favoring and area of enhanced divergence aloft. South central Brasil can expect deep convection over the area with precipitation maxima of 20-35mm. To the north and east, the frontal boundary remains stationary, while the trade winds over north Brasil continue from the east, advecting moisture in north Brasil. As the stationary boundary converges with this easterly flow near the southwest Amazon Basin, and the low level flow continues east towards the western Amazon Basin, expect moisture convergence along the Andean foothills from south Colombia, through east Ecuador, and north-central Peru, where expect maxima of 30-60mm. From Amazonas-Brasil to Mato Grosso, expect maxima of 20-45mm. And from southeast Mato Grosso to Serra do Mar and Espirito Santo, expect maxima of 20-35mm. A secondary frontal boundary is expected to enter in Parana and Sao Paulo, however, due to low available moisture, no significant precipitation is expected. From the central to southern region of the continent, drier conditions are expected over the forecast period as a ridging pattern is expected. As high levels of moisture are not being advected from the north, and the blocking pattern is preventing the entrance of frontal boundaries in Chile, minimal amounts of precipitation are expected on Monday and Tuesday. On Wednesday, the ridging over Argentina propagates to the east, and an upper trough begins to enter through south Chile by Wednesday evening. An associated upper jet and its divergent side enters with a frontal boundary in south and Austral Chile. This front is accompanied by an atmospheric river, with precipitable water values near 20-25mm. This influx of moisture, in association with the frontal boundary will favor moisture convergence in Aysen-Chile, favoring maxima of 15-25mm on Wednesday. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Castellanos...(WPC)