South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 316 PM EST Tue Nov 04 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin for 04 November 2025 at 2000 UTC Across South America, there has been little change in the forecast synoptic pattern, still favoring significant rainfall over portions of the Buenos Aires Province, as well as Central Paraguay and the Amazon basin for the next three days. This is not to say that it will not rain elsewhere, some rainfall is expected elsewhere, albeit with modest amounts, while the aforementioned sections could observe over 25mm of rain over the next 3 days, with isolated higher amounts. The global models are still suggesting that isolated portions of the Buenos Aires Province and central Paraguay, could observe upwards of 40-80 mm with strong thunderstorms over the next three days. In the upper levels there is a low pressure and associated trough across the southeast Pacific, just west of southern Chile. There is an upper jet to its north and northeast, entering central to north Chile and Argentina, though the jet over land areas will relatively zonal from the west peaking at around 110 kt. There is another broad low pressure in the southwestern Atlantic, with an associated trough extending north with its base near 41S/48W, just east of Argentina. This pattern causes a diffluent upper-level flow across portions of Uruguay and southern Brasil, though the weaker winds in that region may cause some upper level convergence. A high pressure over western Brasil into Peru and Bolivia dominates much of the Tropical South America. By Thursday, the Bolivian High is expected to develop, creating a strong pressure gradient further south, and strengthening the upper-level winds across central Chile into central Argentina into Uruguay and southern Brasil. The upper-level winds could reach 150 kt across a small portion of central Chile into Argentina, while winds could be near 120-130 kt further east in central Argentina into Uruguay and southern Brasil. By Friday, the Bolivian High is expected to weaken somewhat, and there will be a tilted trough extending from the southwest Atlantic near the Falkland Islands to the northwest into Central Chile. The mid levels continue to be vertically stacked to the upper levels to a decent degree. The wind speeds in the mid levels appear to be a bit more modest, however. The mid-level winds appear to peak at around 70 kt across isolated areas of northeastern Argentina, Uruguay, and southern Brasil early Friday morning. Otherwise, the winds are expected to be around 50-60 kt across central South America, with weaker winds elsewhere. By Thursday, the high pressure over Bolivia is present but displaced a bit to the east, as well as not as well defined as it is in the upper levels, and by Friday it will displaced even further east into Brasil. Even though the winds in the mid levels are modest, a relatively tight pressure gradient over the central sectors of the continent. In the lower levels, there are still a series of low-level jets and troughs that will contribute to low-level moisture convergence over central South America, which will combine with the mid and upper level dynamics and produce showers and thunderstorms. This is the main reason why portions of northern Argentina and Paraguay could observe significant showers and thunderstorms. The tropical region of South America will also have a few low-level troughs and moisture convergence, and although the mid and upper levels are not particularly favorable for deep vertical development, the combination of the higher than normal moisture with the diurnal heating and local effects will produce daily showers and thunderstorms over the area. The Amazon basin and portions of eastern Brasil possibly observing the highest amounts of moisture and daily showers and thunderstorms. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Alamo...(WPC)