South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 226 PM EDT Fri Sep 19 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 18 September 2025 at 1830 UTC In the upper levels ... A potent upper jet streak max continues to be sustained by a potent vertical stacked upper trough, that is currently centered at 40S 83W, that is interacting with a subtropical upper ridge to the north. East of the Andes, this interaction is favoring a broad diffluent upper level wind pattern across central South America. These conditions may promote upper divergence across this region. Overall, the exit region of the upper trough and jet dynamics will be the driver for the sustenance of divergence across central - Chile and central South America for the next three days. During the day on Saturday, the axis of the upper trough will begin to propagate east of the Andes. After Sunday, the trough axis should be located across central South America and is expected to become positively tilted after Monday morning. The axis will extend as far north as 17S. Another upper jet streak max will begin to develop across northern Argentina, Uruguay, and southern Brasil starting early Sunday morning, favoring jet dynamics across this region. In the mid - levels ... A series of shortwave troughs that are embedded within a broad mid - level trough will continue moving across south - central Chile through the next three days. Its axis, which is currently centered at 39S 83W, will move east of the Andes early Sunday morning and will continue propagating across central South America through Monday. The axis base may extend to 16S. The greatest impact with this feature includes the destabilization of the mid - levels and the added support for vertical ascent. In south - central Chile ... The aforementioned upper level trough and jet dynamics will continue to favor upper divergence across central Chile through Sunday morning. At the mid - levels, a shortwave trough that is accompanied by a vorticity maxima will enter the region starting early Saturday morning. The axis of the broad mid - level trough will be propagating across south - central Chile on Saturday, and will be east of the Andes by late Saturday evening. The mid - level regime will provide additional vertical ascent and destabilization. Prior to the arrival of these mid - level features, a moisture plume will be favoring moisture convergence across central and south Chile. However, the low level wind direction will be from the north - northwest, which may limit orographically enhanced moisture convergence. Precipitable water will decrease across central Chile after Saturday morning, but values should still be above average through Sunday afternoon. Thus, the greatest precipitation impact is expected from Friday morning through Sunday morning and impacts should decrease significantly across south - central Chile thereafter. Expect a total precipitation maxima of 20 - 40mm from Friday morning through Saturday morning. From Saturday morning through Sunday morning, a total precipitation maxima of 20 - 45mm is possible. A risk for cumulonimbus development is present as well as the chance for mountain snow for both days. In central South America ... In the upper levels, the aforementioned upper level trough and jet streak max will help sustain upper divergence across the region. At the surface, a semi-stationary front will meander across central Argentina through Saturday morning. As the upper level trough axis moves east of the Andes on Saturday, expect a drop in low level pressures at the surface and a general increase in low level southerly winds that will be accompanied by high precipitable water values. These conditions will favor cyclogenesis and frontogenesis across north - central Argentina starting Saturday afternoon. At the mid - levels, a series of shortwave troughs and accompanied vorticity maxima may help destabilize the atmosphere and help trigger convective initiation across the region. With passing day, the total precipitation maxima will move north and east with the frontal system and the propagation of the upper and mid - level troughs. From Friday morning through Saturday morning, the areas with greatest precipitation impact will be across the La Pampa - Buenos Aires and Rio de la Plata region, where we expect cyclogenesis to occur. This region will be primed for vertical ascent and enhanced low level moisture convergence as well. Thus, we expect a total precipitation maxima of 25 - 50mm with a risk for severe weather. Another region of interest during this period is northern Argentina and Uruguay. A mid - level shortwave trough may move across this area and may favor the development of precipitation. A low level trough will also be impacting this region, which may further enhance moisture convergence. Thus, a total precipitation maxima of 20 - 35mm is possible in this region. After Saturday, expect the development of a progressive developing cold and occluded front. The cold front is expected to be located across northwest Argentina, Cordoba, and Rio de la Plata on Saturday evening and across the border the Argentina - Paraguay and Rio Grande do Sol on Sunday evening. Moisture pooling is expected ahead of the occluded front. From Saturday morning through Sunday morning, a total precipitation maxima of 40 - 80mm across Rio de la Plata is possible, where severe weather is also expected. Another maxima of 40 - 80mm is possible across southern Brasil, which is attributed to a mid - level shortwave trough and low level trough. As the cold front races northward, the greatest moisture convergence will occur across Paraguay and southern Brasil, where a total precipitation maxima of 40 - 80mm is possible. During this period, another jet streak max will develop across southern Brasil. Thus, we anticipate significant divergent support from the upper levels due to jet dynamics and the location of the trough base, which is expected to be located at 58.6W by Monday morning. In the mid - levels, the base of the aforementioned broad trough will be moving across the region, favoring chances for severe weather. Across northwest Paraguay and Bolivia, the cold front may begin to extend into this region starting early Monday morning. An increase in moisture convergence and local effects may yield a total precipitation maxima of 20 - 35mm from Sunday morning through Monday morning. This region will be affected by the base of the aforementioned upper and mid level trough, which may help generate upper divergence and instability across this sector. In Tropical South America ... The aforementioned subtropical ridge will continue to lose definition, but widespread subsidence is still expected across the interior of tropical South America. Expect a gradual decrease in precipitable water values across the entire basin with passing day. The mid - level flow will also be weak, so minimal shear is expected. If any convection does develop, it will be mostly driven by local and diurnal effects. From Friday morning through Sunday morning, expect a daily total precipitation maxima of 20 - 35mm across the western Amazon basin, where precipitable water values will be highest. Across the Pacific Coast of Colombia, a low level trough will be moving across the Caribbean Sea, which may strenghen the Panamanian low and will favor an increase in moisture convergence across this region. Thus, a total precipitation maxima of 20 - 45mm is possible from Sunday morning through Monday morning. Tinoco-Morales...(WPC)