South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 246 PM EDT Mon Sep 29 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 29 September 2025 at 1845 UTC In Chile... A series of upper and mid - level shortwave troughs and affiliated upper jet streak maxes will propagate across Chile for the next three days. Starting Monday evening, expect an increase in upper level wind speeds affiliated with a developing and strengthening upper jet streak max. Jet dynamics are anticipated to favor an enhancement in upper level divergence, especially across south Chile from Monday evening through Wednesday afternoon. Another upper jet streak max is poised to develop early Tuesday morning and may be accompanied by coupled jet streak dynamics as it could interact with another developing upper jet streak max that will be embedded within the subtropical jet stream. An increase in low level winds, with speeds exceeding 40 knots, will favor a gradual increase in moisture starting Tuesday morning. The low level wind direction will be from the west - northwest during this period and gradually shifting from the north - northwest as the axis of an upper to mid - level trough continues to approach the continent. Precipitable water values may approach 25mm after Wednesday afternoon, where wind speeds will begin to exceed 50 knots across southern Chile. Expect daily total precipitation maxima to exceed 50mm from Tuesday morning through Thursday morning. In central South America... A shortwave upper level trough will begin to interact with weak upper ridging across central Brasil and a jet streak max to the south. This will favor an increasingly diffluent upper level wind pattern across the region from early Tuesday morning through Wednesday morning. This pattern may positively enhance upper divergence. After Wednesday morning, another upper trough will begin to move into the continent and may extend as far north as 15S. Upper divergence will be favorable in its right exit region and at its base. This trough currently has an axis near 85W. In the mid - levels, a series of shortwave troughs may help trigger convection across the region for the next three days. In the lower levels and surface, on Monday, expect an increase in northerly low level winds into northern Argentina, Paraguay, and southern Brasil. These conditions will favor the formation of a propagating surface trough starting late Monday night. A significant increase in moisture pooling may occur across the southeast region of Paraguay, northeast Argentina, and southern Brasil. The combination of upper level diffluence, increasing low level moisture, and a propagating mid - level trough may help increase precipitation chances across this sector from Monday morning through Tuesday morning, where a total precipitation maxima of 40 - 80mm is possible. The surface to low level trough will be located across southern Bolivia on Tuesday evening. By Wednesday evening, it will be across north - central Bolivia. From Tuesday morning through Wednesday morning, precipitable water values may exceed 50mm across Paraguay and southern Bolivia. The combination of mid - level shortwave troughs, upper divergence, and ample moisture will favor total precipitation maxima of 35 to 70mm for the aforementioned time period. Note that the aforementioned upper ridge across central Brasil will disconnect the moisture advection from the tropics into central South America, and thus a decrease in precipitable water is anticipated after Wednesday morning. Despite this, a mid - level trough, that will be propagating across this region through the day on Wednesday may help trigger precipitation and yield a total precipitation maxima of 20 - 35mm from Wednesday morning through Thursday morning across Paraguay. During the same period, across north - central Bolivia, moisture convergence will dominate along and ahead of the surface trough. Expect convection that may be sustained by divergence located at the base of the aforementioned upper trough. These conditions may yield a total precipitation maxima of 30 - 60mm from Wednesday morning through Thursday morning. In Tropical South America... A broad upper ridge across the west Amazon will begin to lose definition through Wednesday morning. Mid - level shear may prohibit the sustenance of deep convection across this region, however, there is still significant precipitable water present in the area. It may diminish slightly from Tuesday morning through Wednesday morning, before increasing again after Wednesday morning. An aforementioned upper trough will begin to move into the southwest portions of tropical South America, helping sustain upper divergence across the southwest Amazon and northern Bolivia. Elsewhere in Bolivia, increasing northerly flow will help in transporting moisture tropical air into the region. The flow may be orthogonal to local topography, which may help favor increased orographic effects through the forecast period. Otherwise, diurnal convection is possible across the western portions of tropical South America for the next three days. Low level troughs will also help trigger convection. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Tinoco-Morales...(WPC)