South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 240 PM EDT Wed Oct 01 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 01 October 2025 at 1845 UTC In the lower mid - latitudes and subtropics... A potent upper trough, that currently has a positively tilted axis from 14S 80W to 33S 67W, will begin to move into the continent through the day Wednesday. This trough will begin to interact with a strengthening subtropical upper ridge to the west and will become elongated after Friday morning. Its base may extend as far north as 13S. Upper divergence will be favorable across its exit region and at its base for the next three days. In the mid - levels, a trough, and embedded shortwave troughs, will be propagating across the region and may help with destabilization and vertical ascent, especially in Paraguay through Friday morning. Thereafter, this mid - level trough will slowly move eastward, affecting southern Brasil and Uruguay for the rest of the forecast cycle. At the surface, a meandering trough will be located across northern Argentina, Paraguay, Bolivia, and southern Brasil from Wednesday morning through Friday morning and will help in triggering precipitation initiation. Overall higher precipitable water values are expected across Paraguay and north - central Argentina, but expect a disconnect with the tropical air mass after Wednesday afternoon. These conditions will yield a gradual daily decrease in total precipitation maxima in this region. In southeast Paraguay and southern Brasil, expect dry mid - levels through Thursday afternoon, which may prohibit the sustenance of deep convection. After Thursday afternoon, expect a gradual increase in northeast low level winds, which will transport a moist air mass from the tropical Atlantic into Sao Paulo, Parana, and Santa Catarina. The change in wind direction may be attributed to a strengthening surface - to - low level high pressure system in the south Atlantic. At the same time, the aforementioned upper trough may help lower pressure values across central South America, favoring an increase in low level northwest winds, which may be accompanied by the advection of moist continental tropical air into the interior of the continent. The increase in moisture advection into southern Brasil and Uruguay will favor moisture convergence across this sector from Thursday afternoon through Saturday morning. A total precipitation maxima of 30 - 60mm is possible from Friday morning through Saturday morning. Another region of interest is northern Bolivia and the southwestern Amazon, where upper divergence will be promoted by the aforementioned upper trough through Friday morning. Precipitable water content may surpass 44mm. Expect minimal low to mid - level shear, which may assist with the vertical development of deep convection across this region. The aforementioned conditions will favor daily total precipitation maxima exceeding 40mm from Wednesday morning through Friday morning. Elsewhere in tropical South America, tropical convection will be favorable across the basin, especially in regions where low - level troughs will be propagating. Note that in the mid - levels, there may be an increase in wind speeds with passing days, while low level wind speeds will decrease. This will result in an increase in shear across the basin and may limit the formation of deep convection after Friday morning. Otherwise, diurnal processes, orographic enhancement, and local effects will influence the initiation and sustenance of precipitation for the next three days in this region. In the high mid - latitudes... Significant total precipitation maxima is expected in south and south - central Chile from Wednesday morning through Friday morning. Through the day on Wednesday, expect low level wind speeds to exceed 40 knots as a frontal system continues to move in-land. An atmospheric river will accompany this frontal system and may have precipitable water values exceeding 25mm through Friday afternoon, weakening afterwards. The low level wind direction will begin to shift to a more north - northwesterly direction with passing days as a cold front nears closer to the continent. Upper and mid - level support will be present in the region. Upper divergence will be plentiful in the area as a series of jet streak maxes will promote jet dynamics through the forecast cycle. Through the day on Wednesday, the left entrance region of a jet streak max, will enhance divergence across south Chile. This jet streak max will be exiting the region thereafter. By Thursday, another progressive jet streak max will begin to strengthen and propagate northward with passing days.Similarly in the mid - levels, a series of shortwave troughs that are embedded within a broad trough will continue to move into Chile through the next three days. Vorticity maxima will accompany the troughs and will further enhance vertical ascent. These conditions will favor a total precipitation maxima of 30 - 60mm from Wednesday morning through Thursday morning, and a maximum of 25 - 50mm from Thursday morning through Friday morning. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Tinoco-Morales...(WPC)