South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 255 PM EDT Tue Sep 23 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 23 September 2025 at 1900 UTC In mid - latitude South America... The days of greatest precipitation impact across south - central Chile are expected to be from Wednesday morning through Friday morning. Beginning Wednesday, a highly amplified upper level trough will begin to propagate across south - central Chile and will be accompanied by a jet streak max. Expect the right exit region of the jet streak max to favor upper divergence across southern Chile from Wednesday morning through late Thursday evening. The jet streak max is anticipated to strengthen and shift northward thereafter. The axis of the upper trough is expected to move east of the Andes after Friday afternoon. In the mid - levels, the axis of a broad trough, which is currently located near 110W, will begin to impact Chile after Thursday evening. It will be accompanied by vorticity advection. Prior to the arrival of the upper and mid - level features, there will be an increase in low level moisture. Beginning Wednesday morning, precipitable water values may exceed 20mm and will again decrease after Thursday morning. Thus, from Wednesday morning through Thursday morning, we expect a total precipitation maxima of 15 - 25mm for southern Chile. From Thursday morning through Friday morning, a total precipitation maxima of 20 - 35mm is possible across south - central Chile due to enhanced upper and mid - level dynamics. In central and tropical South America... A stationary front will continue to be situated across central Brasil and the southwest Amazon from Tuesday evening through Wednesday evening and decaying thereafter. A surface trough will remain across the Central Amazon Basin for Thursday into Friday. In south - central Brasil ... In the upper levels, a trough, that currently has an axis at near 53W, will begin to interact with upper ridging to the north. This interaction will favor the increase in upper diffluence across Pantanal and Mato Grosso from Wednesday morning through Thursday morning. A broad area of divergence may be supported by these upper level features from Mato Gross to Serra do Mar and Espirito Santo during this period. Across southeast Brasil, a mid - level trough and its axis will also be moving south of the region. Some cooling of the mid - levels may be possible, which may promote further destabilization. After Thursday morning, expect a general drying trend across south - central Brasil as a mid - level ridge begins to dominate across this region. The greatest precipitation impacts across Mato Grosso is expected from Wednesday morning through Thursday morning, when the upper and mid - level dynamic regime will be most favorable. Another region of interest is Espirito Santo, where from Tuesday morning through Wednesday morning, the precipitation regime will be primarily influenced by the surface stationary front. There will be a general increase in moisture across this sector and moisture convergence will be favorable. The low level wind direction will begin to shift to a more southeasterly direction after Tuesday evening. This will favor an even further increase in moisture convergence and thus yield to higher total precipitation maxima from Wednesday morning through Thursday morning. Thereafter, the stationary front will transition into a cold front and quickly progress northward, primarily impacting areas along the east - central Atlantic coast of Brasil. Total precipitation maxima should be limited due to the progressive nature of the front. In the central, west, and southwest Amazon... At the low levels, expect an increase in moisture through the forecast cycle across the region. The aforementioned stationary front and a series of low level troughs will continue to favor ascent. In the mid - levels, wind speeds will be relatively strong from Tuesday morning through Thursday morning, potentially exceeding 20 knots. The mid - level wind speeds are projected to decrease after Thursday morning. The elevated mid - level shear may prohibit deep convection from forming. In the upper levels, an increase in upper diffluence is expected from Wednesday morning through Thursday morning and may support upper divergence. Despite the mid - level shear, the low levels will have substantial moisture to favor higher total precipitation maxima. From Tuesday morning through Wednesday morning, conditions may yield a total precipitation maxima of 25 - 50mm. From Wednesday morning through Friday morning, a daily total precipitation maxima of 30 - 60mm is possible. Elsewhere in tropical South America... Low level troughs, local, and diurnal effects will be the main driver for precipitation initiation. Across the Pacific coast of Colombia, there will be an increase in moisture with each passing day. The low level wind speeds will stay within the range of 5 to 10 knots for the forecast cycle and will become orthogonal to the local topography after Thursday morning. Thus, we expect the day of greatest precipitation impact to be 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)