Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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681 FXUS20 KWBC 261714 PMDSA South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 1213 PM EST Wed Nov 26 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin for 26 November 2025 at 1730 UTC Note: Due to the observance of the Thanksgiving holiday, there will be no production of South American charts and discussions for 27 November and 28 November, 2025. Products will resume on 1 December 2025. In the subtropics... Across northeast Brasil, a potent upper trough propagating across the region will promote an enhancement of upper divergence. This upper trough will reflect in the mid-levels and will favor a cooling of the mid-level temperatures, aiding in the destabilization of the environment. In the lower levels, expect the presence of the South American Convergence Zone (SACZ) to persist through Thursday. Precipitable water values will begin to decrease Thursday afternoon but still expect enhanced moisture convergence across the Bahia region through Friday morning due to ongoing onshore low level flow. Thus, the greatest precipitation impacts in this region will be on Wednesday with the presence of the SACZ. Precipitation accumulations will gradually decrease with passing days. In the western Amazon Basin, upper diffluence will dominate across the region prior to the arrival of an upper level trough that is currently located in the Pacific. This trough will begin to move into the continent Wednesday evening and will become negatively tilted as it moves inland. At the same time, a mid-level trough will be developing across northern Bolivia and southern Peru, further enhancing vertical ascent. In the low-levels, troughing will be present across the same region on Wednesday with precipitable water values exceeding 50mm. This will favor increased moisture convergence that may be further enhanced by orographic effects. A total precipitation maxima of 40 - 80mm is expected across northern Bolivia and southern Peru from Wednesday morning through Thursday morning. The aforementioned upper trough will lose its definition quickly once it crosses east of the Andes. However, mid-level troughing will still be present in the region for Thursday. The low level wind direction will also become more northerly early Thursday morning and this wind regime will remain through the weekend. Moisture convergence will be enhanced across the foothills of the Andes in Bolivia. After Thursday, a mid-to-upper ridging pattern will dominate across the interior of the western Amazon, limiting support for deep convection. Thus, the convection that does develop will likely be affiliated with the presence of low level troughs, moisture convergence being enhanced by orographic effects, and the diurnal cycle. Elsewhere in the subtropics, expect seasonal conditions to prevail. The presence of low level troughs will enhance moisture convergence and vertical ascent. In the mid-latitudes... A mid-to-upper level shortwave trough, that currently has an axis near 72W, will continue moving east of the Andes, arriving into central Argentina by early Thursday morning. This feature will help lower pressure values across north-central Argentina, which will assist in the development of a surface cold front in this region. Moisture convergence is anticipated across this boundary, yielding a total precipitation maxima of 15 to 20mm. There will also be a risk for severe weather due to the presence of cooler mid-level temperatures. Some lingering precipitation is anticipated across Rio de la Plata on Thursday morning as the cold front and and the mid-to-upper level trough exit the region. Accumulations during this period will range from 15 to 20mm. Thereafter, an upper jet streak max will develop across central Argentina ahead of the passage of another upper level shortwave trough that is anticipated to move into the continent on Saturday. The presence of the jet streak max will favor the development of shortwave troughs in the mid-levels that may contribute to the initiation of convection across central and northern Argentina on Friday. At the surface, expect the development of meandering surface troughs on Friday and Saturday. The highest precipitation impacts on Friday are expected across the La Pampa and Buenos Aires provinces. Note that there is model disagreement on the location of the surface troughs on Friday across central Argentina. Because the GFS is resolving a more well-defined upper ridge across Bolivia, it is also resolving more intense upper wind speed divergence across northern Argentina and developing a surface trough south of the Chaco region of Argentina where moisture convergence is expected to be most intense. However, both models are indicating the presence of a surface trough across Rio de la Plata and extending into Cordoba/Santa Fe. Considering the aforementioned conditions, we are leaning more towards the ECMWF for the onset of this event. By Saturday, the mid-to-upper level trough will begin to move into north-central Chile and still expect the generation of mid-level troughs east of the Andes due to the ongoing presence of the upper jet streak max. For Saturday, the area of greatest precipitation impact will be across Rio de la Plata and the Mesopotamia regions of Argentina. Low level troughs will be present in this region and there will be a series of mid level troughs moving across this region. With respect to moisture, the South America Low Level Jet (SALLJ) is anticipated to gain definition starting on Thursday and will help transport moist tropical air into Argentina. Expect above normal precipitable water quantities across the region through the weekend. Elsewhere in the mid-latitudes, on Friday, a weak moisture plume will be converging across southern Austral Chile which may yield total precipitation maxima of 15 - 20mm. Otherwise, expect seasonal conditions to prevail across Chile and southern Argentina through Sunday morning. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Tinoco-Morales...(WPC) $$