South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 157 PM EST Thu Nov 06 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin for 06 November 2025 at 1900 UTC The interaction between an upper level subtropical ridge, otherwise known as the Bolivian High, and upper level shortwave troughs embedded within the polar jet stream will play a pivotal role in the development of precipitation across the region. Starting Thursday afternoon, an upper level shortwave trough, that will also be projected into the mid-levels, will begin crossing the Andes and will be located across northern Argentina by early Friday morning. Expect frontogenesis to occur during this period at the surface across northern Argentina, southern Brasil, and Uruguay. With respect to moisture, the aforementioned subtropical upper ridge will support an increased long fetch southerly flow from the subtropics into the mid-latitudes, helping increase available precipitable water across the region. Thus, above average precipitable water values are likely along and ahead of the frontal system that will enhance moisture convergence. Precipitation chances will begin to increase by Thursday afternoon across northeast Argentina and southeast Paraguay and more organized convection is expected overnight Thursday and into Friday. A total precipitation maxima of 30 - 60mm is possible across the Mesopotamia region of Argentina through Friday morning and severe weather will be likely. The highest precipitation accumulations will occur from Friday morning through Saturday morning across southern Brasil and in the vicinity of an occluded front. A total precipitation maxima of 50 - 100mm is expected in this region and there will be a risk for severe weather due to an increase in instability due to cooler mid-levels. After Saturday morning, this frontal system is expected to move off coast into the Atlantic. The aforementioned upper ridge in the subtropics will start to become more well defined on Saturday and will prohibit the surface frontal system from moving farther north. However, low-level moisture convergence will still be present in the region and will favor an increase in precipitation chances, especially across the Cerrado region of Brasil, Mato Grosso, and Tocantins. Note that there will be isolated accumulations exceeding 40mm across this region for Saturday. Another region of interest associated with the aforementioned frontal system is Bolivia. Beginning on Friday, precipitation chances will begin to increase as the cold front migrates into central Bolivia. Speed divergence in the upper levels will be present on Friday and Saturday and there will be a presence of mid-level shortwave troughs that will promote vertical ascent and convective initiation. In the low-levels, expect enhanced southerly flow that will be primarily influenced by the presence of the aforementioned upper subtropical ridge across Brasil. This low level flow pattern will favor an increase in precipitable water as well. Thus, on Friday, a total precipitation maxima of 20 - 35mm is possible across central Bolivia. On Saturday, the cold front is expected to lose definition, but moisture convergence will still be present in the region. These conditions will favor a general total precipitation maxima of 20 - 35mm across northern Bolivia and southern Peru with higher accumulations likely across higher terrain. A secondary upper level shortwave trough will be projected in the mid-levels and will move into the Atlantic ocean late on Friday. During this period, also expect this mid-to-upper level feature to be accompanied by a surface stationary front and low level trough across eastern Brasil. As it exits the region, an upper diffluent pattern will favor the presence of upper divergence through early Saturday morning. There will also be moist onshore flow converging into the coastlines of northern Espirito Santo and Bahia, where precipitable water values will exceed 50mm on Friday. These conditions, coupled with orographic enhancement, may favor a total precipitation maxima of 30 - 60mm from Friday morning through Saturday morning. Moisture convergence will decrease on Saturday along with precipitation accumulations as the upper trough moves offshore. An area of interest on Thursday is northwest Argentina and the Cordoba/Santa Fe provinces. A low pressure system will begin to develop Thursday evening in the area as the axis of a shortwave upper-level trough begins to migrate east of the Andes. This will trigger a lowering of pressure values in the region and the eventual development of the low pressure system. Upper level divergence will also be present in the region and the cooler mid-levels will favor increasing instability. Precipitable water values will be in excess of 31mm through Friday morning and moisture convergence will be likely in this region. Thus, a total precipitation maxima of 25 - 50mm is expected through Friday morning. This low pressure system will quickly lose its definition after Friday morning. In the higher mid-latitudes, seasonable conditions will prevail through the forecast cycle. A series of moisture plumes will continue to move into the area and temporarily increase available moisture through Saturday morning. On Saturday, a strong upper jet streak max will be moving across the area and will be favoring upper divergence. The low level wind flow speeds may exceed 40 knots and will likely promote moisture convergence across austral Chile. These conditions will yield a total precipitation maxima of 20 - 45mm from Saturday morning through Sunday morning. In western and northern tropical South America, the aforementioned upper ridge will promote an increase in easterly and southeasterly low level winds into the Amazon Basin region.There will be a subtle drying trend for Friday across the Basin but expect precipitable water values to increase again for Saturday. Areas with the highest chances for precipitation will be confined to where there is a presence of low-level troughs that will enhance moisture convergence. Additionally, local effects and the diurnal cycle will play a pivotal role in the initiation of precipitation across the region. An area of interest is Venezuela, where a low level trough will be propagating across the country on Thursday and Friday. Daily total precipitation maxima will exceed 40mm. Another region of interest is the Pacific coasts of Colombia and Ecuador for Saturday. There will be an increase in southerly low level flow into the region and this will support an enhancement in moisture convergence. Expect higher total precipitation maxima for this period. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Tinoco-Morales...(WPC)