South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 331 PM EDT Tue Sep 30 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 30 September 2025 at 1925 UTC A highly dynamic synoptic pattern is expected across South America, characterized by strong upper-level jets, a broad mid-level trough, and significant moisture transport into the subtropics. At 200 hPa, a subtropical jet extends from central Chile into northern Argentina with speeds near 120 knots. The jet entrance and exit regions favor areas of divergence and difluence aloft, which are enhancing vertical motion over Paraguay, Uruguay, southern Brazil, and northern Argentina. As the cycle progresses, this jet maximum shifts eastward, maintaining upper-level divergence over the subtropical plains and Rio de la Plata basin. At 500 hPa, a broad trough over the South Pacific is amplifying into southern Chile and central Argentina. This trough, in tandem with ridging over the South Atlantic, promotes sustained ascent over northern Argentina and adjacent areas. By Thursday, the mid-level trough axis sharpens and extends eastward, continuing to produce instability and reinforcing an active convective regime across Uruguay, Paraguay, and southern Brazil. At low levels, the 925 hPa wind field highlights a strengthening northerly low-level jet extending from northern Argentina into Paraguay and southern Brazil. This low-level jet transports warm, moist tropical air into the subtropics, sustaining convergence along a frontal boundary. The overlap of strong low-level moisture advection with jet-induced divergence aloft is creating an environment highly favorable for organized convection. Moisture fields are especially important this week. Precipitable water values climb above 50 mm across the Amazon basin and extend southeastward into Bolivia, Paraguay, and northern Argentina. These values are running between 1 and 2 standard deviations above climatological normal, particularly on Wednesday and Thursday, when a broad plume of anomalously high moisture becomes established. In contrast, negative anomalies persist along the west coast of Peru and Chile, limiting rainfall in those areas. That said, an atmospheric river will be affecting portions of Chile especially on Wednesday onward, starting in Aysen and drifting north into Lagos by late Thursday. Today, convection will develop across northern Argentina, Paraguay, Uruguay, and southern Brazil, where favorable jet dynamics and moisture convergence support rainfall totals of 30 to 80 mm, with isolated higher amounts. On Wednesday, the core of convective activity shifts eastward into Uruguay and southern Brazil, producing again near 30 to 80 mm, with potential for locally strong thunderstorms. By Thursday, widespread thunderstorms will continue across Uruguay, southern Brazil, and adjacent northern Argentina, with rainfall totals of 30 to 60 mm. The environment of high precipitable water, strong low-level convergence, and enhanced divergence and difluence aloft suggests the possibility of severe weather across Uruguay, Paraguay, southern Brazil and nearby areas. This includes potential for hail, strong winds, and locally heavy rainfall. Farther south, successive frontal systems interacting with strong westerly flow will generate heavy precipitation in southern Chile and Patagonia. Daily rainfall amounts of 20 to 40 mm are expected, with locally higher amounts. Snowfall will be particularly heavy in the southern Andes between 40S and 50S, where accumulations of 25 to 50 cm per day are possible. The high snowfall amounts are driven by the combination of strong and persistent westerly winds, abundant mid-level moisture, and orographic uplift along the Andes. Additional lighter snowfall, generally under 10 cm per day, is expected across austral Chile and Argentina. Tropical convection will persist across Colombia, Ecuador, Peru, and western Brazil, with diurnally driven showers and thunderstorms producing daily accumulations of 15 to 35 mm. While not as intense as in the subtropics, these will remain a steady contributor to regional rainfall. Therefore, a highly active early-spring pattern will dominate through Thursday. Strong subtropical jet dynamics, abundant low-level moisture transport, and anomalously high precipitable water values will support widespread convection across northern Argentina, Uruguay, Paraguay, and southern Brazil, with multiple rounds of severe thunderstorms possible. At the same time, persistent frontal activity and strong westerlies will lead to heavy precipitation and significant snowfall in southern Chile and Patagonia, while moderate daily rainfall continues across the tropics and central Andes. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Alamo...(WPC)