


Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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744 FXUS20 KWBC 141916 PMDSA South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 316 PM EDT Mon Jul 14 2025 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ South American Forecast Bulletin from 14 July 2025 at 1915 UTC Model guidance is indicating a relatively significant rainfall event developing across parts of northeastern Argentina into Uruguay from tonight through Thursday morning. A deepening upper-level trough currently located over the southeastern Pacific will shift eastward and evolve into a closed upper low over northern Argentina by Wednesday night. As this feature moves inland, it will place northeastern Argentina and Uruguay under the divergent side of the upper-level flow, enhancing large-scale ascent and supporting thunderstorm development. There is a low-level jet moving into the area, which combined with the moisture convergence and the frontal boundary nearby, it may support the development of MCS. At the same time, mid-level moisture increases steadily, with relative humidity around 700 mb rising to 70 to 90% by Tuesday night into Wednesday. Precipitable water values climb to 45 to 50 mm, indicating a deep moisture plume extending into northeastern Argentina and Uruguay. The aforementioned low-level jet at 850 mb is forecast to develop Tuesday into Wednesday, transporting warm, moist air from the north, southeastward across northern Argentina and into Uruguay. Winds along this jet are expected to reach 30 to 40 knots, helping to saturate the lower atmosphere and increase instability in the region. Near the surface, a low-pressure system and frontal boundary will organize over northern Argentina and shift east northeastward, helping to focus low-level convergence and reinforce upward motion. Rainfall across northeastern Argentina into Uruguay is expected to begin increasing tonight, with the heaviest and most organized precipitation likely from Tuesday night into Thursday morning. Model QPF suggests 72-hour totals of 50 to 100 mm, with localized maxima exceeding 150 mm across northeastern Argentina into southern Uruguay. Southeastern Brazil may also be affected by convective spillover, but the core rainfall axis remains farther southwest. Therefore, from tonight through Thursday morning, northeastern Argentina and Uruguay are expected to experience the most significant impacts from a well-organized upper-level system interacting with deep moisture and favorable surface conditions. The peak of the event is expected between Tuesday night and Thursday morning, with a high potential for heavy rainfall and thunderstorms. Forecast confidence is moderate to high, as the different global model guidance is in general agreement on the synoptic scale features, even though there is some discrepancy between the models in the location of heaviest rainfall, the general consensus is that there will be significant rain in the general area of northeastern Argentina into southern Uruguay over the next 3 days. Over central and southern Chile, the GFS model indicates the presence of a deep upper-level trough offshore tonight into Tuesday, supporting widespread cloudiness and rain along the coast and inland. Enhanced mid and upper-level moisture, along with increasing precipitable water values, suggests an atmospheric river may affect the area, especially from La Araucania to Biobio and parts of Los Rios, especially from Wednesday into Thursday. This significant increase in moisture could bring periods of steady rainfall in the lower elevations, while snow is forecast in the higher elevations. Note: The low-level analysis charts are now expected to be issued regularly. The forecast bulletins are still being issued intermittently. For the latest available charts please visit: https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml Alamo...(WPC) $$