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FXUS20 KWBC 171852
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
252 PM EDT Mon Aug 17 2026

GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/

Forecast Bulletin 17 August 2026 at 1900 UTC:

A potent upper level trough with its base reaching central Peru,
extends over central Bolivia and Chile. An amplified Subtropical
Jet extends over much of central South America, intensifying over
central Argentina and Uruguay throughout the day on Monday.
Another mid to upper level trough over the south Atlantic Ocean is
attempting to close on Monday morning, becoming a major compotent
to the deep convection over Uruguay on Monday morning. In the low
levels, a frontal boundary responds to the persistent upper jet
and propagating upper trough and situates over north Argentina and
into Uruguay in the morning hours, then it will meander in the
same location into the nighttime, and finally propagate northward
by the morning on Tuesday. The persisten location of the frontal
boundary will favor deep convection over the aforementioned
regions, favoring heavy precipitation in northeast Argentina and
Uruguay. In terms of available moisture, the South American Low
Level Jet is transporting moisture from the north and it is
expected to continue to do so into Tuesday. On Tuesday, the upper
level trough continues into the continent, attempting to become a
closed low just west of northern Chile, while the upper jet over
south Boliva and the Chaco region begins to weaken throughout the
day. In the lower levels, the frontal boundary begins to propagate
north and east, expected over central Bolivia, into the
Paraguay/Brasil border, attemps to occlude over Mesopotamia
region-Argentina and Uruguay and exits over Rio Grande do Sul and
Santa Catarina-Brasil into the Atlantic Ocean. A weakening of the
moisture advection is expected as the SALLJ weakens over Bolivia
and into the north Argentina region, favoring a reduction in
precipitation on Tuesday over Uruguay and south Brasil.  On
Wednesday, the upper level trough is located inland, extending
from south Peru/Boliva, into central Argentina and Uruguay. By the
evening on Wednesday, the upper trough begins to tilt negatively,
enhancing the divergence aloft from Paraguay through Uruguay and
south Brasil. At this time, expect the frontal boundary to
continue extending eastward, and favoring cyclonic rotation over
Uruguay and east Argentina. this rotation will favor moist onshore
flow into the Rio de la Plata region and Uruguay. Expect moderate
to locally heavy preciptiation from northeast Argentina and into
Uruguay on Wednesday.

The upper level trough is also favoring frontal boudnaries and
moisture to reach high elevations of the Andes Mountains in Peru
and Bolivia. As the mid to upper trough favors a decrease in
pressures in the region, enhanced moisture transport from Amazon
and along the Pacific Coast is converging over the mountainous
regions in Peru and Bolivia. The higher eleveations can expect
precipitation in the form of snow, while lower elevations could
expect rain showers to develop with some regions exceeding 15mm in
the high evelvations of Bolivia on Monday. Into Tuesday, moisture
remains over the high elevations of the Andes Mountains, but
begins to decrease with the progression of the upper level trough
further inland. A decrease in total precipitation is expected on
Tuesday, with most of the moisture converging along the foothills
of the mountains in central Bolivia and Peru. On Wednesday, with
the upper trough over central South America, expect a decrease in
moisture in the region. This will limit the total preciptiation
for the day, where residual moisture remains in the Selva Alta
region of Peru, where light preciptiation is expected on Wednesday.

To the south, an atmospheric river is entering Austral Chile
throughout the day, with values exceeding 25mm. An upper level
trough extends from the Southern Ocean into the south Pacific
Ocean, enhancing the wind speeds of the North Polar Jet Stream
over Austral Chile and favoring enahnced divergence aloft. Expect
precipitation totals of 20-45mm with locally higher precipitation.
On Tuesday, the atmopsheric river continues to enter southern
Chile, with precipitable water values exceeding 20mm. The region
can expect moderate precipitation on Tuesday. By Wednesday, the
atmopsheric river propagates northward into south-central Chile,
where expect heavy precipitation on Los Lagos and north Aysen.

Castellanos...(WPC)





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