South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
159 PM EDT Mon Oct 27 2025

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

South American Forecast Bulletin for 27 October 2025 at 18000 UTC

A broad upper trough crossing the southern cone will drive an
active weather pattern across northern Argentina, Paraguay,
Uruguay, and southern Brazil during the next three days.
Upper-level divergence and deep moisture ahead of a frontal
boundary will sustain heavy rainfall with maxima between 40 and 80
mm/day, along with a risk of severe convection including strong
winds and hail. Across the southern Andes, successive troughs will
cause some snow showers with accumulations up to 10â€20 cm
through Wednesday. Farther north, the Amazon Basin will observe
widespread convection, with rainfall maxima near 25â€50 mm/day is
possible, while northeast Brazil stays generally dry under
subsident flow.

Over the next three days, a frontal boundary will move north from
northern Argentina and Uruguay today, and by Thursday, the frontal
boundary will progress into southern Brazil and north of Paraguay,
focusing on heavy convection over the area. At upper levels,
divergence and convective development is forecast across much of
tropical South America, especially over the Amazon Basin, northern
Bolivia and eastern Peru. To the south, a robust trough over the
southeast Pacific moves eastward across central Chile and
Argentina on Tuesday and Wednesday. The interaction of this trough
with moist low-level flow from northern Argentina and Paraguay
enhances the chances of widespread convective development. By
Thursday, the trough exits into the South Atlantic, while another
short-wave disturbance approaches from the Pacific, maintaining
unsettled conditions along southern Chile and the Patagonian
Andes.

Ahead of the front across central South America, relatively deep
moisture will be moving south ahead of the front, with
precipitable water values around 45â€60 mm. This moisture aligns
with strong upper divergence and low-level convergence, favoring
mesoscale convective development. Behind the front, a strong
anticyclone builds across southern Chile and Patagonia,
reinforcing subsidence and stable post-frontal conditions.

Southern Andes and Southern Chile:
As the upper trough approaches, scattered precipitation develops
along southern Chile and the Andean region between 38S and 45S.
Through Tuesday and Wednesday, snow accumulations of 10â€20 cm
are expected in higher terrain. Light post-frontal precipitation
persists into Thursday, generally below 15 mm/day.

Northern Patagonia and Central Argentina:
During Tuesday and early Wednesday, convection increases as the
upper trough enhances large-scale ascent. Expect moderate rainfall
with maxima between 15 and 25 mm, mainly over La Pampa and
northern Patagonia. By late Wednesday, convection strengthens
across central and northern Argentina. Organized thunderstorms
will produce rainfall maxima between 35 and 70 mm/day, with
locally heavier amounts. Severe weather is possible, including
damaging winds and hail due to strong instability and favorable
jet coupling.

Paraguay, Uruguay, and Southern Brazil:
By late Tuesday into Wednesday, the frontal system shifts
northeastward, interacting with abundant low-level moisture.
Scattered to widespread convection develops from Paraguay into
southern Brazil and Uruguay, with maxima of 35â€70 mm/day and
embedded thunderstorms capable of producing locally heavy rainfall
and hail.

Brazil and the Western and Central Amazon Basin:
The ridge aloft and deep moisture convergence supports convection
across Colombia, northern Peru, western Brazil, and Bolivia
throughout the period. Daily rainfall maxima ranges from 25 to 50
mm, locally higher near the Andean foothills and zones of strong
diurnal heating. Subsident flow will maintain a dry pattern across
most of eastern Brazil. Only isolated light showers are expected
along easternmost coastal areas, with accumulations generally
below 15 mm/day.

For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Alamo...(WPC)