South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
228 PM EDT Wed Oct 22 2025

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

South American Forecast Bulletin for 22 October 2025 at 1830 UTC

A broad long-wave trough dominates the southeast Pacific,
extending into southern South America, while a subtropical ridge
prevails across the tropical portions of the continent. This
pattern will favor an active regime through Saturday,
characterized by frequent frontal passages across the Southern
Cone, enhanced upper-level divergence, and abundant tropical
moisture feeding into the subtropical region. A strong polar jet
will maintain cool air and unsettled weather over southern Chile
and Patagonia, while a mid-level trough crossing the Andes will
trigger widespread convection and severe weather potential in the
subtropics. North of 20S, the subtropical ridge will help sustain
upper divergence and vigorous diurnal convection across tropical
South America.

Today, a strong upper trough and associated polar jet will move
eastward across southern Chile and Patagonia. Enhanced upper
divergence and strong westerly flow will produce widespread
precipitation across southern Chile and the southern Andes.
Rainfall maxima are expected near 15-35 mm, while higher
elevations of the southern Andes will see 15-35 cm of snow. Strong
westerly winds, with gusts near 45 kt, will accompany the system,
especially across southern Patagonia and Tierra del Fuego.

On Thursday, as the trough moves east into the South Atlantic,
cold air advection behind the surface front will sustain scattered
rain and snow showers. Precipitation amounts will decrease, with
maxima around 5â€10 mm of rain and 5â€10 cm of snow over the
southern Andes. Winds will gradually weaken during the day as
pressure rises over the region. By Friday, another short-wave
trough will enter southern Chile, renewing precipitation south of
45S. Expect light coastal rainfall with maxima of 5-10 mm and snow
accumulations of 5-10 cm in the Andes. Westerly flow will persist,
maintaining cool and unsettled conditions across the austral
region.

Today, the subtropical ridge centered over central South America
will provide some subsidence across northern Argentina and
Paraguay, however, some showers and thunderstorms are still
forecast, generally maxing out at around 35-45. That being said, a
mid-level trough deepening over central Chile will induce
cyclogenesis across northern Patagonia and La Pampa, while moist
northerly flow enhances low-level convergence over central
Argentina. Late in the day, isolated strong convection is expected
across Mendoza, Cordoba, and La Pampa.

On Thursday into Friday, the trough will cross the Andes and
interact with abundant low-level moisture over northern and
central Argentina. Strong upper-level difluence, will create a
favorable environment for widespread convection. Organized
thunderstorms and mesoscale convective systems are possible across
Cordoba, Santa Fe, and surrounding areas, producing 50-100 mm of
rainfall, with a risk of severe weather, including hail, damaging
winds, and locally heavier amounts. Farther west across Cuyo and
northwestern Argentina, convection will be more isolated, with
lower amounts of rain.

Friday into Saturday, the frontal system will shift northeastward
into northern Argentina. Enhanced divergence aloft along the
left-exit region of the upper jet will sustain active convection
with a risk of severe weather over Uruguay and northeastern
Argentina, generating rainfall maxima of up to 75-125mm. Behind
the front, subsidence and cooler air will dominate central
Argentina, leading to clearing skies.

Across tropical South America, broad upper-level ridging will
maintain favorable conditions for diurnal convection throughout
the period. Precipitable water values remain at around 2 standard
deviations above normal, ensuring a deep moisture availability
across the northern and central Amazon Basin. That said, much
drier than normal air mass will be affecting portions of Brazil
from Mato  Grosso eastward to the east coast of Brazil.

Today, convection will concentrate over western Amazonia, from
Bolivia into Acre and Amazonas, with rainfall maxima of 25-35 mm.
On Thursday, convection will intensify over northern and western
Brazil under strong upper divergence and high moisture, producing
25-45 mm across Para, Amazonas, and Roraima. By Friday, widespread
convection will continue over the northern Amazon Basin and the
coastal Guianas, with rainfall maxima of 20-45 mm. Over Peru and
Bolivia, developing mid-level subsidence will begin to limit
convective coverage and rainfall intensity.


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

Alamo...(WPC)