434
FXUS20 KWBC 261857
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
256 PM EDT Mon Oct 26 2026

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

Forecast Bulletin 26 October 2026 at 1900 UTC:

A cold front is present over central Brasil on early Monday
morning, while the Bolivian High is located over the Amazon Basin
with a ridge axis extending into portions of central Brasil. The
periphery of the Bolivian High are fueling the deep convection
present over portions of west Ecuador, south Peru into Bolivia,
and the convection from the eastern Amazon Basin through Para, and
into Bahia-Brasil. As the day progresses, the Bolivian High is
expected to weaken over the western Amazon Basin, while a deep mid
to upper trough enters central Peru by early Tuesday. The ridging
over Brasil strengethens, and the front propagates north and east
while it weakens. To the east of the front, the easterly trades
will be compressed, favoring various low level troughing, where
expect several thunderstorms to develop from Monday to early
Tuesday, and locally moderate to heavy precipitation could
accumulate on Monday. In Bolivia and South Peru, as the mid to
upper level trough enters the continent, expect low level
pressures to decrease and assist with the advection of moisture
along the Andes Mountains. Along the warm, west coast Ecuador and
Peru, moisture advection from the north could reach as south as 5S
and extend further with the entry of the mid to upper level trough
into north-central Peru. Expect the moisture to interact with the
mountainous terrain and favor heavy precipitation into the sierra
region of Ecuador and northern Peru.

On Tuesday, the mid to upper level trough continues entering the
continent, while its divergent side is expected from south Peru
through Bolivia, and Paraguay and Mato Grosso do Sul-Brasil.
Throughout the day, a secondary frontal passage reaches southern
Brasil, where expect instability and deep convection could persist
with a marginal chance of severe weather and heavy precipitation.
Moisture continues to transport along the Andes Mountains in Peru
and Bolivia, where moderate to deep convection is expected on
Tuesday. A slight decrease in total precipitation is expected in
west Ecuador and northwest Peru as a moisture transport from the
north slows down, but with the ample amount of warm Sea Surface
Temperatures in the region, isolated heavy precipitation along the
mountain ranges are expected.

On Wednesday, a new forntal boundary enters the central South
America sector, as the mid to upper trough weakens over Peru and
west Brasil. However, an upper ridge has developed over east
Brasil, where the periphery of the ridge is expected over much of
central Brasil. This region could see disorganized but potent
thunderstorms on Wednesday, with isolated heavy precitation
extending from south Para through Minas Gerais, Sao Paulo, and
Parana. In portions of north and central Bolivia, prefrontal
troughing interacts with the tropical moisture of the region, as
it has ventilation from the mid to upper level troughing to
generate deep convection with heavy precipitation in the region.
In west Ecuador/Peru, the winds that were coming from the north
are now shifting from the south as the upper trough moves away
from the region. An area of low level convergence develops as the
trade winds from the north and those from the south meet offshore
west Ecuador and south Colombia, favoring moist onshore flow into
the continent. This flow can extend from south Colombia into
northwes Peru, where expect locally heavy precipitation on
Wednesday.

In the southern sector of the continent, a drier trend is
developing due to the low amounts of available moisture. A trough
axis enters Chile and quickly propagates into northwest Argentina,
providing some divergence and vorticity as a front enters that
could sustain light precipitation in northwest Argentina on
Monday. On Tuesday, the front is expected to enter the Cordoba and
La Pampa regions, where expect light precipitation throughout the
day. The frontal boundary enters the Buneos Aires region on
Wednesday with an increase in available moisture from the
mid-levels. Buenos Aires can expect moderate preciptiation totals.
It will be until Wednesday that a frontal boundary with an
atmospheric river will enter Austral Chile with enhanced moisture
and a short wave trough aloft that will fuel locally moderate
precipitation in Magallanes and Aysen.

Castellanos...(WPC)



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