South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
100 PM EDT Tue Mar 25 2025

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

South American Forecast Bulletin from 25 March 2025 at 1700 UTC

An upper low centered near 34S/73W is drifting southeast and is
expected to move east of northern Patagonia by Thursday morning.
Meanwhile, an upper trough will approach the coast of Chile by
Thursday afternoon. High pressure currently over southern Chile
into central Patagonia is shifting east-northeast into the
Atlantic, weakening into an upper ridge by Wednesday afternoon
over the western South Atlantic. Further north, the Bolivian High
will remain centered over eastern Bolivia through Thursday,
maintaining upper-level divergence conducive to convective
development across tropical South America.

The mid-level pattern mirrors upper-level dynamics, with a
mid-level low near 33S/74W gradually drifting southeast. A
mid-level high over southern Chile into central Patagonia will
move east-northeast today, ahead of an approaching mid-level
trough set to reach the coast of Chile by Thursday afternoon. Over
the tropical region, a broad mid-level high pressure system
extends from eastern Bolivia into northern Paraguay and the
Pantanal region of Brazil.

Low-level moisture will remain near normal across most of Brazil,
while being below normal across northeastern Brazil and most of
Venezuela through Thursday. However, slightly above-normal
moisture will persist over Bolivia and Paraguay, supporting
localized convective activity. More significantly, well
above-average moisture is observed over Argentina today and
Wednesday, shifting into Uruguay by late Wednesday. This is driven
by north low-level winds advecting tropical moisture into the
region. Meanwhile, a cold front will move into southern and
Austral Chile by Wednesday afternoon, bringing drier air behind it.

Across Tropical South America, diurnal heating will enhance
showers and thunderstorms over central Brazil, western Ecuador,
and Colombia, where persistent onshore flow interacts with the
mountain range. Across Bolivia & Southern Peru, low-level
convergence east of the Altiplano, coupled with slightly anomalous
moisture, will support daily convective development. In Argentina,
strong thunderstorms are expected late tonight into Wednesday
morning, especially across La Pampa and Buenos Aires, where higher
than normal moisture interacts with upper-level divergence. In
Chile, the approaching cold front will bring drier air and
potentially suppress convective development in the south.

In terms of rainfall, the model guidanceâ€including dynamic,
ensemble, and AI-driven forecastsâ€shows good agreement on the
heaviest rainfall locations from today into Wednesday morning.
Mato Grosso into Para, central Bolivia, and southern Peru could
observe 20-50mm daily due to persistent convective activity.
Western La Pampa into Cuyo (Argentina), up to 50mm, with potential
for strong thunderstorms due to enhanced upper divergence. Some
models suggest that Western Ecuador and Colombia can observe up to
80mm in today, with 3-day accumulations reaching 100-150mm in
isolated areas.


For associated QPF graphical information:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Alamo...(WPC)