South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
310 PM EST Wed Mar 04 2026

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

Forecast Bulletin 04 March 2026 at 2010 UTC:

The model guidance did not make many significant changes compared
to yesterday. Over the next three days, the areas with the most
consistent rainfall - possibly amounting to 100mm and isolated
higher - is still expected across a significant portion of Brazil,
Peru, northwestern Bolivia, western Ecuador and western Colombia.
Across Brazil, the areas with the most significant rain could be
in and around Acre and Amazonas, to the east across Para into
Tocantins and Maranhao. The GFS model is very aggressive with an
area up to around 200mm across Amapa, but the ECMWF model is
forecasting much less rain. The latest low-level analysis, as well
as the satellite imagery does show some significant convection
near that area, associated with enhanced moisture with the ITCZ.
Therefore, the 3-day accumulations across that isolated area could
be over 150mm. Over Peru, the most significant rainfall is
expected east of the Andes, extending into northwestern Bolivia.
The rainfall over north central Argentina will be mainly due to
expected thunderstorms near low-level troughs and moisture
convergence, with 3-day accumulations that would be mainly up to
100mm but isolated higher is possible. Farther south, periods of
rain and mountain snow are expected across the southern half of
Chile, with possible water-equivalent amounts up to 50-75mm or so.

For today and tonight, convection across the northeastern quadrant
of Brazil could produce rainfall up to 100mm across isolated
areas. Meanwhile, portions western Brazil, northwestern Bolivia
and northeastern Peru could observe rainfall totals may up to
70mm. Across northern Argentina, particularly along a low-level
trough, rainfall amounts of 60 to 70 millimeters are possible,
with a marginal risk of severe weather in the vicinity of enhanced
upper-level support.

Thursday into Friday, the convection over Brazil is still expected
across the same areas as today, with the daily rainfall maxima of
up to 50 to 80mm, but Amapa could observe up to 100mm near the
ITCZ. Northern Argentina may again receive up to 70mm of rain in
association with another low-level trough and favorable
upper-level divergence. The shower and thunderstorm activity
expected across the Amazon Basin will once again be largely driven
by abundant moisture, diurnal heating, and low-level convergence.

Friday into Saturday, areas north of 30S are expected to receive
significant shower and thunderstorm activity, as several low level
troughs will cause low-level convergence across tropical South
America, while the Bolivian high causes a relative upper level jet
over northern Argentina into southeastern Brazil and Uruguay. The
rainfall totals would be up to 70 to 80mm over northern Argentina
and central Brazil. Lower amounts of rain are expected elsewhere,
but the rainfall coverage overall will be widespread.

In terms of moisture availability over the next few days,
above-normal precipitable water values are expected across much of
Brazil, especially across the areas with highest expected
rainfall, as well as eastern Peru. Slightly above normal rainfall
extends across northern Argentina, northern Bolivia, southern
Paraguay, and Uruguay. That said, much higher than normal moisture
is forecast to move south into central Argentina by Friday into
Saturday. In contrast, northwestern Brazil into southern Venezuela
will experience a pocket of notably drier air, as well as a small
portion of northeastern Brazil.

In the mid to upper levels, a trough will be moving into northern
Chile and Argentina from late Thursday into Friday. Some
Divergence and diffluence is expected over northern Argentina
during that time, which will contribute to the risk of strong
storms in that region. Farther south, a pronounced ridge will be
moving in late Friday into Saturday. Across tropical South
America, winds remain comparatively light under the Bolivian High,
with the exception of a jet in the southern periphery of the high
pressure.

In the low levels, as mentioned previously, several troughs are
present over the continent. Today, a trough extends from southern
Bolivia and western Paraguay into northwestern Argentina, with
another trough across central Brazil. By Thursday into Friday, a
trough develops across northwestern Argentina, northeastern
Argentina into Uruguay, and additional low-level troughs are
present across eastern Brazil. Several low-level troughs will also
be present over the continent on Friday into Saturday, continuing
the low-level moisture convergence.


Alamo...(WPC)