South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
348 PM EDT Fri Mar 20 2026

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

Forecast Bulletin 20 March 2026 at 1945 UTC:

The overall weather pattern will favor significant showers and
thunderstorms across different sections of South America this
weekend. Some areas to note are Amapa of Brazil, French Guiana,
eastern Peru in to western Bolivia, northeastern Argentina,
Uruguay, and the southern half of Chile. The reasons for
significant precipitation vary across these areas. In other areas,
some rainfall is still expected, but the latest guidance and
pattern would suggest more modest amounts. That said, most of
Venezuela, portions of central to northern Amazonas of Brazil, and
northern Chile are forecast to observe little to no precipitation
through the weekend.

in the upper levels, the Bolivian high will slowly drift
southeast, likely being centered from southeast Bolivia today to
southeastern Brazil by Monday. At the same time, there will be a
series of upper-level troughs moving through the southern part of
the continent. This setup will allow for very strong winds in the
periphery of the Bolivian High, mainly in and near the Buenos
Aires province through the weekend. Diffluence aloft in that
section of Argentina is expected today and tonight, then turning
into an easterly jet on Saturday. Tropical South America has
weaker winds aloft, but there is some divergence over Para and
Amapa of Brazil today into tonight. The upper-level winds across
Tropical South America will become more zonal on Saturday onward,
but there will be some divergence and a slight cyclonic curvature
across northeastern Brazil late Sunday.

The mid-levels will also feature a series of troughs moving across
the southern portions of South America, that is mainly south of
20S. The areas north of 20S are a bit more disorganized, as
reflected with a low pressure over Bolivia, but a high pressure
over southeastern Brazil. Locally strong mid-level winds will be
observed over Para into Amazonas of Brazil, while some divergence
can be observed over Amapa today and tonight. The winds over
southern South America are also expected to be relatively strong
through the weekend.  This particular setup of the mid and upper
levels would provide ventilation and support for vertical
development across portions of the Buenos Aires and surrounding
areas today and tonight, as well as some areas of Amapa and
adjacent areas.

In the lower levels, there will be a series of fronts and troughs
that will cause moisture convergence in key areas, which will
combine with the mid and upper level support to develop showers
and thunderstorms. At this time, above normal moisture can be
expected across eastern Peru, Bolivia, Paraguay into northeastern
Argentina and Uruguay, as a low-level jet transports moisture from
the north, allowing for significant moisture pooling over
northeastern Argentina today and tonight. There is slightly above
normal low-level moisture across central and eastern Brazil, while
western Brazil could be observing much drier than normal moisture
over Amazonas. In addition, there is a frontal boundary across the
southeastern Pacific that will bring a band of moisture to Chile
through the weekend.

After considering the current setup, the area in and near Buenos
Aires is prime for significant showers and thunderstorms,
including locally heavy downpours and even a chance of hail for
the rest of today into tonight. Therefore, we have noted a
moderate risk of severe weather over the area today. Also for
today, there will be some moisture pooling and heavy rainfall near
the end of the ITCZ across northern Para and Amapa, as that area
will also have some mid-level support for thunderstorm
development. For Saturday into Sunday, a cold front will continue
to bring a narrow band of moisture across southern Chile, causing
showers. Meanwhile, a stationary front over central South America
will provide sufficient moisture to combine with the strong mid
and upper level winds and provide adequate environment for showers
and thunderstorms, with Uruguay having the highest amounts of
rain, gradually decreasing amounts to the northwest. Most of
Brazil will observe some showers and thunderstorms, but they will
be mainly locally induced due to the combination of moisture and
diurnal heating. For Sunday into Monday, a low level though will
cause moisture convergence across Uruguay, which will combine with
the continued and expected mid to upper level jets. Therefore,
additional showers and thunderstorms are expected over the area.
Once again, for most of Brazil, the showers and thunderstorms
expected would be due to the available moisture and diurnal
heating. That said, a low-level trough could cause locally higher
amounts of rain across central Brazil.

Alamo...(WPC)