South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
258 PM EDT Mon Jun 01 2026

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

Forecast Bulletin 01 June 2026 at 1900 UTC:

Starting Monday afternoon, expect the exit region of an upper
level trough into southern Chile, which is expected to enhance
upper divergence. During this time, there will be an increase in
north-northeasterly low level winds and precipitable water. The
low level wind speeds will increase for Tuesday, surpassing 30
knots and will be accompanied with precipitable water values of up
to 30mm. A jet streak max and a mid-level shortwave trough will
move into the continent during this period as well, helping
destabilize the environment and lead to the increase in vertical
ascent in southern Chile. Thereafter, a broad and short-lived
mid-to-upper level ridging pattern will return for Wednesday.
Thus, expect the most intense total precipitation maxima for
Tuesday, where moderate rainfall accumulations will be likely.

Another region of interest for the next three days is
north-central Argentina. The propagation of multiple upper level
shortwave troughs and the presence of a low level ridge, that is
currently over north-central Argentina and will continue to move
into the Atlantic with passing days, is helping promote a
northerly low level wind flow into the interior of Argentina.
There will be brief periods of upper divergence for the next three
days as well. Expect mostly scattered showers to be present across
much of the region that will be reinforced by local and orographic
effects. Two frontal boundaries will also be present in the
region. The first will be a warm front which will develop on
Monday during the day. The secondary frontal boundary will be
intensifying across the central Argentinian region for Wednesday.
These two boundaries will assist in the enhancement in low level
moisture convergence. Thus, expect light to moderate total
precipitation maxima with these features.

In southeast Brasil, a broad upper trough, which is embedded
within the subtropical jet stream, will slowly move into the
Atlantic through the next three days. Along its base, expect the
development of multiple mid-to-upper level shortwave troughs that
will support brief periods of upper divergence. Another upper
trough will be off the coast of Rio Grande do Sul and Uruguay.
This trough will support the sustenance of a cold front along the
Atlantic coasts of the aforementioned two regions. As this front
advances northward, expect moisture pooling to be present along
and ahead of the boundary. This will lead to the increase in
moisture convergence and thus rainfall accumulations in the
region. On Tuesday, the cold front will be located in the vicinity
of Rio de Janeiro. By Wednesday, it will extend into Espirito
Santo. Overall, anticipate light to moderate accumulations in the
vicinity of this frontal boundary for the next three days.

Meanwhile across the Bahia region of Brasil, expect the presence
of onshore flow for the next three days. At the same time, there
will be mid-level confluence that will reinforce moisture
convergence at the low levels for the next three days and an upper
jet streak max will support upper divergence on Monday. Anticipate
moderate to enhanced daily total precipitation maxima for the
forecast cycle with this pattern. Another region of interest is
the Nordeste region of Brasil, where a low level trough will be
approaching the area for Thursday. This feature will be
accompanied with precipitable waters values of up to 50mm. Expect
enhanced total precipitation maxima with the arrival of this
trough into the region.

Elsewhere in tropical South America, a series of low level troughs
and tropical waves will be moving across the continent for the
next three days. The first feature of interest is a tropical wave
that is currently moving across the Guianas region. As it
continues moving into Venezuela on Tuesday and Wednesday, expect
an enhancement in orographic effects and the increase in surface
to low level troughing that will reinforce the development of
convection. On Monday, A diffluent upper level wind pattern will
be overhead as well and easterly mid-level winds will favor the
transport of deep moisture into the region. Though, the easterly
mid-level winds will subside after Monday. Overall, expect the
most intense total precipitation maxima in the vicinity of the low
level features for the next three days. Meanwhile along the
Pacific coast of Colombia and Ecuador, the region will continue to
have precipitable water values near 50mm, but the low level wind
speeds will be weak for Monday and Tuesday. By Wednesday, expect
the resurgence in westerly low level winds with the approach of
the tropical wave into northern Colombia. This will lead to the
intensification of cyclonic rotation across the region, though the
highest accumulations will be along the north-central coast of
Colombia for Wednesday.


Tinoco-Morales...(WPC)