South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
249 PM EDT Thu Apr 30 2026

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

Forecast Bulletin 30 April 2026 at 1850 UTC:

Across the mid-latitudes...

For the next three days, a potent mid-to-upper trough will
influence the precipitation patterns across southern/austral Chile
and north-central Argentina. Starting on Thursday, the exit region
of the trough and accompanying jet streak max will be converging
into austral/southern Chile, promoting upper divergence through
the early Friday morning. During this period, a surface frontal
boundary will be moving into the continent and it will be
accompanied by a long fetch moisture tongue and moderate low level
winds speeds that will be approaching 30 knots. These conditions
will favor moderate total precipitation maxima along the border of
Los Lagos and Aysen. Seasonal conditions will return for Saturday
and Sunday in southern and austral Chile.

The aforementioned upper trough will move over the Andes early on
Friday. Meanwhile in north-central Argentina, a jet streak max
will be intensifying in the area late on Thursday and will be
interacting with the aforementioned upper trough as the trough
gains latitude. Overall, expect an enhancement in upper divergence
with the aforementioned upper level pattern and the increase in
low level northerly winds that will be responsible for the
transport of moist tropical air. The dynamic mid-to-upper level
pattern across central South America will also support the
development of a mid-to-upper shortwave trough that will be
projected into the low levels across the Mesopotamia region of
Argentina. This will favor the development of precipitation and
yield moderate total precipitation maxima in the Mesopotamia
region.

On Friday, a surface frontal boundary will be developing across
the north-central Patagonia region of Argentina. The cold front
will be located across the Buenos Aires/La Pampa province by
Friday evening. As the frontal boundary moves into southeastern
Paraguay/Brasil and the Mesopotamia region, moisture convergence
will increase significantly due to the presence of high
precipitable water values (reaching 44mm). The axis of the
mid-to-upper trough will be exiting the continent by late Saturday
evening. Thus, the period with the greatest precipitation impact
will be during the morning and day on Saturday. Thereafter,
moisture convergence will still be present along the frontal
boundary but the mid-to-upper level support will subside. With
respect to precipitation accumulations, an enhanced total
precipitation maxima is likely across southeast Brasil and the
Mesopotamia region (Argentina) for Saturday. By Sunday, moderate
total precipitation maxima is likely across southeast Brasil. 

On Saturday across northern Paraguay, a mid-to-upper level
shortwave trough will be developing during the day. At the same
time, the aforementioned frontal system will be moving into the
area, providing surface lift and increasing moisture convergence.
Expect the development of precipitation in the afternoon, which
will yield moderate total precipitation maxima for Saturday.

Across the tropics...

Across the northern coastal sectors between Ecuador and Colombia,
a low-level trough is observed. This feature will support
instability and moisture influx throughout the entire forecast
period, with the greatest intensity expected on Friday and
Saturday. In the Amazon region, primarily between Peru, Colombia,
and Brazil, the continuous moisture transport combined with an
increasing amplitude of a low-level trough will enhance rainfall,
especially through the weekend.

For the northern and northeastern coast of Brazil, the arrival and
movement of low-level troughs are anticipated. These, combined
with the steady influx of moisture, will favor increased
precipitation over the three-day forecast period. Additionally,
the Intertropical Convergence Zone (ITCZ) remains present over the
Amazon Delta, which will continue to support rain development in
that sector.


Tinoco-Morales...(WPC)
Garay-Marzano...(SENAMHI-PERU)