South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
311 PM EDT Tue Apr 28 2026

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

Forecast Bulletin 28 April 2026 at 1915 UTC:

NOTE: The International Desk products will be paused for April 29,
2026. Services will resume at full capacity on April 30, 2026.
Apologies for the inconvenience.

Across central South America...

A series of shortwave troughs will be propagating across the
region for the next two days. On Monday, a positively tilted
shortwave trough will be embedded within the subtropical and polar
jet streams. As the trough approaches northern Argentina/Paraguay,
expect this trough to split where a shortwave trough will remain
in its respective jet stream. The first shortwave trough will be
moving across Uruguay and southeast Brasil on Tuesday afternoon.
The secondary shortwave trough will be moving across northern
Argentina and Paraguay. Of the two, expect the secondary shortwave
trough to impact precipitation patterns in the region. Starting
overnight Tuesday into Wednesday, a mid-level trough will be
moving across Paraguay/southeast Brasil and it will be accompanied
by vorticity advection. A pre-existing stationary front will
continue to meander in the region through Thursday morning and it
will promote surface to low level convergence. Its structure will
lose definition thereafter. Also expect above normal precipitable
water values.

With respect to precipitation, expect an increase in precipitation
chances by the afternoon across Paraguay as the mid-to-upper level
trough moves overhead. Moderate total precipitation maxima are
likely through Tuesday morning. On Wednesday, there will be an
increase in vorticity advection that will also support mid-level
divergence. This pattern will favor localized enhanced total
precipitation maxima from Wednesday morning through Thursday
morning. Lapse rates will steepen with the traversal of the
mid-level trough, leading to a risk for severe weather late
Tuesday through the day on Wednesday as well.

On Thursday, an upper level shortwave trough and associated jet
streak max will be moving across Uruguay, northeast Argentina, and
southeast Paraguay. At the same time, an upper level trough will
be crossing east of the Andes and into southern Argentina. This
will lead to the development of a surface frontal boundary across
southern Argentina, particularly for Friday, and the increase in
northerly low level winds into the region. Also anticipate the
increase in precipitable water values. Thus, there will be an
increase in precipitation chances early Friday morning and
continuing through the day. The most intense total precipitation
maxima will be after Friday morning.

Across the southern regions of South America...

On Tuesday, an upper level trough will be moving into austral
Chile and it will favor upper divergence. The axis of the trough
will be moving east of the Andes by late Tuesday afternoon,
leading to a decrease of divergence in Chile. During this period,
a frontal boundary will be moving into the continent as well and
it will support moisture convergence and orographic enhancement. A
moisture plume will also accompany the front, though this feature
is expected to weaken after Tuesday afternoon.  Thereafter, there
will be weak moisture plumes that will continue to converge into
austral and southern Chile, though the precipitation impact will
be minimal.

Across Peru, Colombia, and the Amazon...

On Tuesday, an upper-level trough will be promoting divergence
over the northern and central highlands of Peru. Combined with
moisture arriving from the Amazon, rainfall accumulations of
around 20 mm are expected. In the middle levels, a ridge located
southeast of Brazil is facilitating the influx and accumulation of
moisture toward the jungles of Colombia, Ecuador, and Peru. The
highest values are expected in the southern Peruvian jungle due to
the formation of an upper-level shortwave.

Along the coasts of Ecuador and Colombia, an increase in low-level
northwesterly flows will allow moisture to enter, leading to
rainfall with maximum accumulations between 25 and 50 mm. For the
Amazon Delta, the presence of the ITCZ (Intertropical Convergence
Zone) will be injecting moisture and supporting rainfall across
its surroundings on the east coast and interior sectors for the
next three days. The ITCZ will shift northward for Thursday.

By Wednesday, the upper-level trough will shift further east over
the northern and central highlands (sierra) of Peru. This,
combined with the influx of moisture from the Amazon, will favor
precipitation across these highland sectors.
In the middle levels, the ridge southeast of Brazil remains in
place, continuing to pump moisture into the jungles of Colombia,
Ecuador, and Peru. As a result, the highest accumulation values
are anticipated in the central and southern Peruvian jungle,
particularly in southern Peru, due to the additional support from
the formation of small shortwave troughs. Along the coasts of
Ecuador and Colombia, the intensity of low-level northwesterly
flows will diminish; therefore, a decrease in rainfall is expected
compared to the previous day.

By Thursday, an upper-level trough will be favoring divergence
over the northern highlands (sierra) of Peru. This, combined with
moisture arriving from the Amazon, will support the occurrence of
precipitation. In the middle levels, a ridge southeast of Brazil
continues to facilitate the influx and accumulation of moisture
toward the jungles of Colombia, Ecuador, and Peru. Enhanced total
precipitation maxima is expected across the northern and southern
jungle.



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