610
FXUS20 KWBC 061906
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
305 PM EDT Wed May 06 2026

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

Forecast Bulletin 06 May 2026 at 1905 UTC:

In the mid-latitudes and subtropics...

A potent mid-to-upper level trough currently has an axis near 85W.
In its entrance region, there is also an upper level jet streak
max that will intensify on Tuesday afternoon and it will support
the development of a negative tilt in the trough before it starts
moving inland in the late afternoon. This will reinforce upper
divergence across south-central Chile.  The trough axis will move
into the continent early Wednesday morning. At the surface, a
frontal boundary will be impacting the region and it will be
accompanied by a weakening long-fetch moisture plume that will
contain precipitable water values of up to 25mm. Precipitable
water values will begin to decrease during the day on Wednesday
with the decay of the frontal system. Thus, expect enhanced
precipitation chances through the day on Tuesday and decreasing
after Wednesday morning. A moderate total precipitation maxima is
likely across southern Chile for Tuesday and light total
precipitation maxima is likely across south-central Chile for
Wednesday. Note that the frontal boundary will re-project east of
the Andes, across central Patagonia, during the day on Tuesday as
a warm front develops and there will be an enhancement in moisture
convergence. Light total precipitation maxima is likely in this
region with the presence of this feature.

Also on Tuesday morning, expect the intensification of an upper
level jet streak max across northern Argentina/Paraguay that will
favor the development of a mid-level shortwave trough, which will
be located near Rio de la Plata during the morning. This trough
will continue propagating northeastward during the day, providing
an increase in instability and vertical ascent. With the decrease
in surface pressure levels across the mid-latitudes, expect the
intensification of the South American Low Level Jet (SALLJ) that
will support the transport of moist tropical air into the region.
A surface warm front will be developing near Rio de la Plata late
Tuesday afternoon and moisture convergence will be enhanced along
this boundary. These conditions will favor moderate total
precipitation maxima with a risk for severe weather.

From Wednesday to Thursday, the potent upper trough axis will
slowly propagate over the Andes, which will induce frontogenesis
across central South America. Its axis will be east of the Andes
by Thursday morning. Expect enhanced upper diffluence across the
exit region of the trough on Wednesday, where mid-level shortwave
troughs are expected to develop. Two surface frontal boundaries
will influence precipitation patterns along the Atlantic coasts of
north-central Patagonia and Buenos Aires/La Pampas/Rio de la Plata
for Wednesday. The first cold front will be developing in the late
evening on Wednesday across Buenos Aires/ La Pampa and
Cordoba/Santa Fe. By Thursday evening, this cold front will be
across Rio Grande do Sul, Paraguay, and extending into southern
Bolivia. The cold front will move into the tropics by the weekend.
In the upper levels for Thursday, expect the aforementioned upper
trough to obtain a negative tilt as the axis moves into the Rio de
la Plata region. There will be multiple shortwave troughs in the
mid-levels that will be accompanied by vorticity advection which
will further enhance mid-level divergence and instability. The
SALLJ will support the increase of moisture pooling along the cold
front and yield precipitable water values of up to 50mm. The
aforementioned conditions will support elevated total
precipitation maxima across Buenos Aires/La Pampa for Wednesday
along the developing frontal boundary. By Thursday, elevated total
precipitation maxima will occur across Uruguay while enhanced
maxima are likely across Bolivia and Paraguay. There will also be
a risk for severe weather.

The second frontal system of interest will be over north-central
Patagonia on Wednesday. On Wednesday evening, the cold front will
be near northern Patagonia and into the Cuyo region. The warm
front will be located along the Atlantic coasts of north-central
Patagonia. By Thursday evening, the cold front will be across
Buenos Aires/La Pampa/Rio de la Plata. The warm front will be off
the coast of central Patagonia. This warm front will retrograde
into the continent thereafter due to its interaction with the
aforementioned first frontal system. This will lead to a
resurgence in precipitable water values in central Patagonia, with
values approaching 12mm. The most significant total precipitation
maxima will be coastal.

In the tropics...

A subtropical upper ridge is currently centered near northern
Bolivia and is expected to lose amplitude by Thursday evening. The
presence of this feature will continue to favor easterly
low-to-mid level winds across the Tropical Basin. Thereafter, an
upper-level trough will be entering the southeast Amazon Basin
along the subtropical jet stream and it will enhance subsidence as
it enters the region on Thursday and Friday. Another mid-to-upper
level trough axis, which will be embedded within the polar jet
stream and it will begin to extend into central Bolivia from
Thursday to Friday. This upper trough will support a surface cold
front, which is expected to extend into Bolivia for Thursday into
Friday. As the cold front and upper level trough approach Bolivia,
there will be an increase in northeasterly low level winds across
the western and southwest Amazon Basin. This pattern will lead to
an increase in total precipitation maxima across central Bolivia
along the surface cold front from Thursday into Friday. Meanwhile
across the rest of the southwest Amazon Basin, expect moderate
total precipitation maxima. Other areas of interest will be in the
vicinity of low level troughs. These troughs will support the
increase in moisture convergence and cyclonic rotation. A low
level trough of interest is currently moving across the Guianas
and will continue to move westward across the Amazon Basin for the
next three days, though, upper level support will be most
favorable for Tuesday. Expect enhanced total precipitation maxima
for Tuesday and moderate total precipitation maxima on Wednesday
with this low level trough. Other regions of interest are along
the northeast Brasilian coast and the Amazon Delta where the
Intertropical Convergence Zone (ITCZ) will continue to converge
into the region and helping sustain daily diurnal showers and
thunderstorms.



Tinoco-Morales...(WPC)




$$