932
FXUS20 KWBC 221839
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
239 PM EDT Tue Sep 22 2026

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

Forecast Bulletin 22 September 2026 at 1840 UTC:

In Chile, a jet streak max will continue moving across the region
and support upper divergence through Tuesday afternoon.
Thereafter, a zonal jet streak max is expected to move into the
continent. The upper level wind direction will be directly from
the west, or orthogonal to the local topography. This pattern will
favor the enhancement in upper divergence in the region starting
today and continuing through the rest of the forecast cycle. This
dynamic upper level pattern is sustaining a surface frontal system
that is also being accompanied with a long fetch moisture plume,
or a weak atmospheric river. The warm front will be converging
into southern Chile during the day today and a weak cold and
stationary front will slowly move across southern Chile before
approaching south-central Chile on Thursday. In addition, enhanced
north-northwesterly low level wind flow will be ongoing for the
next three days and it will support moisture convergence. Thus,
elevated total precipitation maxima is expected across this region
for the next three days with the region of greatest precipitation
impact shifting northward with passing days. On Thursday, the
moisture plume will weaken as it starts to enter south Chile but
the dynamic mid-to-upper level pattern will continue to support
the development of precipitation. A secondary frontal system will
be enhancing precipitation across austral Chile on Tuesday as
well. This system will be supported by the presence of a shortwave
mid-to-upper level trough. These conditions will lead to moderate
total precipitation maxima across austral Chile. Drier conditions
will return thereafter.

Another region of interest is Brasil where a surface frontal
system will continue moving across the southeast Atlantic coast.
This frontal system will be moving across Serra do Mar today and
extend into Espirito Santo by Wednesday morning. The frontal
boundary will meander in this region through the rest of the
forecast cycle. At the same time, an upper level trough will be
moving across the region through the next three days, sustaining
upper divergence across southeast Brasil, providing upper level
ventilation for convection. However, the moisture content in the
region is expected to decrease as the long fetch moisture plume
extending from the tropics begins to weaken. This is due to the
arrival of a mid-to-upper level ridge into the continent from the
Pacific. Thus, the period of greatest precipitation impacts will
be for Tuesday.

The frontal boundary is expected to also extend northward in
Bolivia today before losing its amplitude on Wednesday. This
period will be conducive for precipitation initiation and
sustenance. The environment will be moist and unstable due to the
transport of moisture from the deep tropics, which is being
supported by the presence of a broad anticyclonic flow across
north-central Brasil. Expect enhanced total precipitation maxima
across central Bolivia.

In the tropics, expect diurnal convection across the Pacific
coasts of Ecuador and Colombia for the next three days. The period
of greatest precipitation impact is expected for Tuesday and
Wednesday when the low level wind flow will be converging into
northwest Ecuador and south Colombia. Thereafter, precipitation
accumulations will decrease slightly as there will be a general
change in the low level wind regime; a low level southerly flow
will be dominating across the coast. Another region of interest is
Venezuela. Through the day today, expect a tropical wave to
traverse the Guianas today. This tropical wave will be entering
Venezuela on Tuesday night. The tropical wave will be moving
across Venezuela through the rest of the day on Wednesday before
exiting into Colombia on Thursday. There will also be upper level
support across the region for Wednesday and Thursday due to the
presence of an upper level trough in the region. This will
reinforce the development and sustenance of deep convection.

Diurnal showers and thunderstorms are likely across the rest of
the continent. Local and orographic effects will be the main
triggers for precipitation.


Tinoco-Morales...(WPC)





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