305
FXUS20 KWBC 191721
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1220 PM EST Wed Nov 19 2025

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

South American Forecast Bulletin for 19 November 2025 at 1725 UTC

In the mid-latitudes...

A mid-to-upper level shortwave trough that has a positively tilted
axis near 77W will begin to move into southern Chile starting
Wednesday afternoon. During this period, also expect a surface
cold front and occlusion to move into the continent. There is a
weak moisture plume affiliated with this frontal system that will
help increase moisture convergence across the region. Orography
and local effects will help motivate further surface to low level
convergence that will assist in the initiation of precipitation.
Thus, expect a total precipitation maxima of 15 - 25mm from
Wednesday morning through Thursday morning across southern Chile.
The axis of the aforementioned mid-to-upper level shortwave trough
will be located east of the Andes by Thursday evening. The
decrease in pressure at the surface and increasing baroclinicity
will favor the development of a surface cold front on Thursday
evening across the Cordoba/Santa Fe and Rio de la Plata regions of
Argentina. This front be located across southeastern Brasil by
Friday evening.

Prior to the development of the cold front, a surface-to-low level
high pressure system and the falling surface pressure values
associated with the eastward propagation of the aforementioned
upper trough will favor increasing low level northerly winds into
central Argentina. This low level wind flow pattern will enable
the transport of moist tropical air into the mid-latitudes and
help in increasing precipitable water. As the cold front develops
Thursday evening, moisture pooling will be prominent along the
cold front, further increasing available moisture. The presence of
the mid-to-upper level trough will also help sustain divergence
across the region and further destabilize the environment.

With respect to precipitation impacts, precipitation and
affiliated convection will begin to develop late Thursday evening
and into Friday morning across central Argentina. The highest
total precipitation maxima will be confined to the Rio de la Plata
regions of Argentina and Uruguay where a low pressure system will
also be located. By Friday, precipitation accumulations will be
significantly less in this region due to the decrease of moisture
in the region. Another cold front is expected to develop in
southern Bolivia, Paraguay, and southeastern Brasil on Thursday,
primarily being influenced by the arrival of another mid-level
shortwave trough. By Friday evening, this cold front will be
located across southern Bolivia and Paraguay. The combination of
the presence of the tropical air mass, the incoming newly formed
cold front, and support from the mid to upper levels will assist
in the development of convection across this region. Precipitation
chances are expected to increase after mid-day Friday. We
anticipate a total precipitation maxima to exceed 40 - 80mm across
southern and central Bolivia and a maxima of 60 - 125mm across the
foothills of the Andes in central Bolivia.

In Chile, after Wednesday, expect a seasonal precipitation pattern
to prevail for the rest of the forecast cycle. On Thursday, a
mid-to-upper level trough will move across Austral Chile, however,
precipitable water values will be below 20mm. Thus, if any
precipitation does develop in this region, expect isolated total
precipitation maxima of 15mm.

In Tropical South America...

A surface cold front will continue to move northward into Espirito
Santo and the Bahia region of Brasil on Wednesday. This cold front
will transition into a stationary front on Thursday and will be
located in the same region for the rest of the forecast cycle.
Moisture pooling is anticipated along the cold front, where total
precipitable water values will exceed 50mm for the next three
days. Expect enhanced low level moisture convergence to be further
enhanced by local effects and orography. On Wednesday, the area of
greatest precipitation impact will be across Espirito Santo and
the southern Bahia regions, where total precipitation maxima of
35-70mm is likely. By Thursday, expect a significant increase in
precipitation accumulations across the northeastern Bahia region.
A total precipitation maxima of 50 - 100mm is expected. Low level
troughing will also extend into the Interior Nordeste region of
Brasil where an increase in precipitable water and precipitation
accumulations is also anticipated. By Friday, an upper level
trough will begin to extend into the area and will promote upper
divergence at its base, which is expected to be located across 47W
on Friday evening. The region of greatest precipitation impact
will thus be across Mato Grosso and the Tocantins regions of
Brasil. Some moisture convergence will still be present across the
northeast Bahia region and will yield a total precipitation maxima
of 30 - 60mm from Friday morning through Saturday morning.

Elsewhere in tropical South America, low level onshore flow is
still expected across the Pacific coasts of Colombia and Ecuador
for Wednesday and Thursday. Speed and directional shear may limit
the ability for deep convection to develop. However, upper
divergence and moisture convergence will be present in the area
and it may promote the sustenance of convection. East of the
Andes, low level troughs will be the main driver for precipitation
initiation. Local effects, the diurnal cycle, and orographic
enhancement will help sustain precipitation and convection.

For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Tinoco-Morales...(WPC)


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