068
FXUS20 KWBC 171652
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1251 PM EDT Fri Oct 17 2025

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

South American Forecast Bulletin from 17 October 2025 at 1655 UTC

In central South America...

Through the day on Friday, the axis of an upper level trough will
begin to move east of the Andes. Ahead of the axis, there is an
upper jet streak max that will continue to promote divergence
across the region. To the north of the jet streak max, upper level
diffluence is probable and may positively contribute to the
enhancement of upper divergence. This trough axis will become more
amplified and may develop a positive tilt as it continues its
migration eastward. By Monday morning, the axis should be located
near southeast Brasil. A series of shortwave mid - level troughs
will also continue to propagate across the region through the
forecast period and will be sustained by the dynamics of the upper
levels and a potent mid - level trough. These mid - level features
will support vertical ascent and destabilization.

Through Saturday morning, an area of significant precipitation
impact will be the Chaco region. Expect the development of a
surface low pressure system and an associated cold front that will
continue racing northward on Sunday. The lowering pressures in
this region will favor the increase in northerly low level wind
flow into the region and help transport moist tropical air, where
precipitable water values may exceed 50mm. Significant moisture
convergence is also possible across the foothills of the Andes in
northwest Argentina and south Bolivia, where enhanced
southeasterly low level winds will dominate on the southern
periphery of the low pressure system, especially overnight Friday
into Saturday. These conditions, coupled with the mid and upper
level regime, will yield total precipitation maxima of 60 to 125
mm across el Chaco and a secondary maxima of  30 - 60mm across
northwest Argentina.

From Saturday to Sunday morning, the cold front will continue to
advance northward, enhancing precipitation chances across Bolivia.
Note that the connection to the moist tropical air will become
disconnected after Saturday evening. Precipitable water values may
continue to surpass 50mm on Saturday and will decrease on Sunday.
Some speed divergence may be present across central and northern
Bolivia on Saturday that will be affiliated with the eastern
periphery of a subtropical ridge. These conditions and orographic
effects may favor a total precipitation maxima of 30 - 60mm from
Saturday morning through Sunday morning across the southwest
Amazon and a localized 60 - 125mm across the Yungas region of
Bolivia. Another region of interest is southeast Bolivia and
northern Paraguay, where a mid - level shortwave trough will help
trigger convection. This may yield a total precipitation maxima of
50 - 100mm from Saturday morning through Sunday morning. By
Sunday, the decrease in precipitable water will limit
precipitation accumulations compared to the prior days across this
region. Regardless, upper level speed divergence and diffluence
will still be present across north - central Brasil. The cold
front will transition into a stationary front after Saturday
evening and will continue supporting an enhancement in low level
cyclonic rotation, favoring a generalized total precipitation
maxima of 15 - 25mm across Mato Grosso. In the southwest Amazon,
moisture convergence across the foothills of the Andes may still
be possible as the weakening stationary front could still provide
vertical ascent for convective initiation. This may yield a total
precipitation maxima of 30 to 60mm for this region from Sunday
morning through Monday morning.

In southeast Paraguay and Brasils Parana Basin, anticipate the
left entrance region of the aforementioned jet streak max to
promote upper divergence across this region through Saturday
evening. The mid - to - upper level pattern will support the
sustenance of a preexisting cold front, that will be draping
across the Argentinian Chaco region, Mesopotamia, and Rio do Sul
by Friday evening, and a stationary front that is located across
Pantanal, the Parana Basin, and Serra do Mar. Between these two
surface features, a low level trough will be located across
southeast Paraguay and the southwest areas of the Parana Basin on
Friday and Saturday. The stationary front is expected to
transition into a cold front by Saturday afternoon with the
arrival of the upper level trough axis. These features will help
provide additional lift for precipitation initiation and
sustenance.

Starting early Saturday morning, a low level low pressure system
will begin to develop over southeast Brasil and may contribute to
the increase in moisture pooling near the center of the low
pressure system and along and ahead of a transitioning cold front.
A series of mid - level troughs will continue supporting the
destabilization of the environment and supporting the addition of
vertical ascent. The coupling of cyclogenesis and the mid - to -
upper level support will favor a total precipitation maxima of 50
- 100mm across southeast Paraguay and another maxima of 40 - 80mm
across Rio Grande do Sul through Saturday morning. The area of
greatest precipitation impact will continue shifting northeastward
with the northward progression of the aforementioned cold front.
From Saturday morning through Sunday morning, the area of greatest
precipitation impact will be Serra do mar and the southeast areas
of the Parana Basin. There will be an increase in onshore flow
along the coasts associated with the low pressure system. These
conditions will yield total precipitation maxima of 40 - 80mm from
Saturday morning through Sunday morning. After Sunday morning, the
aforementioned upper trough will have its axis located across
central Brasil and Espirito Santo, potentially enhancing upper
level support. Across the mid - levels, still expect
destabilization. Meanwhile, in the low levels, onshore flow will
still dominate along the coasts. Precipitable water values may
still exceed 50mm, especially near the coasts. These conditions
will favor a total precipitation maxima of 40 - 80mm from Sunday
morning through Monday morning.

In the high mid - latitudes...

A moisture plume, that has precipitable water values exceeding
19mm, is currently present in austral Chile and will affect the
region through the entirety of the forecast cycle. In the upper
levels, expect the development of an upper jet streak max in the
exit region of an upper trough that is currently located in the
Pacific. This jet streak max may enhance upper divergence across
the region for Saturday. The trough axis is expected to be moving
across the Andes by Monday. During this period, mid - level
shortwave troughs will move into the area and enhance vertical
ascent. Through Saturday morning, total precipitation maxima of 20
- 35mm are possible across southern and austral Chile, where
northwest low level wind speeds will exceed 40 knots and favor an
enhancement in moisture convergence and orographic effects.
Thereafter, a surface cold front will be moving inland and rapidly
migrate northward starting Saturday through Monday. Expect
increasing moisture convergence along and ahead of this cold front
that is also coupled with the aforementioned moisture plume.
Mountain snow is possible during this period.

In the deep tropics...

An area of interest is Venezuela and the Guianas. A tropical wave
will be moving across the region after Saturday and will provide
additional vertical ascent for convective initiation. The effects
of this tropical wave will be most significant for Sunday into
Monday, when precipitable waters will increase significantly
across Venezuela and southeastern Colombia. A total precipitation
maxima of 30 - 60mm is possible for this period. Another region of
interest is the Pacific coast of Colombia, where onshore flow and
significant moisture content will favor increasing orographic
effects and total precipitation maxima. Through the next three
days, daily precipitation accumulations may exceed 60mm. Elsewhere
in the region, surface to low level troughs, tropical diurnal
convection, and local effects will continue to favor the
development for precipitation across the region.


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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