761
FXUS20 KWBC 161605
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1205 PM EDT Thu Oct 16 2025

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

South American Forecast Bulletin from 16 October 2025 at 1610 UTC

In the higher mid - latitudes...

Upper level ridging will continue to dominate off the coast of
southern Chile through the forecast cycle. Across the exit region
of this ridge, a jet streak max is expected to develop and enhance
upper level divergence for Friday afternoon and evening across
southern Chile. Another upper level feature of interest is a
progressive trough, that is currently located near 112W, and is
projected to move westward with passing days. This feature may
support the increase in wind speeds along its exit region,
favoring the development of another more potent upper jet streak
max for Friday evening. This upper jet streak will begin to affect
the upper level dynamic regime across southern and austral Chile
by early Saturday morning. Note that in the mid - levels,
shortwave troughs and vorticity maxima pulses may reinforce
vertical ascent.

Precipitable water, that is associated to an atmospheric river, is
expected to increase starting Friday afternoon as well. Low level
wind speeds from the north - northwest may exceed 50 knots across
austral Chile for Saturday into Sunday, which may positively
contribute to the enhancement in moisture convergence across this
region. Thus, we anticipate the highest precipitation
accumulations to occur between Friday morning and Sunday morning
across southern and austral Chile, where a generalized daily total
precipitation maxima of 15 to 30mm is possible.

In the lower mid - latitudes...

The axis of an upper trough will begin to move east of the central
Andes starting Friday afternoon. Ahead of the axis, expect the
development of a potent jet streak max that will favor jet streak
dynamics starting on Thursday. Note that throughout the entirety
of the forecast period, mid - level shortwave troughs will be
propagating across this region and may further enhance vertical
ascent. The upper level features will help lower the surface
pressure values and increase baroclinicity across this sector.

At the surface, a cold front will be located across Cordoba and
Rio de la Plata Thursday evening. On Friday evening, this cold
front will be located across the southern Argentinian Chaco and
central Mesopotamia regions. By Saturday evening, this front will
begin to decay over southeastern Brasil. Another frontal system of
interest is a stationary front that will drape across the Chaco
region, central Paraguay, and southern Brasil from Thursday
through Saturday. With the arrival of the upper level trough axis
into the region, this stationary front will begin to transition
into a cold front and quickly migrate northward. This cold front
should be located across from north - central Bolivia, Mato
Grosso, central Brasil, and Serra do Mar by Sunday evening.

In the low levels, through Friday, expect long fetch moisture
transport from the tropics to extend as far south as 30S. Enhanced
moisture convergence is likely across a prefrontal trough ahead of
the cold front. These conditions may yield total precipitation
maxima of 50 - 100mm across southeast Paraguay from Thursday
morning through Friday morning. By Friday, there is a potential
for the development of two areas of broad cyclonic circulation in
the low level wind field, one across El Chaco region and the other
in southeast Brasil. This wind flow pattern may disconnect the
long fetch moisture transport into southeast Brasil and east
Paraguay. However, mid - to - upper level support will still favor
an increase in instability and upper divergence. Moisture
convergence will still be plentiful across this sector. Thus, a
total precipitation maxima of 50 - 100mm is possible across
southeast Paraguay and Rio Grande do Sul. Another maxima of 60 -
125mm possible across El Chaco for Friday morning into Saturday
morning, where orographic effects will be favorable. Across
northwest Argentina, south - southeasterly low level winds will
favor orographic effects and may yield a total precipitation
maxima of 35 - 70mm. Overall, Significant rainfall accumulations
is likely across northern Argentina, Paraguay, and southern Brasil
for Friday.

By Saturday, the stationary front turned cold front will begin to
shift the corridor of highest precipitable water to the north.
Across Bolivia, there will be a significant increase in moisture
convergence along the propagating cold front. This, coupled with
orographic and local effects will favor total precipitation maxima
of 40 - 80mm from Saturday morning into Sunday morning. The other
areas of significant precipitation accumulations will be located
in the vicinity of the low level low pressure systems, located
across El Chaco/Pantanal and off the coast of southeast Brasil.
Onshore moist flow will contribute to increased moisture
convergence along the coasts of Brasil and may favor a total
precipitation maxima of 40 - 80mm. Meanwhile in El Chaco/Pantanal,
a total precipitation maxima of 35 - 70mm is possible where the
base of the aforementioned upper trough will be located over on
Saturday and will support the enhancement of upper divergence.

In the subtropics...

Low level troughs and diurnal tropical convection will continue to
dominate the precipitation regime across the tropics. In the
Amazon Basin, still expect a subtle decrease in precipitable water
values from Thursday to Friday. The highest precipitation
accumulations may be found across the western regions of the
Amazon Basin. Thereafter, a tropical wave will be propagating
across the Guianas and northern Venezuela. This wave will support
the increase in low level moisture, which may favor the increase
in total precipitation maxima. Across the Pacific coast of
Colombia, moisture will be abundant and weak low level westerly
winds may help support convergence through the next three days.
Otherwise, expect seasonal conditions for the rest of the region.


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

Tinoco-Morales...(WPC)



$$