685
FXUS20 KWBC 211755
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1255 PM EST Fri Nov 21 2025

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

South American Forecast Bulletin for 21 November 2025 at 1755 UTC

In the subtropics, a mid-to-upper level shortwave trough will be
propagating across the Chaco region on Friday. This trough will
enable the development of a cold front across southern Bolivia and
Paraguay starting Friday afternoon. The cold front will quickly
lose its definition as it enters northern Bolivia on Saturday
evening. At the low levels, a pre-existing northwesterly low level
wind flow will transport moist tropical air into the region,
increasing available precipitable water. Moisture convergence is
expected to be magnified with the presence of the frontal boundary
across central and northern Bolivia for Friday. Thus, a total
precipitation maxima of 50 - 100mm is possible across the Yungas
region of Bolivia through Saturday morning and precipitation
chances will begin to increase starting late Friday evening.
Across the Chaco region of Bolivia and the western region of
Chiquitania, expect a total precipitation maxima of 30 - 60mm.

Low level troughing will still be present in northern Bolivia on
Saturday. Moisture convergence will be enhanced in this region and
these conditions may support a total precipitation maxima of 30 -
60mm from Saturday morning through Sunday morning. In general, the
subtropics are seeing an increase in precipitable water through
the forecast cycle. The presence of low level troughs will favor
precipitation initiation. Orography, local effects, and the
diurnal cycle may also contribute to the initiation and sustenance
of convection across the region.

Another region of interest is the Interior Nordeste and Bahia
regions of Brasil.  A weakening stationary front will begin to
move southward and low level troughing will persist in the region
for Friday and Saturday. Low level onshore flow will be prominent
through Saturday evening and this will enhance moisture
convergence in the region. Precipitable water values will begin to
decrease substantially for Sunday and precipitation chances will
decrease. For Friday, a total precipitation maxima of 40 - 80mm is
possible across Interior Nordeste and Bahia. By Saturday, a maxima
of 25 - 50mm is expected across Bahia. Moisture convergence will
still be favorable across Espirito Santo on Sunday and this may
favor total precipitation maxima of 20 - 45mm.

By Saturday, the aforementioned mid-to-upper level trough will be
moving across central Brasil. This trough will begin to interact
with a broad upper ridging pattern to the north and will favor an
increase in upper diffluence and the development of a jet streak
max across south-central Brasil through Monday morning. This will
favor the development of low level troughs in the region and
moisture pooling along these boundaries. The cooler mid-level
temperatures will also increase instability and will help sustain
convection in this region. Isolated to widespread convection is
anticipated across central Brasil, where a total precipitation
maxim of 15 - 25mm is possible. Meanwhile, a total precipitation
maxima of 30 - 60mm is expected across Mato Grosso and the Cerrado
for Saturday. On Sunday, a total preicpitaiotn maxima of 25 - 50mm
is possible across the same region.

Along the Pacific coasts of Colombia and Ecuador, onshore flow
will be persistent through the next three days. The Panamanian low
will gain definition on Saturday and will persist through Monday.
This will favor a general low level cyclonic wind flow pattern in
the region that may favor the initiation of precipitation across
the region. Expect daily precipitation accumulations to exceed
40mm through the forecast cycle.

Another feature of interest is a cold front that is currently
located across northern Argentina and extending across Uruguay. By
Friday evening, this cold front will be located across
southeastern Brasil and losing its definition across northern
Argentina. Expect a daily northward propagation of this frontal
boundary. The greatest precipitation impacts associated with this
cold front will be on Sunday. During this period, expect the
arrival of the axis of a mid-to-upper level trough to begin to
support upper divergence and destabilization of the atmosphere.
There will also be an increase in onshore flow across Serra do
Mar. These conditions will favor a total precipitation maxima of
30 - 60mm from Sunday morning through Monday morning.

In the mid-latitudes, a mid-to-upper level trough will begin to
move into austral Chile on Saturday. At the surface, a potent cold
front that is accompanied by a long fetch moisture plume will
begin to converge with the continent starting Saturday morning.
This will increase low level moisture convergence. Low level wind
speeds will exceed 50 knots and the direction of the winds will be
from a north-northwest direction. The mid-to-upper level support
and above normal moisture will lead to total accumulations of 20 -
40mm across the northern regions of austral Chile. By Sunday,
still expect enhanced moisture convergence as the low level wind
direction will be from a more westerly direction, which is
favorable for moisture convergence and orographic effects. Expect
a total precipitation maxima of 15 to 20mm from Sunday morning
through Monday morning across the northern regions of austral
Chile and southern Chile.

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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