South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
157 PM EST Thu Nov 06 2025

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

South American Forecast Bulletin for 06 November 2025 at 1900 UTC

The interaction between an upper level subtropical ridge,
otherwise known as the Bolivian High, and upper level shortwave
troughs embedded within the polar jet stream will play a pivotal
role in the development of precipitation across the region.
Starting Thursday afternoon, an upper level shortwave trough, that
will also be projected into the mid-levels, will begin crossing
the Andes and will be located across northern Argentina by early
Friday morning. Expect frontogenesis to occur during this period
at the surface across northern Argentina, southern Brasil, and
Uruguay. With respect to moisture, the aforementioned subtropical
upper ridge will support an increased long fetch southerly flow
from the subtropics into the mid-latitudes, helping increase
available precipitable water across the region. Thus, above
average precipitable water values are likely along and ahead of
the frontal system that will enhance moisture convergence.

Precipitation chances will begin to increase by Thursday afternoon
across northeast Argentina and southeast Paraguay and more
organized convection is expected overnight Thursday and into
Friday. A total precipitation maxima of 30 - 60mm is possible
across the Mesopotamia region of Argentina through Friday morning
and severe weather will be likely. The highest precipitation
accumulations will occur from Friday morning through Saturday
morning across southern Brasil and in the vicinity of an occluded
front. A total precipitation maxima of 50 - 100mm is expected in
this region and there will be a risk for severe weather due to an
increase in instability due to cooler mid-levels. After Saturday
morning, this frontal system is expected to move off coast into
the Atlantic. The aforementioned upper ridge in the subtropics
will start to become more well defined on Saturday and will
prohibit the surface frontal system from moving farther north.
However, low-level moisture convergence will still be present in
the region and will favor an increase in precipitation chances,
especially across the Cerrado region of Brasil, Mato Grosso, and
Tocantins. Note that there will be isolated accumulations
exceeding 40mm across this region for Saturday.

Another region of interest associated with the aforementioned
frontal system is Bolivia. Beginning on Friday, precipitation
chances will begin to increase as the cold front migrates into
central Bolivia. Speed divergence in the upper levels will be
present on Friday and Saturday and there will be a presence of
mid-level shortwave troughs that will promote vertical ascent and
convective initiation. In the low-levels, expect enhanced
southerly flow that will be primarily influenced by the presence
of the aforementioned upper subtropical ridge across Brasil. This
low level flow pattern will favor an increase in precipitable
water as well. Thus, on Friday, a total precipitation maxima of 20
- 35mm is possible across central Bolivia. On Saturday, the cold
front is expected to lose definition, but moisture convergence
will still be present in the region. These conditions will favor a
general total precipitation maxima of 20 - 35mm across northern
Bolivia and southern Peru with higher accumulations likely across
higher terrain.

A secondary upper level shortwave trough will be projected in the
mid-levels and will move into the Atlantic ocean late on Friday.
During this period, also expect this mid-to-upper level feature to
be accompanied by a surface stationary front and low level trough
across eastern Brasil. As it exits the region, an upper diffluent
pattern will favor the presence of upper divergence through early
Saturday morning. There will also be moist onshore flow converging
into the coastlines of northern Espirito Santo and Bahia, where
precipitable water values will exceed 50mm on Friday. These
conditions, coupled with orographic enhancement, may favor a total
precipitation maxima of 30 - 60mm from Friday morning through
Saturday morning. Moisture convergence will decrease on Saturday
along with precipitation accumulations as the upper trough moves
offshore.

An area of interest on Thursday is northwest Argentina and the
Cordoba/Santa Fe provinces. A low pressure system will begin to
develop Thursday evening in the area as the axis of a shortwave
upper-level trough begins to migrate east of the Andes. This will
trigger a lowering of pressure values in the region and the
eventual development of the low pressure system. Upper level
divergence will also be present in the region and the cooler
mid-levels will favor increasing instability. Precipitable water
values will be in excess of 31mm through Friday morning and
moisture convergence will be likely in this region. Thus, a total
precipitation maxima of 25 - 50mm is expected through Friday
morning. This low pressure system will quickly lose its definition
after Friday morning.

In the higher mid-latitudes, seasonable conditions will prevail
through the forecast cycle. A series of moisture plumes will
continue to move into the area and temporarily increase available
moisture through Saturday morning. On Saturday, a strong upper jet
streak max will be moving across the area and will be favoring
upper divergence. The low level wind flow speeds may exceed 40
knots and will likely promote moisture convergence across austral
Chile. These conditions will yield a total precipitation maxima of
20 - 45mm from Saturday morning through Sunday morning.

In western and northern tropical South America, the aforementioned
upper ridge will promote an increase in easterly and southeasterly
low level winds into the Amazon Basin region.There will be a
subtle drying trend for Friday across the Basin but expect
precipitable water values to increase again for Saturday. Areas
with the highest chances for precipitation will be confined to
where there is a presence of low-level troughs that will enhance
moisture convergence. Additionally, local effects and the diurnal
cycle will play a pivotal role in the initiation of precipitation
across the region. An area of interest is Venezuela, where a low
level trough will be propagating across the country on Thursday
and Friday. Daily total precipitation maxima will exceed 40mm.
Another region of interest is the Pacific coasts of Colombia and
Ecuador for Saturday. There will be an increase in southerly low
level flow into the region and this will support an enhancement in
moisture convergence. Expect higher total precipitation maxima for
this period. 
    

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

Tinoco-Morales...(WPC)