279
FXUS20 KWBC 051921
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
221 PM EST Wed Nov 05 2025

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

South American Forecast Bulletin for 05 November 2025 at 1925 UTC

The dynamic upper-level pattern across tropical and central South
America will be a driving force to precipitation across the region
for the next three days. An upper level ridge, otherwise known as
the Bolivian High, will act as a blockage to the progression of
shortwave upper level troughs across central South America through
Thursday. After Thursday, this upper ridge is expected to begin
moving eastward and may lose definition, allowing for the
shortwave upper level troughs to start crossing east of the
central Andes Mountains. Prior to the eastward propagation of the
upper ridge, another upper level trough will be located across
eastern Brasil and will interact with the southeastern periphery
of the aforementioned upper ridge. This interaction may lead to an
increase in a diffluent upper level wind regime and may positively
enhance upper divergence. The axis of this trough will be located
near 43W Wednesday night and is expected to move out into the
Atlantic Basin by Thursday. This upper level trough will promote
upper divergence on Wednesday and Thursday and will also be
sustaining a weak surface trough and stationary front across
central Brasil and Espirito Santo. A mid-level trough will also
help provide additional ascent and instability across this region
through Thursday. These conditions will support a total
precipitation maxima of 20 - 35mm across central Brasil.

Meanwhile, across Espirito Santo and the southern Bahia region,
there will be a significant increase in precipitable water through
Thursday, which will enhance low level moisture convergence and
orographic effects. On Wednesday, the area of greatest
precipitation impact will be Espirito Santo where a total
precipitation maxima of 20 - 35mm is expected. By Thursday
afternoon, the stationary front will meander into southern Bahia
and will yield a total precipitation maxima of 25 - 50mm from
Thursday morning through Friday morning. The significant increase
in accumulations is likely due to this period having the highest
precipitable water values, which may exceed 50mm. On Friday,
accumulations across the coast should decrease while an increase
in precipitation accumulations is expected in Mato Grosso and
central Brasil. The upper-to-mid level pattern will not be as
suitable for the sustenance of deep convection. However, there
will be ample low level moisture and instability to promote higher
isolated total precipitation maxima.

Another region of interest is Paraguay, Uruguay, Southern Brasil,
and northern Argentina. Upper level jet streak maxes will be
present across much of central South America for the entirety of
the forecast cycle while the upper wind flow pattern will remain
mostly zonal through Thursday. Upper divergence will be favorable
across the left entrance and right exit regions of these jet
streak maxes. Ahead of the passage of the aforementioned shortwave
upper trough across the Andes early on Friday, expect long fetch
southerly flow to extend as far south as 38S. On Wednesday
afternoon, there will be two weak frontal systems present, one
across northern Argentina and the other across El Chaco region,
central Paraguay, and southern Brasil regions. In general, total
precipitation maxima will exceed 30mm from Wednesday morning
through Thursday morning in these regions due to enhanced moisture
convergence along the surface-to-low-level boundaries, the diurnal
cycle, and local effects. By late Thursday night, a surface low
pressure system will begin to develop across the El Chaco region
of Paraguay and will cut off moisture transport from the deep
tropics into the central regions of Argentina. Frontogenesis will
be likely with the arrival of the axis of a shortwave upper trough
overnight Thursday into Friday. A warm front is expected to
develop near the Mesopotamia region of Argentina, northern
Uruguay, and southern Brasil while a cold front will begin to
develop across the El Chaco region. From Thursday morning through
Friday morning, a total precipitation maxima of 35 - 70mm is
likely across southeastern Paraguay and the Mesopotamia region of
Argentina and a risk for severe weather is possible. On Friday,
the area of greatest precipitation impacts will be concentrated in
the vicinity of the low pressure system, which is expected to be
located near Rio Grande do Sul. Onshore flow of moisture will lead
to an increase in moisture convergence and will likely favor total
precipitation maxima of 50 - 100mm across this region. Instability
will be plentiful, thus a risk for severe weather is likely.
Higher accumulations are possible along the aforementioned frontal
system that is developing overnight Thursday into Friday.

The cold front associated with the aforementioned frontal system
will favor higher precipitation accumulations across north-central
Bolivia. The cold front is expected to have a fast propagation
speed, where it will be located across southern Bolivia on Friday
night and will be extending into north-central Bolivia by Saturday
morning. The greatest precipitation impacts may occur overnight
Friday into Saturday. Convection will be primarily driven by the
low and mid-levels. From Friday morning through Saturday morning,
a total precipitation maxima of 20 - 45mm is possible.

Another region of interest is Cordoba/Santa Fe and northwest
Argentina. A weakening cold front and a moist long fetch low level
flow will lead to above normal precipitable water anomalies
starting Wednesday. The presence of the cool mid-levels and upper
divergence may help in the sustenance of convection in this
region. Thus, a total precipitation maxima of 15 - 30mm is likely
on Wednesday and a risk for severe weather is possible. By
Thursday afternoon, a low level pressure system will begin to
develop in this region and precipitable water values may increase
to 30mm. A mid-level trough will also be moving into the region
and will be accompanied by vorticity advection. From Thursday
morning through Friday morning, a total precipitation maxima of 30
- 60mm is possible Cordoba/Santa Fe and will be further enhanced
by orographic effects and the diurnal cycle. Precipitation impacts
will decrease after Thursday.

Elsewhere in tropical South America, seasonal conditions will
persist for the entirety of the forecast cycle. There will be a
drying trend across the western portions of the Amazon due to a
decrease in precipitable water. Diurnal tropical convection will
still be present in the region, especially in areas where there is
a presence of low level troughs. Moisture convergence, the diurnal
cycle, and local effects will be the main drivers for
precipitation initiation. Along the Pacific coast, there will be
ample moisture available, however, the low level flow is weak and
moisture convergence will be limited across Colombia, Ecuador, and
northern Peru.

In the high mid-latitudes, seasonal conditions will also prevail
across southern Chile. A series of weak fronts will propagate
across the region for the next three days and may be accompanied
by a subtle increase in precipitable water. If any precipitation
does develop, total precipitation maxima will be on the moderate
side.


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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