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FXUS20 KWBC 051704
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1204 PM EST Fri Dec 05 2025

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

South American Forecast Bulletin for 05 December 2025 at 1715 UTC

In the lower mid-latitudes, the exit region of a mid-to-upper
level trough, that is currently centered near 26S 81W, will begin
to affect the continent on Friday. Upper diffluence will be
plentiful east of the Andes, leading to the sustenance of upper
divergence. East of the Andes, expect an increase in long-fetch
northerly and north-easterly low level winds after Saturday
afternoon, which will assist in transporting moist tropical air
into the Cordoba and northwest regions of Argentina. Mid-level
shortwave troughs will be developing prior to the arrival of the
trough axis and this will help in the development of convection
for Friday and Saturday. Thereafter, the trough axis will be over
the continent early Sunday morning. This will trigger the
formation and advancement of a surface cold front across northern
Argentina. Convective processes will be driven by the presence of
the cold front. Total precipitation maxima will exceed 50mm across
the El Chaco region of Argentina for Sunday. The risk for severe
weather will also be present for Saturday and Sunday as the
mid-level temperatures will be below -9 degrees Celsius. The
northward progression of this cold front will be limited due to
the presence of the Bolivian High to the north.

In the Buenos Aires region, the axis of an upper level trough will
be directly overhead on Friday and will be accompanied by a
mid-level shortwave trough that is expected to move across this
region starting Friday afternoon and into the evening. The
presence of these features will enable the development of a
surface cold front for Friday evening. Moisture pooling will be
present along and ahead of this front and precipitable water
values will exceed 38mm during this period. These conditions will
favor a total precipitation maxima of 15 - 30mm and there will be
a risk for severe weather. This front will quickly move offshore
and quiet conditions will remain for the rest of the weekend
across Buenos Aires and Uruguay.

In the higher mid-latitudes, a surface high pressure system will
begin to develop off the coast of central Chile starting Friday
afternoon and will limit the northward progression of cold fronts.
This semi-stationary high pressure system will allow for a long
fetch moisture tongue associated with a cold front to converge
into the northern regions of austral Chile through the entirety of
the forecast cycle. The interaction between these two surface
features will favor a persistent north-westerly low level wind
flow. Any precipitation that does develop will be influenced by
the diurnal cycle, local effects, and orographic enhancement. The
most favorable mid-to-upper level conditions will remain farther
south, primarily across the southern regions of austral Chile.
Regardless, expect a total precipitation maxima of 20 - 40mm
across the northern region of austral Chile on Friday. On
Saturday, another broad area of high precipitable water that is
affiliated to a dying occluded front will move into austral Chile
and affecting the entire region. This will yield a broad total
precipitation maxima of 20 - 35mm. By Sunday, the area of greatest
precipitation impact will again be across northern austral Chile
due to the aforementioned surface conditions favoring ongoing
northwesterly low level flow.

In the western subtropics, a broad upper level ridging pattern,
otherwise known as the Bolivian High, will continue to gain
definition through the weekend. In the low to mid levels, there
will be a trough residing across central Bolivia through Sunday
evening and it will provide additional low level circulation and
enhance moisture convergence. The low level trough is expected to
decay after Sunday evening. Expect daily total precipitation
maxima to exceed 40mm. Farther north, the nocturnal low level jet
will be developing daily across eastern Colombia, favoring a
decrease in available moisture and overall accumulations across
the northwest Amazon Basin. To the east, cyclonic rotation will be
enhanced across the southwest regions of the central Amazon Basin
on Friday and Saturday and will increase the chances for
precipitation and convection.

Across Brasil, the South America Convergence Zone (SACZ) will
continue to weaken on Friday across central Brasil and Espirito
Santo. Low level troughing will remain in its wake for Saturday.
With respect to moisture, available moisture will begin to
decrease by Saturday afternoon. The low level flow will also shift
to a more northeasterly direction and will limit moisture
convergence along the coast of Espirito Santo. Thus, the period of
greatest precipitation impact will be on Friday and accumulations
will decrease substantially with passing days. Across central
Brasil, moisture convergence will still persist due to the
presence of low level troughs and upper level diffluence through
the weekend.

Elsewhere in tropical South America, expect seasonal conditions to
persist. In the Guianas, a long fetch moisture plume extending for
the central Atlantic Basin will continue to converge into the
area. Embedded within this moisture plume will be regions of low
level cyclonic rotation in the wind field that may develop into
low level troughs. These troughs will move into the region through
the weekend and will favor an increase in precipitation chances
with its passage. Otherwise, Local effects, the diurnal cycle, and
orographic enhancement will favor increasing precipitation
accumulation across the subtropical 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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