South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
311 PM EDT Mon Oct 20 2025

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

South American Forecast Bulletin from 20 October 2025 at 1915 UTC

On Monday, the upper air systems include a ridging pattern over
northwest South America, extending into portions of the northeast
Brasil region. The base of an upper level trough is extending into
the south-central region of Brasil, from the south Atlantic ocean
by Monday evening. This system has favored the northward
progression of a frontal boundary from the Atlantic Ocean,
entering the southern Brazilian Nordeste, continuing through the
interior Nordeste and weakening in south Tocantins by the evening
hours on Monday. This will lead to continuous moist onshore flow
to enter into the coastal regions of Brazil, expected to persist
throughout the day on Monday, and continue over the next three
days. The regions that can expect the heaviest precipitation are
just south of frontal boundary, expected to remain stationary from
Alagoas to north Bahia from Monday to Wednesday. On Monday, the
southern Nordeste region into Bahia can expect maxima of 30-60mm,
while south Bahia can expect maxima of 20-35mm. The central
portions of Bahia can expect maxima of 30-60mm. While from
Tocantins through Mato Grosso, expect maxima of 20-45mm. Locally
higher precipitation totals are possible. On Tuesday, the frontal
boundary weakens over the Interior Nordeste region of Brasil,
while the stationary front remains on the coast. North Bahia can
expect maxima of 25-50mm, while the eastern Amazon Basin into
north Tocantins can expect similar amounts with the remnants of
the frontal boundary remain in a moist atmosphere and are affected
by the diurnal cycle. Moisture to the west pools over the central
Amazon Basin into the southern countries of Venezuela and
Colombia, favoring maxima of 20-35mm. On Wednesday, the stationary
front continues to favor moist onshore flow into Bahia, where
increased moisture will favor maxima of 35-70mm. Moisture is being
advected to the west along the easterly trade winds and continue
to affect the central and western Amazon Basin, where expect
maxima of 20-45mm.

In the southern regions of the continent, a high amplitude upper
level trough pattern is propagating from the west over the south
Pacific Ocean into Chile on Monday into Tuesday. As this upper
trough enters the region, an accompanying cold front has entered
Chile, becoming weak from the Pacific into south-central Chile,
and continues over the Patagonia region of Argentina, exiting into
south Atlantic Ocean by Monday evening. Moisture associated with
the frontal boundary will interact with the terrain and see
orographic lift, favoring maxima of 15-30mm in Los Lagos and north
Aysen. On Tuesday, the frontal boundary continues to push north
and east, located in the Cuyo region of Argentina, and Buenos
Aires. Since Monday, moisture has been transported from the north
with the help of low level jets reacting to the lower pressures in
the Patagonia. As the colder air mass interacts with the moist and
warmer air mass, expect a potential for severe weather from the
the Cuyo region into Buenos Aires. The Buenos Aires province can
expect moderate total precipitation of 20-45mm. In the Austral
region of Chile, a new entrance of moisture and an approaching
cold front and occlusion will favor maxima of 20-35mm with
mountain snow in the elevated regions. By Wednesday, the upper
level trough is favoring divergence aloft over the central and
northern regions of Argentina during the overnight hours into
early Thursday, as the divergent exit region of an upper jet is
present. As low level jets continue to transport moisture from
Bolivia and Paraguay into the foothills of the Andes, the
Cordoba/Santa Fe region into Buenos Aires can expect a risk of
severe weather with maxima of 20-45mm. Similar amounts are
expected from southern Bolivia into the Chaco regions of Argentina
and west Paraguay. A potential for locally heavier precipitation
is possible.

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

Castellanos...(WPC)