878
FXUS20 KWBC 141831
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
231 PM EDT Fri Aug 14 2026

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

Forecast Bulletin 14 August 2026 at 1830 UTC:

Over South America, much of the region is seeing dry conditions in
the lower latitudes, where the lack of moisture and increased
subsidence is favoring light precipitation over the next three
days and into the next work week. In the larger scale, the
divergent phase of the Madden-Julian Oscillation (MJO) is forecast
to remain over the Pacific Ocean as it struggles to continue
propagating eastward. Convergence in the upper levels is fueling
subsidence over much of northern South America. An atmospheric
Kelvin Wave is forecast to propagate from the east Pacific and
enter the continent over the weekend, adding divergent support
aloft. Of note, the Antarctic Oscillation is currently in its
negative phase, and could remain negative into the next week. The
combination of these variables and other components could support
the strengthening of the deep mid to upper level trough that is
extending from the Southern Ocean into the South-eastern Pacific
Ocean on Friday, easing the northward extent of its base and the
Subtropical Jet at around 25S to 20S.

On Friday morning, the exit region of the upper jet is entering
north Chile almost perpendicularly, while embedded shortwave
troughs traveling along the large trough are enhancing lower
pressures aloft and lift of moist air from the lower levels as a
frontal boundary forms from the remnants of past fronts and
developing troughs in relation to the upper air conditions. The
entry of the frontal boundaries contains precipitable water values
that are above normal for the region, with values exceeding 25mm.
The moisture is expected along the Chilean coast as the low level
boundaries extend south into the north Austral region. The
moisture will converge along the Andes Mountains and go through
orographic lift in the areas of enhanced upper level divergence
due to the short wave troughs and right side of the large upper
trough. The higher elevations can expect snowfall accumulations,
particularly in Central Chile. The lower can expect liquid
precipitation ranging from 15-30mm in Central Chile, and maxima of
20-45mm in South Chile into Los Lagos-Chile.

On Saturday, the upper level trough is impacted by a developing
ridge to its west, and as it interacts with ridging over central
Brasil, extending northward into central Peru, while the
Subtropical Jet propagates into central Argentina and exits
through Uruguay, south Brasil, and the Rio de La Plata region.
This long extension of the Subtropical Jet begins to bring
divergent conditions from the Mesopotamia region in Argentina, and
along the Atlantic coasts from Buenos Aires-Argentina through
Santa Catarina-Brasil. While troughing in the low levels continue
entering Chile, the low level flow converges along the Andes
Mountains, where expect heavy snowfall in the higher elevations of
Central Chile, while moderate precipitation is expected from the
Sur Chile region into the Austral Chile region. Over the Atlantic
sector of the continent, the divergence entering the area will
enhance the frontal boundaries present in the region, favoring
light to moderate precipitation in Rio Grande do Sul and Uruguay,
with locally heavier precipitation with the influx of moisture
from the tropical region as the South American Low Level Jet
(SALLJ) forms over central Bolivia and reaches north Argentina by
early Sunday morning.

By Sunday, the upper trough is impacted by the ridge to its west,
as the ridge digs southward into Austral Chile throughout the day
on Sunday and into early Monday, allowing for a jet streak to
develop to the left side of the trough over the south-East
Pacific. These conditions will favor the elongation of the upper
trough and two distinct trough axis One axis extends from central
Argentina and into the Pacific Ocean, while the second main axis
is shorter and becomes negatively tilted in the Patagonia and into
the south Atlantic Ocean by Sunday evening. This note is important
because the upper trough axis over the Pacific that will enter
Chile will begin to transition to a closed upper level low during
the day on Monday and will tilt negatively, becoming the support
needed to produce precipitation in the high elevations of north
Chile and Bolivia. This upper level cutoff low will also support
heavy precipitation and severe weather in Uruguay in the early
part of next week. We are monitoring closely this upcoming event.
In terms of precipitation on Sunday, expect heavy precipitation in
Uruguay with a moderate risk of severe weather. The southern
Mesopotamia region in Argentina can expect moderate to heavy
precipitation with a moderate risk of severe weather.

Castellanos...(WPC)





$$