084 

FXUS20 KWBC 151850
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
249 PM EDT Wed Apr 15 2026

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

Forecast Bulletin 15 April 2026 at 1850 UTC:

Tropical South America...
The Bolivian High is situated over the western Amazon Basin on
Wednesday morning, interacting with another ridging system in the
upper levels over northern Brasil and the Guianas, and a weak
upper trough over north Colombia and Venezuela. These conditions
in the upper levels will provide ventilation on Wednesday for deep
convection, the northern coast of Brasil and the western Amazon
Basin due to propagating troughs bringing moisture and instability
to the region. To the south, the Subtropical Jet will be
interacting with the Bolivian High, where its left side entrance
region will remain over south Peru and into central Bolivia. This
will enhance divergent conditions aloft while moisture continues
to travel along the foothills of the Andes Mountains due to the
presence of a weak South American Low Level Jet (SALLJ) extending
into north Argentina. Expect moderate to heavy precipitation in
the southern Selva Alta region of Peru and into the Yungas region
of Bolivia. By Thursday, the anticyclonic circulation associated
with Bolivian High has weakened over the southwest Amazon Basin,
becoming more disorganized ridging over the area. To the north,
the upper level trough over the west Caribbean deepens and enters
deeper into northern Colombia, enhancing upper divergence aloft
over Venezuela and into southern Colombia. In the lower levels,
propagating troughs continue over the Amazon Basin, with higher
amounts of precipitable water favoring moderate to heavy
precipitation on Thursday. The Subtropical Jet has strengthened
over Bolivia, with the entrance region of the jet max still
present over Bolivia favoring deep convection in the
aforementioned regions of Bolivia and Peru on Thursday. By Friday,
troughing in the mid levels over portions of central Peru, into
south Amazonas/Acre-Brasil, and into north Bolivia, will favor
deep convection throughout the day Friday into the early morning
hours of Saturday. Moderate amounts of precipitation is expected
in the region, with localized heavier amounts expected. In the
western Amazon Basin, moist easterly winds in the low levels are
expected to converge along the foothills of the Andes Mountains,
favoring convection on Friday. To the east, in southern Venezuela
and eastern Colombia, troughs embedded in the trades will favor
precipitation throughout the day. Some locally higher amounts of
precipitation is expected.

Extra-tropical and Sub-tropical South America...
On Wednesday, a potent shortwave trough is located over northeast
Argentina and entering into Uruguay and south Brasil by the
evening hours. Although weakening, the upper trough will provide
the divergent support for deep convection throughout the day.
Expect the lower levels to continue with the cyclonic circulation
over the region favoring moist onshore flow from the east as well
as moisture from the north, enhanced by the moisture transport of
the weakening SALLJ. In southern Chile, an atmospheric river is
expected to enter the region by late evening into Thursday,
favoring light to moderate precipitation totals for Wednesday. The
Polar Jet will enhance some of the precipitation totals, and
locally higher amounts are expected. On Thursday, a generally
zonal flow is expected in the upper levels over much of northern
Argentina and northern Chile, with ridging behind it, favoring
less precipitation in the region. The atmospheric river that had
entered Austral Chile propagates northward into the central region
of the country, favoring light to locally moderate precipitation
totals, enhanced by an upper level jet, entering perpendicularly
into the region. By Friday, a propagating upper level trough
enters Austral Chile, and extends over the Patagonia throughout
the day, favoring a decrease in pressures in the lower levels.
This will favor the development of the SALLJ over northern
Argentina, increasing the available moisture over portions of
Cordoba/Santa Fe. Expect light to locally moderate precipitation
totals in the region for Friday. In Central Chile, expect light to
moderate precipitation as the moisture associated with the
northward moving atmospheric river reaches Central Chile, with
support from the upper level jet.

Castellanos...(WPC)



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