451
FXUS20 KWBC 152000
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
400 PM EDT Tue Sep 15 2026

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

Forecast Bulletin 15 September 2026 at 2005 UTC:

Low-level moisture convergence and orographic forcing will support
an increase of moisture availability in the mid-to-lower
atmospheric layers, from Orinoquia to the Western Amazon Basin, as
well as along the western Colombia coast into northern Ecuador.
Precipitable water (PW) levels will steadily exceed three standard
deviations above normal for this time of year over portions of
Colombia and Ecuador for the next two days. These significantly
high moisture levels and atmospheric dynamics will lead to
widespread thunderstorms today, especially from Choco, Colombia to
northern Ecuador, where maximum rainfall accumulations of 15-35 mm
are likely today while locally higher amounts are expected. On the
Amazon side, the highly moisture-saturated mid-level layers will
extend from Orinoquia to Selva Alta in Peru today, constrained in
its extension to the south by a mid-level ridge extending over
northern Bolivia. On Wednesday, the ridge will weaken and shift
eastward while a trough will begin to increase its influence over
the region, increasing mid-level potential vorticity and
northwesterly wind speed over the Bolivian Altiplano. An increase
in mid-level moisture transport from the Amazon basin into the
Altiplano and Chaco is forecast on Wednesday and Thursday. Daily
maximum rainfall accumulation over the Altiplano will reach 15-20
mm on Wednesday and up to 30 mm on Thursday, with a marginal risk
of severe weather across the Chaco.

Today through tomorrow, expect a mid-level trough to extend over
southern Brazil into Paraguay will provide an increase in
potential vorticity on its eastern flank over southeastern Brazil.
This pattern will aid a plume of moisture transported from the
Amazon basin to become reorganized and saturated over the region.
Strong upper-level winds will provide ventilation, and the arrival
of a cold front from its southwest will provide low-level forcing
and trigger thunderstorms along the Sao Paulo and Rio de Janeiro
coasts into Minas Gerais today and tomorrow (Wednesday). Higher
maximum rainfall accumulations are expected today, topping out at
up to 35 mm; then a mid-to-upper level ridge will enter the region
to dry out mid-levels, limiting higher rainfall accumulations over
the region on Wednesday and Thursday as deep convection is forced
to move offshore into the Atlantic.

A well-structured trough will move across the southern continent
to increase mid-level moisture availability dramatically over
Austral Chile today. Strong upper-level winds will provide
ventilation to sustain upward motions, and abundant mid-level
vorticity availability will sustain storm organization. Daily
maximum rainfall accumulations of 15-20 mm and mountain snowfall
are expected throughout the next two days. A second trough will
arrive on Thursday to reactivate the thermodynamic setup over the
region, leading this time to higher maximum rainfall accumulations
of 20-45 mm and significant mountain snowfall totals.



Sanchez-Perez...(WPC)
Tinoco-Morales...(WPC)





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