South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
235 PM EDT Wed Mar 19 2025

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

South American Forecast Bulletin from 19 March 2025 at 18:15 UTC

ANALYSIS AND FORECAST OF LARGE SCALE CONDITIONS
-MJO and Tropospheric Waves: The MJO is propagating in phase 4,
while large scale upper convergence still persists over the
Americas. A tropospheric Kelvin is forecast to arrive early next
week to provide enhancement to convection in tropical regions.
-SST: Important positive anomalies continue west of Ecuador and
Colombia, which will continue to fuel convection and precipitation
amounts during the next several days.

SYNOPTIC ANALYSIS AND FORECAST FOR TROPICAL SOUTH AMERICA
Analysis: The subtropical upper ridge centers on a cell off the
coast of southern Peru and continues ventilating convection over
most of west and central tropical South America. Drier conditions
are occuring south of 19S given advection of a drier air mass from
the Pacific by mid-upper westerlies. In the low-levels, enhanced
moisture convergence centers in eastern Amazonas-Brasil, Rondonia
and north Bolivia, where widespread deep convection is occurring.
Increasing low-level westerlies in coastal Ecuador are stimulating
moisture convergence.

Western Tropical South America Forecast: The upper ridge is
forecast to slowly roll eastward into southern Peru starting
today, to maintain ventilation north of 10S while continuing to
advect dry air into the southern Altiplano, limiting convection in
southern Bolivia. In the low levels, high values of available
moisture in Amazonas, Rondonia and north Bolivia will favor heavy
precipitation on Wednesday, decreasing thereafter. Another region
of interest east of the Andes is Ecuador and southern Colombia,
where southeasterly low-level winds will enhance moisture
convergence on Wednesday, and the arrival of the moist plume
thereafter. In coastal Ecuador expect weak low-level westerlies
and increasing precipitable water to sustain active convection
through the cycle, peaking on Thursday.

Noteworthy QPF:
-West and Central Amazon: Maxima of 35 - 70mm and a risk of MCS in
central Brasil on Wednesday, and maxima of 25 - 50mm in the
oriente ecuatoriano and south Colombia. On Thursday maxima
increases to 35 - 70mm in oriente Ecuatoriano/southern Colombia,
increasing further to 40 - 80mm on Friday.
-Altiplano/Peruvian Andes: Seasonal convection, generally
producing maxima of 15mm from northwest Bolivia into northern Peru
on a daily basis.
-Pacific basin of Ecuador: Maxima of 25 - 50mm on Wednesday,
increasing to 35 - 70mm on Thursday, decreasing to 20 - 45mm on
Friday. Risk of large rainfall rates.

Eastern and southern Tropical South America Forecast: ITCZ
convergence will be the primary factor enhancing convection,
particularly along the north coast of Brasil/Amazon delta. A weak
perturbation is propagating along Ceara/Piaui on Wednesday and is
forecast to reach the Amazon delta on Friday, to provide some
enhancement in convection and associated accumulations. In
southeast Brasil, a mid-upper trough is forecast to move slowly
from Parana-MG Sul to western Sao Paulo-southeastern MG by Friday
evening. This will stimulate diurnal convection from Espirito
Santo/Rio de Janeiro into southeastern MG on a daily basis. The
largest amounts are expected on Wednesday where onshore flow and
enhanced upper divergence are expected to couple.

Noteworthy QPF:
-Northern coast of Brasil: Max. of 20 - 40mm on Wednesday
decreasing after.
Interior nordeste/Tocantins: Peak on Thursday when expect max. of
30 - 60mm.
-Southeastern Brasil: Max. of 35 - 70mm on Wednesday, decreasing
after.

SYNOPTIC ANALYSIS AND FORECAST FOR EXTRATROPICAL SOUTH AMERICA:
Analysis: A broad mid-upper trough in the southern stream
associates with an atmospheric river that is entering Aysen in
Austral Chile. Although it contains 20-25mm of precipitable water,
it is losing moisture, wind speeds and support in the upper
troposphere. In the northern stream, a short wave mid-upper trough
extends along 86W. It is relevant given its potential role in
triggering strong convection in the Chaco on Friday.

Forecast: The atmospheric river will still favor moderate amounts
in Austral Chile on Wednesday, gradually decreasing thereafter
while chances of mountain snow increase as a cooler air mass moves
in. East of the Andes, expect a gradual activation of convection
in Argentina as several mid-upper troughs stimulate northerly
low-level winds. The SALLJ will likely increase precipitable water
to near 50mm in the Chaco on Friday, which will favor moderate
precipitation and a marginal risk for severe convection on Friday
into Saturday.

Noteworthy QPF:
-Austral Chile: Max of 20 - 35mm on Wednesday, decreasing after.
-Interior NW Argentina/Chaco: Maxima of 20 - 35mm on Thursday. Max
of 30 - 60mm in the chaco on Friday with a marginal risk for
severity.

For associated QPF graphical information:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Galvez...(WPC)
Alamo...(WPC)