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

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

South American Forecast Bulletin from 18 March 2025 at 17:15 UTC

ANALYSIS AND FORECAST OF LARGE SCALE CONDITIONS

MJO and Tropospheric Waves: The MJO continues propagating across
phase 3, consistent with large scale upper convergence established
over South America. A tropospheric Kelvin wave is forecast to
arrive during the weekend/early next week, to provide some minor
enhancement in the coverage and intensity of deep convection in
tropical regions.

SST: Important positive anomalies continue along equatorial
portions of the Eastern Pacific and the Gulf of Panama. These are,
however, superficial and could be rapidly altered by changes in
surface winds. Still, they will continue to fuel convection and
associated rainfall amounts in Ecuador and west Colombia.


SYNOPTIC ANALYSIS AND FORECAST FOR TROPICAL SOUTH AMERICA

Analysis: A cell of the subtropical upper ridge continues
strengthening off the coast of southern Peru. Speed divergence
north of the ridge is venting widespread deep convection in far
west Brasil/SE Colombia and NE Peru. Another cell centers over
Espirito Santo but has limited impacts in precipitation given the
presence of a mid-level ridge and associated dry air over east
Brasil. Over the equatorial Atlantic, the subequatorial upper
ridge extends along 05-07N. enhanced diffluence and divergence
south of this axis is venting convection along the Atlantic ITCZ,
located mostly off the north coast of Brasil and into the Amazon
Delta.

Western Tropical South America Forecast: The upper ridge is
forecast to reamin almost stationary off the southern coast of
Peru through Wednesday, moving east into south Peru on Thursday.
This will sustain upper divergence in most of west Brasil, Peru
and Ecuador through the forecast cycle. In the low-levels, a moist
air mass of total precipitable water exceeding 55mm is propagating
across the central Amazon. It is forecast to reach the andes of
Ecuador, north Peru and south Colombia on Thursday, to favor an
increase in rainfall amounts. In the Pacific basin, an increase in
the onshore component of low-level winds is expected on Wednesday,
to favor moderate rainfall amounts in Ecuador once again. An
additional increase in onshore flow and in precipitable water will
support higher accumulations on Thursday across the entire
Ecuadorian coast and far southwest Colombia.

Noteworthy quantitative precipitation forecasts:
-Pacific basin of Ecuador: Isolated maxima of 20 - 35mm on
Tuesday, increasing to maxima of 25 - 50mm on Wednesday and to 35
- 70mm on Thursday, with a risk of heavy thunderstorms.
-West and Central Amazon: Maxima of 20 - 45mm in Amazonas-Brasil
into Peru/Oriente Ecuatoriano on Tuesday. On Wednesday maxima in
the 20 - 50mm/Day range. On Thursday, maxima of 25 - 50mm west
Amazonas into south Colombia and north Peru.
-Altiplano/Peruvian Andes: Maxima generally in the 15 - 20mm/day
range or seasonal values.

Eastern and southern Tropical South America Forecast: Of interest
is a mid-upper trough forecast to amplify while propagating across
south Brasil and Paraguay/SE Bolivia late on Tuesday. This will
first yield to an increase in ventilation in the southwest Amazon
into southern Minas Gerais/Sao Paulo on Tuesday. As the trough
moves east on Wednesday, its divergent tier will enhance ascent in
southeast Brasil to focus heavy precipitation in Rio de
Janeiro/southern Minas Gerais and east Sao Paulo in combination
with moist southerly flow along coastal areas. On Thursday, expect
the heaviest precipitation in Espirito Santo/SE Minas Gerais. In
northern/northeast Brasil, scattered convection and moderate
amounts are expected on a daily basis in association with the
Atlantic ITCZ and the NET. A perturbation in the easterly trades
is forecast to favor an increase in rainfall totals from Piaui
into east Para on Thursday, as it is expected to associate with
precipitable water values approaching 60mm and enhanced trade wind
convergence.

Noteworthy quantitative precipitation forecasts:
-Bolivia/Paraguay/MG Sul: Maxima in the 20 - 50mm/Day range on
Tuesday decreasing thereafter.
-Southeast Brasil: Maxima of 20 - 35mm/Day on Tuesday increasing
to maxima of 40 - 80mm in Rio de Janeiro/S Minas Gerais and S
Espirito Santo on Wednesday. Maxima of 30 - 60mm in N Rio de
Janeiro/Espirito Santo/SE Minas Gerais on Thursday.
-North coast of Brasil/northern Nordeste: Maxima of 20 - 45mm in
Maranhao/Amazon Delta on Tuesday and Wednesday. Increases to
maxima of 25 - 50mm on Thursday.


SYNOPTIC ANALYSIS AND FORECAST FOR EXTRATROPICAL SOUTH AMERICA:

Analysis: A broad trough in the Bellingshausen sea is hosting a
potent negatively tilted short wave trough near 90W and south of
45S on Tuesday morning. This includes the cold exit of an upper
jet, which is highlighting ascent. The mid-upper trough and jet
are interacting with an atmospheric river of total precipitable
water values near 30mm, and low-level winds exceeding 55kt south
of the front. This system will have an important impact in
precipitation in Austral Chile as it moves inland later today.
Otherwise, generally fair weather and dry conditions are present
from northern Argentina-Rio Grande do Sul into central Patagonia.
An exception is a front propagating across the eastern Buenos
Aires province, which is sustaining strong thunderstorms this
morning.

Forecast: The system of interest is the atmospheric
river/negatively tilted upper trough, forecast to enter Austral
Chile later on Tuesday. The largest accumulation is expected in
interior portions of southern Aysen/far northern Magallanes on
Tuesday, although remaining convective stability will sustain cold
air cumulus showers through Thursday, to produce lesser amounts.
Generally fair weather is expected elsewhere with the exception of
isolated diurnal thunderstorms in the Argentinean central and
northern Andes on a daily basis; and the Buenos Aires Province on
Tuesday with the front. A short wave trough crossing the Andes
will favor an increase in the coverage and intensity of diurnal
convection along eastern flanks of the Andes on Thursday,
especially between south Bolivia and central Argentina. This will
also highlight precipitation in northeast Patagonia sustaining a
risk for heavy thunderstorms on Thursday into early Friday.

Noteworthy quantitative precipitation forecasts:
Austral Chile: Maxima of 25 - 50mm on Tuesday from north
Magallanes into central Aysen. Maxima of 20 - 35mm in the same
region on Wednesday. Maxima of 15 - 25mm and mountain snow from
central Aysen into west Magallanes on Thursday.
Interior NW Argentina: Maxima of 20 - 35mm on Thursday.
NE Patagonia: Maxima of 15 - 20mm on Thursday with a risk for
heavy TS.


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

Galvez...(WPC)
Ledesma...(WPC)