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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1700Z Dec 06, 2017)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1200 PM EST WED DEC 06 2017

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

MODEL DISCUSSION (VALID FROM DEC 06 AT 0000 UTC): THE GLOBAL
MODELS ARE IN BETTER AGREEMENT...FOLLOWING SIMILAR EVOLUTION
THROUGH 132-144 HRS. DURING THE LATTER PART OF THE FORECAST...THE
GFS AND UKMET MADE PATTERN CORRECTIONS...AND THEY NO LONGER
FORECAST A LOW TO CLOSE OVER THE SOUTHERN STATES OF BRASIL. THIS
IS SIMILAR TO WHAT THE ECMWF PREVIOUSLY FORECASTED...WITH THE
ENSEMBLE MEMBERS PROVIDING MORE SUPPORT TO THIS SOLUTION.

A BROAD MID/UPPER LEVEL TROUGH EXTENDS OVER THE ATLANTIC ALONG 20W
TO 20S. CONSISTENT WITH PREVIOUS FORECAST...THIS IS TO GRADUALLY
WEAKEN AS IT MEANDERS EAST ACROSS THE SOUTH ATLANTIC. AT LOW
LEVELS...THE TROUGH ALOFT SUSTAINS AN ELONGATED FRONT THAT EXTENDS
ACROSS THE SOUTH ATLANTIC TO THE SOUTHERN STATES OF BRASIL. THE
SEGMENT OVER SOUTHERN BRASIL IS FORECAST TO FRONTOLIZE LATER ON
THURSDAY...WHILE THE REMNANTS OVER THE ATLANTIC DISSIPATE LATER ON
SATURDAY. AN OLD BOUNDARY LIES TO THE NORTH...EXTENDING ACROSS THE
SOUTH ATLANTIC TO RIO DE JANEIRO/ESPIRITO SANTO IN BRASIL. THIS IS
TO ALSO WEAKEN/NEARLY DISSIPATE ON THURSDAY TO FRIDAY. OVER
SOUTHEAST BRASIL...THE LATTER SUSTAINS A LOW/MID LEVEL TROUGH
ACROSS ESPIRITO SANTO-MINAS GERAIS TO MATO GROSSO DO SUL. ALTHOUGH
THE FRONT TENDS TO DISSIPATE...THE TROUGH IS TO PERSIST THROUGH
THE END OF THE WEEK. AT 250 HPA AN ANTICYCLONICALLY CURVED
SUBTROPICAL JET EXTENDS ACROSS NORTHERN ARGENTINA-SOUTHERN BRASIL.
THE JET ALOFT FAVORS AN UPPER DIVERGENT PATTERN THAT IS TO VENT
DEEP CONVECTION ALONG THE LOW LEVEL TROUGH. MOST ACTIVE CONVECTION
IS TO CLUSTER ACROSS MINAS GERAIS-ESPIRITO SANTO AND BAHIA. IN
THIS AREA THE ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR MCS TO
FORM DURING THE NEXT FEW DAYS...WITH DAILY RAINFALL AMOUNTS OF
40-80MM WITH LOCALLY HIGHER AMOUNTS IN EXCESS OF 125MM HIGHLY
PROBABLE. ON FRIDAY TO SATURDAY THE FOCUS OF THE HEAVY CONVECTION
SHIFTS TO BAHIA-TOCANTINS/GOIAS IN NORTHEAST BRASIL.

IN A TIGHT BELT OF MID LATITUDE WESTERLIES...PROGRESSIVE MID LEVEL
PERTURBATIONS ARE TO STREAM ACROSS THE SOUTHEAST PACIFIC TO THE
SOUTHERN CONE OF SOUTH AMERICA. ONE MOVES ACROSS 50W EARLY IN THE
CYCLE AS ANOTHER MAKES LANDFALL OVER SOUTHERN CHILE EARLY ON
THURSDAY MORNING. AT LOW LEVELS POLAR FRONT MOVES NORTH ACROSS LA
PAMPA TO THE BUENOS AIRES PROVINCE LATER TODAY...REACHING RIO DE
LA PLATA BASIN/URUGUAY ON THURSDAY. ANOTHER FRONT
FOLLOWS...ENTERING SOUTHERN CHILE EARLY ON THURSDAY
MORNING...CROSSING PATAGONIA TO LA PAMPA-CENTRAL CHILE LATER IN
THE DAY. UNDER INFLUENCE OF THESE FRONTS...SCATTERED CONVECTION
ACROSS EXTREME SOUTHERN PORTIONS OF CHILE IS TO SUSTAIN RAINFALL
AMOUNTS OF 00-05MM/DAY AND MAXIMA OF 10MM. ACROSS CENTRAL
ARGENTINA EXPECTING MAXIMA OF 20-35MM EARLY IN THE CYCLE...WHILE
ON THURSDAY IT DECREASES TO 00-05MM. ON FRIDAY MOST ACTIVE IS TO
CLUSTER BETWEEN MENDOZA-BUENOS AIRES PROVINCE AND URUGUAY...WITH
MAXIMA OF 15-25MM.

NORTH OVER THE CONTINENT...AT 200 HPA...BROAD CELL OF THE
SUBTROPICAL RIDGE CONTINUES TO DOMINATE THE FLOW TO THE NORTH OF
30S...WITH AXIS ANCHORING ON A CLOSED HIGH OVER MATO GROSSO IN
BRASIL. AN ELONGATED TROUGH LIES TO THE NORTH OF THIS
RIDGE...EXTENDING ACROSS NORTH COAST OF BRASIL TO THE GUIANAS. THE
TROUGH ALOFT FAVORS AN UPPER CONVERGENT PATTERN THAT IS TO INHIBIT
DEEP CONVECTION ALONG THE NORTH COAST OF BRASIL TO PARA. THE RIDGE
ALOFT IS TO VENT CONVECTION TO THE SOUTH. MOST INTENSE IS TO
CLUSTER ACROSS MINAS GERAIS-BAHIA-TOCANTINS/GOIAS WHERE THE DAILY
MAXIMA IS TO RANGE BETWEEN 40-80MM AS MCS FORM OVER NORTHEAST
SOUTH AMERICA. ON THE CENTRAL/SOUTHERN SIERRA OF PERU TO THE
ALTIPLANO OF BOLIVIA THIS IS TO FAVOR MODERATE RAINFALL AMOUNTS
WITH ACCUMULATION OF 20-35MM/DAY. OVER EASTERN ECUADOR/SOUTHERN
COLOMBIA-NORTHERN JUNGLE OF PERU INITIALLY EXPECTING RAINFALL
AMOUNTS OF 15-20MM/DAY. ON THURSDAY-SATURDAY THIS WILL INCREASE TO
20-30MM/DAY IN SCATTERED DEEP CONVECTION.

OVIEDO...SHN (ARGENTINA)
POMIER...SENAMHI (BOLIVIA)
RIVAS...SENAMHI (PERU)
DAVISON...WPC (USA)