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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1533Z Jul 13, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1133 AM EDT TUE JUL 13 2021

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

FORECAST DISCUSSION FROM JUL 13/16UTC: A SHORT WAVE TROUGH ON THE
NORTHERN STREAM FLOW IS TO PULL TOWARDS NORTHERN CHILE EARLY IN
THE CYCLE...WITH A VIGOROUS SUBTROPICAL JET MAXIMA TO ACCOMPANY
THIS TROUGH AS IT MAKES LANDFALL. EARLY ON WEDNESDAY MORNING THE
TROUGH IS TO SPILL ACROSS THE ANDES INTO NORTHERN ARGENTINA...WITH
THE SUBTROPICAL JET COUPLING ON ITS DIVERGENT EXIT REGION WITH THE
DIVERGENT SIDE OF A NORTHERN POLAR JET STREAMING ACROSS THE BUENOS
AIRES PROVINCE TO THE WESTERN ATLANTIC. THE ENSUING AREA OF
DIVERGENCE WILL VENT DEEP/POTENTIALLY SEVERE CONVECTION OVER THE
RIO DE LA PLATA BASIN. THROUGH THE DAY THE SHORT WAVE TROUGH IS TO
STREAM ACROSS NORTHERN ARGENTINA...AND AS IT EXITS THE COAST OF
URUGUAY/BRASIL ON THURSDAY MORNING IT IS TO PHASE WITH ANOTHER
SHORT WAVE TROUGH TO THE SOUTH.

AS THE MID LEVEL TROUGH MAKES LANDFALL OVER NORTHERN CHILE...IT IS
TO TRIGGER MODERATE TO HEAVY SNOWFALL AMOUNTS...WITH ACCUMULATION
OF 10-15CM/DAY. OVER NORTHERN  ARGENTINA IT WILL INTERACT WITH A
MEANDERING FRONT THAT STRETCHES ACROSS THE SOUTH ATLANTIC TO
SOUTHERN BRASIL/URUGUAY-NORTHERN ARGENTINA...SUSTAINING A FRONTAL
WAVE THAT IS TO GRADUALLY INTENSIFY AS IT PROPAGATES ACROSS
CORRIENTES/MISIONES TO URUGUAY LATER ON WEDNESDAY. THIS WILL DRIVE
THE FRONT NORTH ACROSS MID SECTIONS OF SOUTH AMERICA WHILE
TRIGGERING ORGANIZED CONVECTION. INITIALLY THE MOST ACTIVE IS
EXPECTED ACROSS URUGUAY-ENTRE RIOS/CORRIENTES WITH MAXIMA OF
20-45MM IN POTENTIALLY SEVERE CONVECTION. ON WEDNESDAY THE RISK OF
SEVERE CONVECTION EBBS...BUT THE MAXIMA INCREASES TO 30-60MM.
ACROSS PARAGUAY TO SOUTHERN BRASIL THIS IS TO THEN SUSTAIN
MODERATE RAINFALL AMOUNTS...WITH MAXIMA OF 20-35MM/DAY ON
WEDNESDAY AND THURSDAY. AS THE FRONT MEANDERS NORTH ACROSS SAO
PAULO TO MATO GROSSO DO SUL ON FRIDAY...THE MAXIMA INCREASES TO
20-45MM. OVER CENTRAL BOLIVIA EXPECTING LIGHT RAINFALL AMOUNTS ON
WEDNESDAY TO BUILD IN PREFRONTAL CONVECTION...WITH ACCUMULATION OF
05-10MM. ON THURSDAY THIS INCREASES TO 20-30MM WHILE BUILDING
NORTH AND WEST INTO THE SOUTHERN JUNGLE OF PERU.

ON THE SOUTHERN STREAM...AT MID/UPPER LEVELS...A PROGRESSIVE SHORT
WAVE TROUGH MAKES LANDFALL LATER TODAY. A DEEPER PERTURBATION IS
TO CLOSELY FOLLOW...ENTERING THE SOUTHERN CONE LATER ON THURSDAY
AFTERNOON. AS IT PULLS ACROSS THE SOUTHERN CONE TO THE WESTERN
ATLANTIC THE TROUGH IS TO RAPIDLY AMPLIFY...WITH AXIS BOTTOMING
OUT OVER SOUTHERN BRASIL/URUGUAY LATER ON SATURDAY. THIS THEN
EVOLVES INTO A DEEP/CLOSED LOW OFF THE COAST OF URUGUAY/BUENOS
AIRES LATER DURING THE WEEKEND...WITH BOTH EUROPEAN MODELS
FORESEEING A DEEPER LOW THAN WHAT THE GFS SUGGESTS. THEIR SOLUTION
ITS WELL SUPPORTED BY THE MEAN OF THE GLOBAL ENSEMBLE
MEMBERS...BUT THE SPREAD IS TOO HIGH TO ESTABLISH CONFIDENCE IN
THE LONGER RANGE FORECAST GUIDANCE. AT LOW LEVELS THIS WILL
SUSTAIN A BROAD TROUGH ACROSS THE SOUTHERN CONE LATER ON
THURSDAY...TO SUSTAIN MODERATE CONVECTION ACROSS SOUTHERN REGIONS
OF CHILE WITH MAXIMA OF 20-35MM. ON FRIDAY EXPECTING ADDITIONAL
RAINFALL AMOUNTS OF 10-15MM TO CLUSTER BETWEEN ISLA DE CHILOE AND
CONCEPCION. LATER DURING THE WEEKEND...AS A POLAR RIDGE BUILDS
ACROSS ARGENTINA AND A DEEPENING FRONTAL LOW OCCLUDES TO THE
EAST...THIS WILL SUSTAIN A SOUTHERLY LOW LEVEL JET THAT ADVECTS
COLD POLAR AIR NORTHWARD ACROSS ARGENTINA/RIO DE LA PLATA TO
SOUTHERN BRASIL...WITH NEAR FREEZING TEMPERATURES LIKELY ACROSS
THE BUENOS AIRES PROVINCE TO RIO GRANDE DO SUL IN BRASIL.

NORTH OVER THE CONTINENT...AT 250 HPA...A SHORT WAVE TROUGH IS TO
SLOWLY LIFT ACROSS THE NORTHEAST STATES OF BRASIL TO THE WESTERN
ATLANTIC LATER ON WEDNESDAY EVENING. ON ITS WAKE...A BROAD/LOW
AMPLITUDE RIDGE IS TO ESTABLISH OVER THE CONTINENT TO THE NORTH OF
13S/15S. THE EVOLVING UPPER LEVEL PATTERN WILL VENT DEEP
CONVECTION OVER THE CONTINENT...WITH MOST ACTIVE TO INITIALLY
CLUSTER TO THE NORTH OF 05S. ACROSS PARA AND AMAZONAS IN BRASIL
THIS WILL INITIALLY FAVOR MAXIMA OF 15-30MM IN SCATTERED DEEP
CONVECTION. THROUGH THURSDAY...HOWEVER...ACTIVITY WEAKENS TO
10-15MM/DAY IN ISOLATED TO SCATTERED CONVECTION. ACROSS NORTHERN
PERU TO EASTERN ECUADOR...MEANWHILE...ACTIVITY IS TO GRADUALLY
BECOME BETTER ORGANIZED THROUGH THE WEEKEND. ON FRIDAY IT
INCREASES TO 10-15MM...WHILE ON SATURDAY THE MAXIMA PEAKS AT
15-25MM. ON SUNDAY THE MODELS SUGGEST POTENTIALLY HIGHER RAINFALL
AMOUNTS...WITH MAXIMA OF 20-40MM IN SCATTERED DEEP CONVECTION.

 ..WITH RAINFALL MAXIMA OF 15-20MM/DAY EXPECTED THROUGH WEDNESDAY.
LATER IN THE WEEK IT WEAKENS TO MAXIMA OF 10-15MM.

DAVISON...WPC (USA)