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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1547Z Jul 14, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1147 AM EDT WED JUL 14 2021

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

FORECAST DISCUSSION FROM JUL 14/16UTC: THE GLOBAL MODELS
CONSISTENTLY FORECAST A MID/UPPER LEVEL SHORT WAVE TROUGH ON THE
SOUTHERN STREAM TO LIFT ACROSS SOUTHERN CHILE TO PATAGONIA IN
ARGENTINA EARLY IN THE CYCLE...WHILE TO THE NORTH A SHORT WAVE
TROUGH SPILLS ACROSS THE CENTRAL/NORTHERN ANDES OF CHILE TO
ARGENTINA. EARLY ON THURSDAY MORNING THESE PERTURBATIONS ARE TO
PHASE OVER THE WESTERN ATLANTIC...COMBINING INTO A BROAD/LOW
AMPLITUDE TROUGH AS THEY MEANDER FARTHER EASTWARD. AT 250 HPA
COUPLED JETS ACCOMPANY THESE TROUGHS...WITH THE JETS TO INITIALLY
FAVOR A BROAD AREA OF UPPER LEVEL DIVERGENCE ACROSS URUGUAY/RIO DE
LA PLATA BASIN IN ARGENTINA. ON THURSDAY THE FOCUS OF THE UPPER
DIVERGENCE SHIFTS TO SOUTHERN BRASIL/SOUTHEAST PARAGUAY TO MATO
GROSSO DO SUL IN BRASIL. THE FAVORABLE UPPER LEVEL JET DYNAMICS
WILL HELP SUSTAIN A RAPIDLY TO INTENSIFY FRONTAL LOW OVER
URUGUAY/RIO GRANDE DO SUL IN BRASIL...WITH THE SURFACE LOW TO
RAPIDLY OCCLUDE AS IT ENTERS THE WESTERN ATLANTIC EARLY ON
THURSDAY MORNING. IN RESPONSE...THE ASSOCIATED FRONT SURGES ACROSS
SOUTHERN BRASIL-PARAGUAY LATER TODAY...REACHING SAO PAULO-MATO
GROSSO DO SUL/NORTHERN BOLIVIA LATER ON THURSDAY. BUT AS THE UPPER
TROUGH MEANDERS AWAY...BOUNDARY IS EXPECTED TO STALL ON
FRIDAY-SATURDAY. MEANWHILE...THE DEEPENING FRONTAL LOW...AS IT
INTENSIFIES EARLY IN THE CYCLE...WILL SUSTAIN HEAVY RAINFALL
AMOUNTS ACROSS URUGUAY TO ENTRE RIOS IN ARGENTINA...WITH MAXIMA OF
35-70MM...WHILE OVER PARAGUAY EXPECTING MAXIMA OF 20-40MM IN
PREFRONTAL CONVECTION. OVER CENTRAL BOLIVIA...THE SURGING FRONT
WILL RESULT IN RAINFALL MAXIMA OF 20-30MM ON THURSDAY TO
FRIDAY...WHILE OVER CENTRAL/SOUTHERN JUNGLE OF PERU EXPECTING
ACCUMULATION OF 05-10MM IN PREFRONTAL CONVECTION.

ON THE SOUTHERN STREAM...A DEEPENING MID/UPPER LEVEL TROUGH LIFTS
ACROSS THE DRAKE PASSAGE TO TIERRA DEL FUEGO LATER ON THURSDAY. AS
IT ENTERS THE SOUTHERN CONE THE SHORT WAVE AXIS IS EXPECTED TO
RAPIDLY INTENSIFY AS IT MERGES WITH A SHORT WAVE TROUGH TO THE
NORTH. ON FRIDAY TO SATURDAY THIS IS TO THEN EVOLVE INTO A HIGH
AMPLITUDE LONG WAVE TROUGH ALONG THE COAST OF ARGENTINA TO
SOUTHERN BRASIL...WITH A CLOSED LOW FORMING OFF THE COAST OF
URUGUAY AND ALONG THE APEX OF THE MID LEVEL TROUGH LATER DURING
THE WEEKEND. IN THIS CYCLE THE GFS JOINS THE EUROPEAN MODELS IN
FORECASTING A DEEPER LOW/TROUGH TO RAPIDLY BECOME THE DOMINANT
FEATURE EARLY NEXT WEEK. AT LOW LEVELS...A DEEPENING FRONTAL LOW
ENTERS SOUTHERN CHILE/TIERRA DEL FUEGO LATER ON THURSDAY. THIS
COMBINES WITH A BUILDING POLAR RIDGE TO THE WEST TO DRIVE A
SURFACE FRONT NORTH ACROSS THE RIO DE LA PLATA BASIN/CENTRAL
ARGENTINA. ON SATURDAY THIS RACES ACROSS SOUTHERN BRASIL-PARAGUAY
TO SAO PAULO-NORTHERN BOLIVIA. ON SUNDAY THE FRONT CONTINUES
FARTHER NORTH AND EAST ACROSS BRASIL/BOLIVIA WHILE TRIGGERING A
PREFRONTAL SHEAR LINE TO THE NORTHWEST. OVER SAO PAULO TO MATO
GROSSO DO SUL...FRONTAL CONVECTION ON SATURDAY WILL SUSTAIN
RAINFALL AMOUNTS OF 05-10MM/DAY DURING THE WEEKEND...WHILE OVER
CENTRAL BOLIVIA AND THE SOUTHERN JUNGLE OF PERU EXPECTING MAXIMA
OF 20-40MM. LATER DURING THE WEEKEND...FOCUS OF THE ORGANIZED
CONVECTION SHIFTS TO NORTHWEST SOUTH AMERICA AS ENHANCED BY THE
FRONT AND THE PREFRONTAL SHEAR LINE. OVER THE PERUVIAN JUNGLE TO
ACRE/AMAZONAS IN BRASIL THIS WILL SUSTAIN MODERATE RAINFALL
AMOUNTS WITH MAXIMA OF 20-3OMM IN SCATTERED DEEP CONVECTION.
HIGHER AMOUNTS ARE EXPECTED ACROSS EASTERN ECUADOR TO SOUTHERN
COLOMBIA...WITH MAXIMA OF 30-60MM. NOTE THAT DURING THIS
PERIOD...A FEW OF THE HIGHER RESOLUTION GFS ENSEMBLE MEMBERS SHOW
LOCALIZED MAXIMA RANGING BETWEEN 75-150MM...SO THERE IS AN
ENHANCED RISK OF HIGHER RAINFALL AMOUNTS.

NORTH OVER THE CONTINENT...AT 250 HPA...A BROAD/LOW AMPLITUDE
RIDGE IS TO ESTABLISH OVER CONTINENTAL AREA TO THE NORTH OF 12S.
THIS IS TO THEN HOLD THROUGH THE END OF THE WEEK...WITH SOME
INTENSIFICATION POSSIBLE LATER DURING THE WEEKEND. THE UPPER RIDGE
PATTERN WILL THEN VENT DIURNAL CONVECTION ACROSS EQUATORIAL SOUTH
AMERICA...WITH ACTIVITY BUILDING OVER BRASIL TO THE NORTH OF 05S.
EARLY IN THE CYCLE EXPECTING RAINFALL AMOUNTS OF 15-20MM/DAY WITH
LOCALIZED MAXIMA OF 20-30MM CLUSTERING TO THE NORTHWEST. THROUGH
THURSDAY THIS DECREASES TO 10-15MM IN SCATTERED CONVECTION WHILE
SIMULTANEOUSLY BUILDING WEST AND SOUTH ACROSS EASTERN ECUADOR TO
NORTHERN PERU. BUT AS PREVIOUSLY INDICATED...UNDER THE INFLUENCE
OF A SURGING FRONT TO THE SOUTH...CONVECTION TO THE NORTHWEST IS
EXPECTED TO INTENSIFY DURING THE WEEKEND.

DAVISON...WPC (USA)