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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1620Z Jul 23, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1220 PM EDT FRI JUL 23 2021

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

FORECAST DISCUSSION FROM JUL 23/16UTC: A MID/UPPER LEVELS TROUGH
IS INITIALIZED OVER THE EASTERN PACIFIC WITH AXIS NORTH BETWEEN
90W-80W TO 15S...WHILE A BROAD RIDGE LIES TO THE EAST. AS THE
TROUGH PRESSES AGAINST THE RIDGE TO THE EAST...IT IS TO SHEAR
SHORT WAVE ENERGY UNDER THIS AXIS. IN THIS PROCESS THE TROUGH
SPLITS IN TWO...WITH SOUTHERN PORTIONS TO ACCELERATE ACROSS
PATAGONIA TO THE WESTERN ATLANTIC WHILE THE REMAINING SEGMENT
STALLS OFF THE CENTRAL/NORTHERN COAST OF CHILE. THROUGH SATURDAY
THIS IS TO THEN WEAKEN WHILE SHEARING SHORT WAVE ENERGY ACROSS THE
CENTRAL/NORTHERN ANDES OF CHILE TO ARGENTINA. AT 250 HPA A
SUBTROPICAL/NORTHERN POLAR JET ARE TO ACCOMPANY THIS TROUGH AS IT
SPILLS ACROSS THE ANDES...WITH THE JETS COUPLING ON THEIR
DIVERGENT ENTRANCE/EXIT REGIONS OVER ENTRE RIOS IN
ARGENTINA/URUGUAY. AT LOW LEVELS THIS SUSTAINS A FRONT THAT MOVES
NORTH ACROSS CENTRAL ARGENTINA EARLY IN THE CYCLE...WITH THE MID
LEVEL TROUGH/FAVORABLE JET DYNAMICS ALOFT TO SUSTAIN CYCLOGENESIS
ALONG THIS BOUNDARY OVER URUGUAY/ENTRE RIOS ON SATURDAY. THIS...IN
TURN...WILL TRIGGER SCATTERED/POTENTIALLY SEVERE CONVECTION ACROSS
URUGUAY-ENTRE RIOS...WITH MAXIMA OF 20-40MM ON SATURDAY TO SUNDAY.

CONSISTENT WITH PREVIOUS FORECASTS...A MID/UPPER LEVEL SHORT WAVE
TROUGH IN THE SOUTHERN STREAM FLOW IS TO CLOSELY FOLLOW...WITH THE
MID LEVEL VORTEX RACING ACROSS 90W LATER THIS EVENING. BUT AS THE
SHORT WAVE TROUGH ENTERS THE SOUTHERN CONE ON SATURDAY EVENING...A
BUILDING RIDGE TO THE WEST WILL THEN INDUCE THE NORTHWARD
AMPLIFICATION OF THIS TROUGH ACROSS THE SOUTHERN CONE. INFLOW OF
SUCCESSIVE VORTICES WILL THEN HELP THIS SYSTEM EVOLVE INTO A
HIGHLY AMPLIFIED LONG WAVE TROUGH OVER THE WESTERN ATLANTIC EARLY
NEXT WEEK...WITH AXIS CENTERING ON A DEEP/CLOSED LOW TO THE
NORTH-NORTHEAST OF THE MALVINAS/FALKLAND ISLANDS. THROUGH EARLY
NEXT WEEK...THE RIDGE IS TO GRADUALLY AMPLIFY TO THE SOUTHEAST
ACROSS EXTREME SOUTHERN CHILE/DRAKE PASSAGE TO THE WEDDELL
SEA...WITH AXIS RIDGE UNDERCUTTING THE LONG WAVE TROUGH TO THE
EAST. BY MIDWEEK THE POSITIVELY TILTED TROUGH IS TO THEN EXTEND TO
THE NORTHWEST ACROSS THE CENTRAL SOUTH ATLANTIC TO NORTHERN
ARGENTINA. AT LOW LEVELS...THE TROUGH ALOFT SUSTAINS A PROGRESSIVE
FRONTAL TROUGH THAT LIFTS ACROSS THE SOUTHEAST PACIFIC TO THE
SOUTHERN CONE/WESTERN ATLANTIC LATER ON SATURDAY...TO CONTINUE
NORTH ACROSS PATAGONIA AND LA PAMPA TO THE BUENOS AIRES
PROVINCE/CENTRAL ARGENTINA EARLY ON SATURDAY. EARLY NEXT
WEEK...THIS EVOLVES INTO A BROAD/OCCLUDED TROUGH OVER THE
SOUTHWEST ATLANTIC...WHILE A POLAR RIDGE OVER ARGENTINA FAVORS A
COLD/SOUTHERLY FLOW THAT DRIVES THE FRONT NORTH TO SOUTHERN
BRASIL-PARAGUAY/SOUTHERN BOLIVIA ON MONDAY AFTERNOON. ON TUESDAY
TO WEDNESDAY THE FRONT THEN SURGES ACROSS SOUTHEAST BRASIL-MATO
GROSSO TO NORTHERN BOLIVIA/ACRE IN WESTERN BRASIL/THE SOUTHERN
JUNGLE OF PERU.

AS IT MAKES LANDFALL OVER SOUTHERN CHILE THIS WILL TRIGGER
MODERATE RAINFALL AMOUNTS...WITH RAINFALL MAXIMA OF 20-35MM
CLUSTERING BETWEEN ISLA DE CHILOE AND TIERRA DEL FUEGO TO THE
SOUTH. ON SATURDAY EXPECTING SIMILAR AMOUNTS AS THE FRONTAL LOW
LIFTS ALONG THE SOUTH TO CENTRAL COAST OF CHILE...WHILE OVER
TIERRA DEL FUEGO TO THE SOUTH IT PHASE CHANGES TO MODERATE
SNOWFALL AMOUNTS. OVER THE BUENOS AIRES PROVINCE AND ALONG THE
COAST OF PATAGONIA THE OCCLUDING LOW WILL SUSTAIN A MIX OF
SNOW/RAIN SHOWERS ON MONDAY THROUGH TUESDAY...WITH ACCUMULATION OF
05-10MM/DAY...WHILE OVER SOUTHERN BRASIL/SOUTHEAST PARAGUAY AND
CENTRAL BOLIVIA THIS IS TO RESULT IN MODERATE TO HEAVY RAINFALL
AMOUNTS WITH MAXIMA OF 20-40MM.

AT 250 HPA...OVER NORTHERN SOUTH AMERICA...A BROAD/LOW AMPLITUDE
RIDGE CONFINES TO EQUATORIAL SOUTH AMERICA. THROUGH THE WEEKEND
THIS IS TO SLOWLY BUILD SOUTH ACROSS BRASIL TO NORTHERN
BOLIVIA...RESULTING IN A TIGHT GRADIENT AS IT PRESSES AGAINST THE
AMPLIFYING POLAR TROUGH TO THE SOUTH. THIS WILL THEN FAVOR A TIGHT
BELT OF LOWER LATITUDE WESTERLIES ACROSS NORTHERN CHILE/ARGENTINA
TO PARAGUAY-SOUTHERN BRASIL...HIGHLY CONDUCIVE TO GENERATION OF
MOUNTAIN WAVE TURBULENCE OVER MID SECTIONS OF SOUTH AMERICA. IN
THIS PATTERN...DIURNAL CONVECTION IS TO INITIALLY CONFINE TO
EQUATORIAL SOUTH AMERICA. BUT AS THE UPPER RIDGE PATTERN
INTENSIFIES...CONVECTION IS TO SPREAD SOUTH AND WESTWARD ACROSS
AMAZONAS IN WESTERN BRASIL TO THE NORTHERN/CENTRAL JUNGLE OF PERU.
BY THE MIDDLE OF NEXT WEEK THIS WILL COMBINE WITH PREFRONTAL
CONVECTION BUILDING ACROSS PERU-ACRE IN BRASIL TO RESULT IN A
BROAD AREA OF MODERATE RAINFALL AMOUNTS.

DAVISON...WPC (USA)