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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1536Z Jul 21, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1136 AM EDT WED JUL 21 2021

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

FORECAST DISCUSSION FROM JUL 21/16UTC: IN A HIGHLY AMPLIFIED
SINUSOIDAL WAVE PATTERN...AT MID/UPPER LEVELS...A LONG WAVE TROUGH
EXTENDS NORTH OVER THE PACIFIC ALONG 100W/110W TO 20S...WHILE A
RIDGE TO THE EAST HUGS THE COAST OF CHILE ON ITS WAY TO THE
BELLINGSHAUSEN SEA. THROUGH THURSDAY THE RIDGE RELOCATES ACROSS
THE SOUTHERN CONE WHILE YIELDING TO THE LONG WAVE TROUGH. BUT IN
THIS PROCESS THE TROUGH WILL START TO SHEAR SHORT WAVE ENERGY
ACROSS SOUTHERN CHILE/TIERRA DEL FUEGO TO PATAGONIA IN ARGENTINA.
IN THIS PROCESS THE TROUGH IS EXPECTED TO GRADUALLY PULL WHILE THE
RIDGE TO THE EAST FLATTENS. AT LOW LEVELS...A BROAD TROUGH WILL
EXTEND OVER THE SOUTHEAST PACIFIC LATER TODAY..WITH AXIS TO SLOWLY
STREAM ACROSS THE DRAKE PASSAGE/SOUTHERN CONE TO THE WESTERN
ATLANTIC EARLY ON FRIDAY MORNING...WITH THE ASSOCIATED FRONT
BRIEFLY BOTTOMING OUT OVER PATAGONIA-SOUTHERN CHILE. ON
FRIDAY...AS A POLAR RIDGE BUILDS TO THE SOUTH...A PAMPERO LOW
LEVEL JET DRIVES THE FRONT NORTH ACROSS LA PAMPA TO THE BUENOS
AIRES PROVINCE...WITH THE FRONT REACHING URUGUAY/NORTHERN
ARGENTINA ON SATURDAY. MEANWHILE...AS THE FRONTAL TROUGH PUSHES
ACROSS THE SOUTHERN CONE...IT WILL FOCUS A PLUME OF SUBTROPICAL
MOISTURE OVER THE SOUTHERN REGIONS OF CHILE...WHERE IT IS TO
SUSTAIN MODERATE TO HEAVY RAINFALL AMOUNTS WITH ACCUMULATION OF
20-45MM/DAY THROUGH FRIDAY MORNING. AS THE FRONT SURGES ACROSS THE
RIO DE LA PLATA BASIN ON SATURDAY...UNDER INFLUENCE OF COUPLED
JETS ALOFT...CONDITIONS WILL BECOME FAVORABLE FOR ORGANIZED DEEP
CONVECTION TO BUILD ACROSS URUGUAY TO ENTRE RIOS IN ARGENTINA. IN
THIS AREA EXPECTING RAINFALL MAXIMA OF 25-50MM IN POTENTIALLY
SEVERE CONVECTION.

A PROGRESSIVE SHORT WAVE TROUGH ON THE SOUTHERN STREAM IS TO
FOLLOW...WITH THE MID LEVEL VORTEX RACING ACROSS 90W LATER ON
FRIDAY. BUT AS IT ENTERS THE SOUTHERN CONE ON SATURDAY...A
BUILDING RIDGE TO THE WEST IS TO THEN INDUCE ITS NORTHWARD
AMPLIFICATION ACROSS CHILE/ARGENTINA TO MIDSECTIONS OF SOUTH
AMERICA. THE BROAD TROUGH IS TO THEN ENVELOP MOST OF THE SOUTHERN
CONE/WESTERN ATLANTIC THROUGH EARLY NEXT WEEK. AT LOW
LEVELS...THIS SUSTAINS A PROGRESSIVE FRONTAL TROUGH THAT LIFTS
ACROSS THE SOUTHEAST PACIFIC TO THE SOUTHERN CONE LATER ON
SATURDAY...WITH AXIS TO TRIGGER MODERATE RAINFALL AMOUNTS ACROSS
CHILE TO THE SOUTH OF 40S...WITH MAXIMA OF 15-20MM. THROUGH SUNDAY
THE FRONTAL TROUGH IS TO THEN AMPLIFY OVER THE WESTERN ATLANTIC
WHILE SIMULTANEOUSLY A POLAR RIDGE BUILDS NORTH ACROSS ARGENTINA.
THIS WILL SUSTAIN A COLD ADVECTIVE PATTERN WHILE SIMULTANEOUSLY
DISPLACING THE FRONT NORTH ACROSS ARGENTINA TO MID SECTIONS OF
SOUTH AMERICA. OVER TIERRA DEL FUEGO/SOUTHERN PATAGONIA THIS WILL
THEN FAVOR SOLID PRECIPITATION...WITH SNOWFALL AMOUNTS OF 10-15CM.
ACROSS SOUTHERN BRASIL TO MISIONES IN ARGENTINA THIS WILL SUSTAIN
MODERATE RAINFALL AMOUNTS WITH MAXIMA OF 15-25MM ON SUNDAY TO
MONDAY...WITH SIMILAR AMOUNTS EXPECTED ACROSS SOUTHERN BOLIVIA.

AS SUCCESSIVE POLAR PERTURBATIONS STREAM ACROSS SOUTHERN/CENTRAL
PORTIONS OF SOUTH AMERICA...A BROAD/LOW AMPLITUDE RIDGE IS TO
REMAIN OVER EQUATORIAL SOUTH AMERICA/NORTHERN STATES OF BRASIL.
THE RIDGE ALOFT IS TO VENT CONVECTION ACROSS NORTHERN BRASIL TO
THE NORTHERN JUNGLE OF PERU/EASTERN ECUADOR. IN THIS AREA
EXPECTING MAXIMA OF 30-60MM EARLY IN THE CYCLE...TO GRADUALLY
WEAKEN TO MAXIMA OF 20-35MM ON FRIDAY. OTHER CONVECTION IS
EXPECTED TO THE EAST TO CLUSTER ACROSS EASTERN BAHIA...WHERE UNDER
INFLUENCE OF A FRONTAL/PREFRONTAL SHEAR LINE EXPECTING RAINFALL
AMOUNTS OF 20-35MM/DAY. THIS IS TO PERSIST DURING THE NEXT 48-72
HRS...THEN WEAKENS TO 15-25MM/DAY ON FRIDAY AS THE SURFACE FRONT
WEAKENS AND THE SHEAR LINE GRADUALLY BECOME ILL DEFINED.

DAVISON...WPC (USA)