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South American Forecast Discussion
 
(Latest Discussion - Issued 1522Z Sep 18, 2020)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1122 AM EDT FRI SEP 18 2020

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

FORECAST DISCUSSION FROM SEPT 18/15UTC: AT MID/UPPER LEVELS...A
LONG WAVE RIDGE IS BUILDING SOUTH OVER THE EASTERN PACIFIC BETWEEN
95W-75W AND TO THE NORTH OF 70S. UNDER INFLUENCE OF THIS RIDGE...A
BROAD TROUGH IS DEEPENING TO THE EAST...WITH AXIS ENVELOPING AREA
BETWEEN ARGENTINA-30S AND TO THE SOUTH OF 20S EARLY IN THE CYCLE.
THE SLOW TO EVOLVE SYSTEM IS TO HOLD ITS GROUND OVER THE WESTERN
ATLANTIC THROUGH SUNDAY...THEN WEAKEN WHILE PULLING TO THE
EAST-SOUTHEAST. AT LOW LEVELS...THE TROUGH ALOFT SUSTAINS A BROAD
TROUGH OVER THE WESTERN ATLANTIC...WHILE A POLAR RIDGE IS BUILDING
NORTH ALONG THE LE OF THE ANDES OVER ARGENTINA. UNDER INFLUENCE OF
THESE A SOUTHERLY/PAMPERO LOW LEVEL JET DOMINATES ACROSS THE
SOUTHERN CONE. AS THE RIDGE BUILDS AND THE LOW LEVEL JET
STRENGTHENS...A PROGRESSIVE FRONT SURGES ACROSS ARGENTINA EARLY IN
THE CYCLE...MOVING TO THE SOUTHEAST STATES OF BRASIL...MATO GROSSO
TO NORTHERN BOLIVIA LATER ON SATURDAY. ON SUNDAY-MONDAY...AS THE
UPPER TROUGH STARTS TO PULL AWAY...THE SURFACE FRONT STALLS ACROSS
RIO DE JANEIRO-MATO GROSSO DO SUL-NORTHERN BOLIVIA. EARLY NEXT
WEEK THE TRAILING END OF THE FRONT OVER THE CONTINENT WILL
FRONTOLIZE WHILE RETROGRESSING ACROSS PARAGUAY TO NORTHERN
ARGENTINA...WHILE SEGMENT OVER THE CENTRAL ATLANTIC/SOUTHEAST
BRASIL PERSISTS. AS THE FRONT SURGES ACROSS ARGENTINA ONLY
EXPECTING A FEW CONVECTIVE CELLS ALONG THE COAST OF PATAGONIA-RIO
DE LA PLATA BASIN. BUT AS IT MOVES OVER THE WARMER WATERS OFF THE
SOUTHEAST COAST OF BRASIL ON SATURDAY-SUNDAY...UNDER FAVORABLE
UPPER LEVEL JET DYNAMICS...CONVECTION ALONG THIS BOUNDARY WILL
INTENSIFY. ON SATURDAY EXPECTING MAXIMA OF 15-30MM...INCREASING TO
30-60MM IN POTENTIALLY SEVERE CONVECTION ON SUNDAY. ANOTHER ROUND
OF HEAVY CONVECTION IS EXPECTED ON TUESDAY AS ENHANCED BY A
MID/UPPER LEVEL TROUGH STREAMING ACROSS PARAGUAY TO SOUTHEAST
BRASIL...WITH MAXIMA DURING THIS PERIOD INCREASING TO 40-80MM.
OTHER CONVECTION IS EXPECTED TO THE WEST...TO AFFECT CHACO
PARAGUAYO AND SOUTHEAST BOLIVIA ON SATURDAY. IN THIS AREA...AS
ENHANCED BY THE PAMPERO LOW LEVEL JET...EXPECTING RAINFALL MAXIMA
OF 35-70MM IN POTENTIALLY SEVERE CONVECTION. ACROSS NORTHERN
BOLIVIA-ACRE IN BRASIL TO THE SOUTHERN JUNGLE OF PERU IT IS TO
THEN SUSTAIN ACCUMULATION OF 20-40MM ON SATURDAY TO SUNDAY.

THE MID/UPPER LEVEL RIDGE AMPLIFYING OVER THE EASTERN
PACIFIC/ALONG THE COAST OF CHILE IS FORECAST TO HOLD ITS GROUND
THROUGH SUNDAY MORNING. LATER IN THE DAY/EARLY NEXT WEEK IT IS
EXPECTED TO COLLAPSE UNDER PRESSURE FROM SUCCESSIVE SHORT WAVE
PERTURBATIONS STREAMING ACROSS THE SOUTH PACIFIC TO THE SOUTHERN
CONE. ALTHOUGH THE GLOBAL MODELS GENERALLY AGREE ON THIS EVOLUTION
AND IMPACT THEY ARE TO HAVE...THEY DIVERGE ON THE TIMING OF THESE
PERTURBATIONS. AS A RESULT...OUR CONFIDENCE IN THE FORECAST OVER
THE EASTERN PACIFIC IS LOW BEYOND 72/84 HRS. AT LOW
LEVELS...EXPECTING PROGRESSIVE FRONTS TO STREAM ACROSS THE EASTERN
PACIFIC TO THE SOUTHERN REGIONS OF CHILE/PATAGONIA IN ARGENTINA
LATER DURING THE WEEKEND/EARLY NEXT WEEK. THESE ARE TO SUSTAIN
MODERATE RAINFALL AMOUNTS WITH MAXIMA OF 15-20MM/DAY EARLY NEXT
WEEK.

AT 250 HPA...THE SUBTROPICAL RIDGE DOMINATES THE FLOW OVER THE
CONTINENT TO THE NORTH OF 20S...WITH AXIS CENTERING ON A CLOSED
HIGH OVER WESTERN BRASIL. AS THE RIDGE HOLDS TO THE NORTH...IT IS
TO STEER SHORT WAVE PERTURBATIONS EAST ACROSS THE NORTHERN ANDES
OF CHILE INTO BOLIVIA/MID SECTIONS OF SOUTH AMERICA. THE INFLOW OF
SHORT WAVE ENERGY WILL HELP SUSTAIN THE AFOREMENTIONED CONVECTION
ACROSS THE SOUTHEAST STATES OF BRASIL..AND IT IS TO ALSO SUSTAIN
AN INCREASE IN DIURNAL CONVECTION ACROSS PARA-AMAZONAS-ACRE AND
RONDONIA IN BRASIL EARLY NEXT WEEK...WITH THE GDI SHOWING
POTENTIAL FOR SCATTERED DEEP CONVECTION AND MODERATE RAINFALL
AMOUNTS...WITH MAXIMA INCREASING FROM 15-20MM/DAY TO 20-35MM/DAY
EARLY NEXT WEEK.

DAVISON/GALVEZ...WPC (USA)