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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1511Z Sep 17, 2020)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1111 AM EDT THU SEP 17 2020

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

FORECAST DISCUSSION FROM SEPT 17/14UTC: CONSISTENT WITH PREVIOUS
FORECAST...A BUILDING MID/UPPER LEVEL RIDGE OVER THE EASTERN
PACIFIC IS TO INDUCE THE RAPID AMPLIFICATION OF A LONG WAVE TROUGH
OVER THE WESTERN ATLANTIC. ON FRIDAY-SATURDAY THE TROUGH REACHES
MAXIMUM INTENSITY...WHEN IT IS TO EXTEND BETWEEN 65W-20W AND TO
THE SOUTH OF 20S. THIS FEATURE IS TO THEN HOLD THROUGH THE WEEKEND
TO EARLY NEXT WEEK...WHEN IT IS TO SLOWLY WEAKEN WHILE PULLING TO
THE EAST-SOUTHEAST. AT LOW LEVELS THIS SUSTAINS A DEEP/BROAD
TROUGH OVER THE SOUTHERN CONE...WITH AXIS FORECAST TO MEANDER EAST
ACROSS THE SOUTHERN CONE TO THE WESTERN ATLANTIC ON
THURSDAY-FRIDAY. SIMULTANEOUSLY...A POLAR RIDGE BUILDS ACROSS
ARGENTINA TO MID SECTIONS OF THE CONTINENT ON FRIDAY-SATURDAY. THE
DEEP TROUGH WILL BE ACCOMPANIED BY STRONG BOUNDARY LAYER
WINDS...PEAKING AT 35-50KT ACROSS PATAGONIA IN ARGENTINA/SOUTHERN
REGIONS OF CHILE AND TIERRA DEL FUEGO DURING THE NEXT TWO DAYS. AS
THE POLAR RIDGE BUILDS...IT IS TO THEN SUSTAIN A PAMPERO LOW LEVEL
JET ACROSS ARGENTINA. THIS...IN-TURN...WILL DRIVE A PROGRESSIVE
FRONT NORTH ACROSS CENTRAL TO NORTHERN ARGENTINA ON FRIDAY. ON
SATURDAY-SUNDAY THE FRONT SURGES ACROSS SOUTHERN TO SOUTHEAST
BRASIL WHILE TRAILING ACROSS MATO GROSSO TO NORTHERN BOLIVIA.
EARLY IN THE CYCLE THIS WILL SUSTAIN MODERATE CONVECTION ACROSS
SOUTHERN CHILE...WITH MAXIMA OF 15-20MM. ON THE SOUTHERN ANDES AND
TIERRA DEL FUEGO EXPECTING SOLID PRECIPITATION...WITH SNOWFALL
AMOUNTS OF 10-15CM DURING THE DAY TODAY...AND AN ADDITIONAL
05-10CM ON FRIDAY-SATURDAY. 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...THIS
IS TO THEN SUSTAIN SCATTERED DEEP CONVECTION WITH MAXIMA
INCREASING FROM 15-30MM ON SATURDAY TO 30-60MM ON MONDAY. OVER
CENTRAL PARAGUAY-CHACO THIS WILL ALSO TRIGGER DEEP CONVECTION ON
SATURDAY...WITH ACCUMULATION OF 20-30MM IN SEVERE CONVECTION. OVER
CENTRAL BOLIVIA EXPECTING HIGHER AMOUNTS...WITH MAXIMA OF 25-50MM.
BUT AS IT SPREADS NORTH ACROSS RONDONIA TO ACRE IN BRASIL/SOUTHERN
JUNGLE OF PERU THE MAXIMA DECREASES TO 20-40MM.

NORTH OVER THE CONTINENT...AT 250 HPA...A CELL OF THE SUBTROPICAL
RIDGE DOMINATES AREA TO THE NORTH OF 20S. INITIALLY THIS IS TO
CENTER ON A CLOSED HIGH OVER TOCANTINS/GOIAS IN BRASIL. BUT
THROUGH SATURDAY...THE HIGH RELOCATES ACROSS BRASIL TO
ACRE/AMAZONAS TO THE WEST. AS THE RIDGE PATTERN ALOFT EVOLVES...IT
IS TO STEER SHORT WAVE PERTURBATIONS EASTWARD ACROSS THE NORTHERN
ANDES OF CHILE INTO BOLIVIA...WITH THE PERTURBATIONS AMPLIFYING
OVER SOUTHEAST BRASIL AS THE HIGH SETTLES TO THE WEST. OVER THE
NEXT COUPLE OF DAYS DIURNAL CONVECTION IS TO CLUSTER OVER AMAZONAS
IN BRASIL-CENTRAL/NORTHERN JUNGLE OF PERU TO EASTERN
ECUADOR...WITH MAXIMA OF 15-20MM/DAY. AS THE UPPER LEVEL RIDGE
PATTERN EVOLVES...THIS WILL GRADUALLY SHIFT EASTWARD TO MATO
GROSSO-SOUTHERN PARA...WHILE THE MAXIMA INCREASES TO 20-35MM/DAY
IN SCATTERED DEEP CONVECTION.

DAVISON/GALVEZ...WPC (USA)