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South American Forecast Discussion
 
(Latest Discussion - Issued 1612Z Sep 19, 2017)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1212 PM EDT TUE SEP 19 2017

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

MODEL DISCUSSION (VALID FROM SEP 19 AT 0000 UTC): GLOBAL MODELS
REMAIN IN GENERAL AGREEMENT THROUGHOUT THE SHORT RANGE
PERIOD...THEN START TO DIVERGE ON THE AMPLITUDE/SPEED OF SHORT
WAVE PERTURBATIONS ENTERING THE SOUTH AMERICAN DOMAIN. DIFFERENCES
AMONG THE MODELS GRADUALLY BECOME MORE PRONOUNCED BY 120-144 HRS.
THIS LEADS TO A LOWER CONFIDENCE IN THE FORECAST BEYOND 84-96 HRS.

A 500 HPA TROUGH IS INITIALIZED OVER THE FAR EASTERN PACIFIC AND
ALONG THE COAST OF CHILE. AS IT PUSHES EAST ACROSS CHILE TO
ARGENTINA THE TROUGH WILL SPLIT IN TWO...WITH SOUTHERN HALF TO
GRADUALLY ACCELERATE OVER THE SOUTH ATLANTIC WHILE THE NORTHERN
SEGMENT REMAINS OFF THE CENTRAL/NORTHERN COAST OF CHILE. THE
LATTER IS TO THEN SPILL ACROSS THE NORTHERN ANDES OF CHILE INTO
NORTHWEST ARGENTINA ON THURSDAY EVENING...AND LATER IN THE DAY IT
MOVES ACROSS THE BUENOS AIRES PROVINCE/MESOPOTAMIA VALLEY IN
ARGENTINA. AT LOW LEVELS THIS SUSTAINS AN ELONGATED FRONT ACROSS
SOUTHERN CHILE/ PATAGONIA IN ARGENTINA. LATER THIS EVENING THE
FRONT WILL MOVE ACROSS PATAGONIA TO LA PAMPA IN ARGENTINA...WHILE
TRAINING ACROSS CONCEPCION CHILE TO THE EASTERN PACIFIC. ON
WEDNESDAY...AS A POLAR RIDGE BUILDS TO THE SOUTH AND A PAMPERO LOW
LEVEL JET ESTABLISHES ACROSS ARGENTINA THE FRONT WILL SURGE ACROSS
THE BUENOS AIRES PROVINCE TO CORDOBA/MENDOZA. LATER ON THURSDAY IT
MOVES TO SOUTHERN BRASIL-PARAGUAY TO SANTA CRUZ IN EASTERN
BOLIVIA...AND ON FRIDAY IT IS TO EXTEND ACROSS RIO GRANDE DO
SOUL-PARAGUAY TO SOUTHEAST BOLIVIA. OVER SOUTHERN CHILE THIS IS TO
SUSTAIN SCATTERED CONVECTION BETWEEN TEMUCO-ISLA DE CHILOE...WITH
ACCUMULATION OF 10-15MM THROUGH WEDNESDAY MORNING. OVER LA PAMPA
THIS WILL THEN FAVOR RAINFALL AMOUNTS OF 05-10MM LATER TODAY/EARLY
WEDNESDAY MORNING. THROUGH THE DAY THIS INCREASES TO 20-30MM WHILE
BUILDING ACROSS RIO DE LA PLATA BASIN TO CORDOBA IN ARGENTINA. ON
THURSDAY MAXIMA ACROSS URUGUAY-ENTRE RIOS IN ARGENTINA WILL PEAK
AT 15-20MM...WHILE ON FRIDAY EXPECTING MAXIMA OF 20-30MM OVER
URUGUAY-RIO GRANDE DO SUL TO CORRIENTES/ENTRE RIOS IN ARGENTINA.
OTHER CONVECTION TO THE NORTH IS TO CLUSTER ON THE HIGH
JUNGLE/SIERRA OF SOUTHERN PERU AND PORTIONS OF BOLIVIA...AS
ENHANCED BY THE UPPER LEVEL TROUGH. IN THIS AREA EXPECTING
RAINFALL AMOUNTS OF 10-15MM LATER ON THURSDAY AND FRIDAY.

ALSO AT 500 HPA...A PROGRESSIVE SHORT WAVE TROUGH ON THE SOUTHERN
STREAM WILL FOLLOW...WITH MID LEVEL PERTURBATION ENTERING THE
DRAKE PASSAGE/TIERRA DEL FUEGO EARLY ON WEDNESDAY. ON THURSDAY IT
CONTINUES TO THE WEDDELL SEA/SOUTHWEST ATLANTIC. AT LOW LEVELS
THIS WILL SUSTAIN AN ELONGATED FRONT THAT ENTERS SOUTHERN CHILE ON
WEDNESDAY. THROUGH THURSDAY EVENING THE FRONT MOVES NORTH TO
PATAGONIA IN ARGENTINA. MOST ACTIVE CONVECTION WITH THIS FEATURE
IS EXPECTED OVER CHILE TO THE SOUTH OF 40S...WITH RAINFALL AMOUNTS
OF 05-10MM ON WEDNESDAY AND 10-15MM ON THURSDAY-FRIDAY.

NORTH OVER THE CONTINENT...AT 200 HPA...BROAD CELL OF THE
SUBTROPICAL RIDGE DOMINATES THE FLOW OVER SOUTH AMERICA WHILE
CENTERING ON A CLOSED HIGH OVER BOLIVIA. A TROUGH TO THE EAST...
MEANWHILE...MEANDERS OVER SOUTHEAST BRASIL...WITH AXIS ANCHORING
ON A CLOSED LOW OVER MINAS GERAIS. UNDER INFLUENCE OF THE DEEP
POLAR TROUGH STREAMING TO THE SOUTH THE RIDGE IS TO RAPIDLY FALLEN
THROUGH WEDNESDAY EVENING/THURSDAY MORNING. LATER IN THE PERIOD
THE RIDGE WILL THEN CONFINE TO EQUATORIAL SOUTH AMERICA. THE RIDGE
ALOFT IS TO VENT DEEP CONVECTION ACROSS NORTHERN AMAZONAS IN
BRASIL-SOUTHERN COLOMBIA-NORTHERN JUNGLE IN PERU/EASTERN
ECUADOR...WITH RAINFALL MAXIMA OF 10-15MM OVER THE NEXT COUPLE OF
DAYS. ACTIVITY GRADUALLY WEAKENS ON WEDNESDAY...BUT LATER IN THE
WEAK IT IS TO THEN BECOME MORE ACTIVE/BETTER ORGANIZED....WITH
MAXIMA IN THIS AREA INCREASING TO 20-30MM. ON THE AMAZON RIVER
DELTA REGION INITIALLY EXPECTING LIGHT CONVECTION. LATER ON
WEDNESDAY ACTIVITY SURGES...WITH MAXIMA OF 10-15MM. OTHER
CONVECTION IS TO CLUSTER ALONG THE NORTHEAST COAST OF BRASIL
BETWEEN ESPIRITO SANTO AND PERNAMBUCO. UNDER INFLUENCE OF BRISK
EASTERLY TRADES EXPECTING LIGHT COASTAL CONVECTION TO TRIGGER
RAINFALL AMOUNTS OF 05-10MM. LOCALLY HIGHER AMOUNTS ARE EXPECTED
ON WEDNESDAY WHEN THE MAXIMA INCREASES TO 10-15MM.

GONZALEZ...DMC (CHILE)
VANNUCCI...SMN (ARGENTINA)
DAVISON...WPC (USA)