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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1541Z Apr 29, 2015)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1141 AM EDT WED APR 29 2015

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

MODEL DISCUSSION (VALID FROM APRIL 28 AT 0000 UTC): CYCLE-TO-CYCLE
CONTINUITY DURING MEDIUM RANGE PERIOD IS LOW...WITH MODELS MAKING
PATTERN CORRECTIONS ACROSS THE SOUTHERN CONE. HIGH VARIABILITY
AMONG DETERMINISTIC MEMBERS IS ALSO EVIDENT AMONG THE GLOBAL
ENSEMBLES. AS A RESULT...CONFIDENCE IN THE FORECAST IS LOW BEYOND
84/96 HRS.

BROAD MID/UPPER LEVEL TROUGH OVER THE EASTERN PACIFIC CONTINUES TO
PRESS AGAINST A HIGH AMPLITUDE RIDGE OVER THE WESTERN ATLANTIC.
THE TROUGH IS TO SLOWLY MOVE INLAND ACROSS SOUTHERN CHILE TO
PATAGONIA IN ARGENTINA THROUGH 60-72 HRS...WITH RIDGE TO SLOWLY
YIELD TO THIS TROUGH. INFLOW OF SHORT WAVE ENERGY IS TO THEN
RESULT IN HEIGHT FALLS OF 75-150 GPM ACROSS THE CENTRAL PROVINCES
IN ARGENTINA BY 60-84 HRS. UNDER FAVORABLE JET DYNAMICS THIS IS
LIKELY TO FAVOR SEVERE CONVECTION ACROSS CENTRAL ARGENTINA/RIO DE
LA PLATA BASIN. MODELS THEN DIVERGE ON HOW THIS TROUGH IS TO
EVOLVE...WITH EUROPEAN MODELS SHOWING A BROAD TROUGH DEVELOPING
ACROSS THE SOUTHERN CONE/WESTERN ATLANTIC LATER IN THE CYCLE. THE
GFS...IN CONTRAST...FAVORS A WEAKER PERTURBATION TO MEANDER OFF
THE COAST OF ARGENTINA THROUGH 120-132 HRS. THIS IS ADDING TO THE
UNCERTAINTY AND LACK OF CONFIDENCE IN THE MEDIUM RANGE FORECAST.
AT LOW LEVELS...THE UPPER RIDGE OVER THE WESTERN ATLANTIC SUSTAINS
A BROAD AREA OF HIGH PRESSURE. THIS RIDGE IS TO ANCHOR ON A CLOSED
HIGH TO THE NORTH-NORTHWEST OF THE SOUTH GEORGIA ISLANDS. THIS
RIDGE IS TO FAVOR A WARM/MOIST NORTHERLY FLOW ACROSS PARAGUAY INTO
ARGENTINA. BROAD POLAR TROUGH TO THE WEST... MEANWHILE...IS TO
EXTEND ACROSS THE BELLINGSHAUSEN SEA TO THE EASTERN PACIFIC EARLY.
THIS TROUGH IS TO SLOWLY PUSH EAST ACROSS SOUTHERN CHILE TO
PATAGONIA IN ARGENTINA BY 60-72 HRS. POLAR FRONTS ARE TO REVOLVE
AROUND THIS AXIS. UNDER INFLUENCE OF THE LOW/MID LEVEL RIDGE TO
THE EAST...THE FRONTS ARE TO INITIALLY CONFINE TO EXTREME SOUTHERN
PORTIONS OF CHILE/PATAGONIA IN ARGENTINA. THROUGH 60-72 HRS THE
RIDGE WEAKENS ENOUGH TO ALLOW FRONTAL BOUNDARY TO SURGE ACROSS
PATAGONIA TO LA PAMPA IN ARGENTINA. DEEPENING LOW OFF THE COAST OF
BUENOS AIRES AND BUILDING POLAR RIDGE OVER PATAGONIA IS TO THEN
DRIVE THIS BOUNDARY NORTH ACROSS URUGUAY TO SOUTHERN BRASIL/
NORTHERN PROVINCES IN ARGENTINA...WHERE IT IS TO THEN STALL
THROUGH 120-132 HRS. AS THE FRONT/TROUGH ENTERS SOUTHERN CHILE IT
IS TO TRIGGER SCATTERED CONVECTION WITH ACCUMULATION OF 10-15MM.
HIGHER AMOUNTS ARE EXPECTED OVER CENTRAL ARGENTINA/RIO DE LA PLATA
BASIN... WHERE THE SURGING POLAR BOUNDARY IS TO TRIGGER MODERATE
TO HEAVY CONVECTION WITH ACCUMULATION OF 30-60MM IN SEVERE
CONVECTION BY 60-84 HRS. AS THE FRONT MOVES NORTH ACROSS ARGENTINA
FOCUS OF THE CONVECTION SHIFTS TO SOUTHERN BRASIL-PARAGUAY/MATO
GROSSO DO SUL...WITH MAXIMA IN THIS AREA TO PEAK AT 35-70MM IN
POSSIBLE MCS.

ANOTHER MID/UPPER LEVEL TROUGH LIES TO THE EAST OF THE WESTERN
ATLANTIC RIDGE. THE TROUGH IS TO EXTEND ALONG 40S 40W TO SOUTHEAST
BRASIL THROUGH 72-96 HRS. AT LOW LEVELS IT IS TO PROVIDE SUPPORT
TO A BROAD/OCCLUDING LOW OVER THE CENTRAL ATLANTIC...WITH
ASSOCIATED FRONT TRAILING TO THE WEST/NORTHWEST INTO THE
SOUTHEASTERN STATES OF BRASIL. THE FRONT...AND BROAD RIDGE TO THE
SOUTH...WILL ENHANCE MOISTURE CONVERGENCE ALONG THE SERRA DO
MAR...TO FAVOR RAINFALL MAXIMA OF 15-25MM BY 36 HRS. PREFRONTAL
CONVECTION ACROSS ESPIRITO SANTO-EASTERN BAHIA WILL SUSTAIN HIGHER
AMOUNTS...WITH RAINFALL MAXIMA TO PEAK AT 20-35MM BY 36 HRS...AND
20-45MM AT 36-72 HRS.

NORTH OVER THE CONTINENT...AT 200 HPA...MODELS FORECAST AN OPEN
RIDGE TO EXTEND OVER THE CONTINENT TO THE NORTH OF 20S OVER THE
NEXT COUPLE OF DAYS. LATER IN THE CYCLE RIDGE IS TO CONFINE TO
NORTHWEST SOUTH AMERICA. AS THE RIDGE WEAKENS...A SHORT WAVE
TROUGH IS TO LIFT ACROSS THE NORTHEAST STATES OF BRASIL BY 84-108
HRS. MEANWHILE... MOST ACTIVE CONVECTION OVER THE CONTINENT IS
EXPECTED TO THE NORTH OF 10S...TO CLUSTER ACROSS PARA-AMAZONAS IN
BRASIL TO SOUTHERN COLOMBIA/NORTHERN JUNGLE OF PERU...WHERE DAILY
MAXIMA IS TO PEAK AT 20-35MM OVER THE NEXT 36-48 HRS...AND 15-20MM
LATER IN THE CYCLE. HIGHER AMOUNTS...HOWEVER...ARE LIKELY ACROSS
NORTHERN PERU-SOUTHERN COLOMBIA...WHERE MAXIMA IS TO PEAK AT
20-45MM THROUGH 36 HRS. ACROSS WESTERN ECUADOR...EXPECTING
RAINFALL MAXIMA OF 15-20MM/DAY...THIS INCREASES TO 20-30MM/DAY BY
72-132 HRS.

SCHNEIDER...INMET (BRASIL)
CALLE...UNALM (PERU)
CHARPENTIER...DMC (CHILE)
DAVISON/GALVEZ...WPC (USA)