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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1647Z Sep 10, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1247 PM EDT WED SEP 10 2014

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

MODEL DISCUSSION (VALID FROM SEPTEMBER 10 AT 00UTC): MODELS AGREE
ON PATTERN EVOLUTION THROUGH 84-96 HRS. THEY THEN DIVERGE ON
AMPLITUDE AND SPEED OF SHORT WAVE PERTURBATIONS MOVING ACROSS THE
DOMAIN. HIGH VARIABILITY IS ALSO EVIDENT AMONG THE ENSEMBLE
MEMBERS...LEADING TO LOW CONFIDENCE IN THE FORECAST DURING THE
MEDIUM RANGE PERIOD.

BROAD MID/UPPER LEVEL TROUGH EXTENDS OVER THE WESTERN ATLANTIC...
ENVELOPING AREA BETWEEN 65W-25W AND TO THE SOUTH OF 25S. THE
TROUGH THEN PULLS TO THE EAST-SOUTHEAST AS THE CYCLE
PROGRESSES...WITH AXIS TO GRADUALLY WEAKEN AS IT MIGRATES ACROSS
THE SOUTH ATLANTIC. A SIMILAR EVOLUTION IS EXPECTED AT LOW
LEVELS...WITH DEEP OCCLUDED LOW TO MEANDER EAST OF THE
MALVINAS/FALKLAND ISLANDS EARLY IN THE CYCLE. THROUGH 48-60 HRS
THIS MIGRATES ACROSS THE SOUTH GEORGIA ISLANDS. ASSOCIATED
FRONT...MEANWHILE...TRAILS TO THE NORTHWEST ACROSS THE SOUTHERN
STATES OF BRASIL-PARAGUAY TO BOLIVIA. THE FRONT LOSES UPPER LEVEL
SUPPORT EARLY IN THE CYCLE...AS A RESULT IT STALLS OVER MID
SECTIONS OF SOUTH AMERICA. THROUGH 60-72 HRS IT MEANDERS SOUTH TO
NORTHERN ARGENTINA. MEANWHILE...THIS IS TO SUSTAIN MOISTURE
CONVERGENCE ACROSS RIO GRANDE DO SUL TO CORRIENTES/MISIONES IN
ARGENTINA. IN THIS AREA EXPECTING RAINFALL AMOUNTS OF 20-30MM
THROUGH 36 HRS...AND 10-15MM/DAY AT 36-60 HRS. A SLIGHT INCREASE
IS POSSIBLE BY 60-72 HRS...WHEN MAXIMA IS TO INCREASE TO 15-25MM.

SIMULTANEOUSLY...A PROGRESSIVE MID LEVEL TROUGH ON THE SOUTHERN
STREAM PULLS ACROSS THE SOUTH ATLANTIC...ENTERING SOUTHERN CHILE/
DRAKE PASSAGE BY 48 HRS. AS IT ENTERS THE SOUTHERN CONE IT IS TO
TRIGGER HEIGHT FALLS OF 150-250GPM. AT LOW LEVELS IT WILL SUSTAIN
A PROGRESSIVE FRONT THAT OCCLUDES AS IT ENTERS THE ANTARCTIC
PENINSULA/DRAKE PASSAGE BY 48 HRS...WITH ASSOCIATED POLAR FRONT TO
QUICKLY STREAM ACROSS SOUTHERN CHILE AND PATAGONIA. THIS IS TO
TRIGGER SCATTERED SHOWERS ACROSS SOUTHERN CHILE-TIERRA DEL
FUEGO...WITH SNOWFALL LIKELY ACROSS THE SOUTHERN ANDES OF CHILE.
IN THIS AREA EXPECTING RAINFALL AMOUNTS OF 10-15MM IN MIXED
PRECIPITATION.

A SHORT WAVE TROUGH LIES TO THE NORTH...TO CENTER AT 500 HPA ON A
CLOSED LOW NEAR 37S 87W EARLY IN THE CYCLE. THROUGH 48 HRS THE LOW
LIFTS ACROSS 30S 85W...WITH BROAD TROUGH DOMINATING FLOW OFF THE
CENTRAL/NORTHERN COAST OF CHILE. THROUGH 84 HRS THIS MEANDERS EAST
ACROSS 30S 75W...FOCUSING SHORT WAVE ENERGY ACROSS
CENTRAL/NORTHERN CHILE. AT LOW LEVELS IT SUSTAINS A BROAD TROUGH
AND AN OCCLUDED FRONT. THE BOUNDARY IS TO FRONTOLIZE AS IT ENTERS
CENTRAL CHILE BY 78-84 HRS. MEANWHILE...THIS IS TO FAVOR COASTAL
CONVECTION BETWEEN LA SERENA-SANTIAGO DE CHILE BY 72-84 HRS...WITH
ACCUMULATION OF 05-10MM. IT IS TO THEN SUSTAIN SCATTERED SNOW
SHOWERS ON THE ANDES BETWEEN 34S-27S...WITH ACCUMULATION OF
05-10CM.

MODELS THEN DIVERGE ON HOW FAST/STRONG THE MID LEVEL TROUGH IS TO
REMAIN AS IT SPILLS ACROSS THE ANDES OF CHILE TO ARGENTINA. THE
EUROPEAN MODELS NOW FAVOR A DEEPER/SLOWER PERTURBATION TO SPILL
ACROSS THE ANDES...WHILE THE GFS FAVORS A SLIGHTLY WEAKER/FASTER
WAVE. THERE IS HIGH VARIABILITY AMONG THE ENSEMBLE MEMBER WITH
RESPECT TO THIS FEATURE...BUT THE CONSENSUS SEEMS TO FAVOR THE
SLIGHTLY FASTER GFS OVER THE SLOWER EUROPEAN MODELS. AS THE TROUGH
SPILLS ACROSS THE ANDES IT IS TO INTERACT WITH SURFACE FRONT OVER
NORTHERN ARGENTINA...TRIGGERING CYCLOGENESIS OVER ENTRE RIOS/RIO
DE LA PLATA BASIN BY 84-96 HRS. THIS IS TO THEN DEEPEN AS IT MOVES
OFF THE COAST OF URUGUAY/BUENOS AIRES. AS THE LOW DEEPENS...THE
SURFACE FRONT IS TO THEN ADVANCE ACROSS SOUTHERN BRASIL-PARAGUAY.
AS IT SURGES NORTH...EXPECTING STRONG/POTENTIALLY SEVERE
CONVECTION TO DEVELOP ACROSS URUGAY-ENTRE RIOS-CORRIENTES IN
ARGENTINA...WITH MAXIMA OF 40-80MM.

NORTH OVER THE CONTINENT...AT 200 HPA...THE SUBTROPICAL RIDGE IS
TO CONFINE TO THE EASTERN TROPICAL PACIFIC...TO INITIALLY CENTER
ON A CLOSED HIGH TO THE SOUTH-SOUTHWEST OF THE GALAPAGOS ISLANDS.
WEAK TROUGH ALOFT IS TO THEN EXTEND ACROSS MINAS GERAIS TO PARA IN
BRASIL...WHILE A WEAK RIDGE TRIES TO BUILD TO THE SOUTH ACROSS
MATO GROSSO-SOUTHERN STATES OF BRASIL. MOST ACTIVE CONVECTION IS
EXPECTED FROM SOUTHERN COLOMBIA/NORTHERN AMAZONAS IN BRASIL TO
NORTHERN PERU/EASTERN ECUADOR...WITH RAINFALL MAXIMA TO PEAK AT
15-25MM OVER THE NEXT FEW DAYS. OTHER ACTIVITY IS EXPECTED ACROSS
ILHA DE MARAJO/AMAPA IN BRASIL AS ENHANCED BY THE UPPER
TROUGH...BUT MAXIMA IN THIS AREA PEAKS AT 05-10MM IN ISOLATED
CONVECTION. ACROSS EASTERN BAHIA-PERNAMBUCO EXPECTING ACCUMULATION
OF 05-10MM IN TRADE WIND CONVERGENCE.

JALFIN...SMN (ARGENTINA)
CAROCA...DMC (CHILE)
DAVISON...WPC (USA)