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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1655Z Feb 23, 2015)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1155 AM EST MON FEB 23 2015

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

MODEL DISCUSSION (VALID FROM FEBRUARY 23 AT 00UTC): SYSTEM OF
CONCERN IS A SHORT WAVE TROUGH ON THE NORTHERN STREAM FLOW
INITIALIZED OVER THE PACIFIC OCEAN TO THE WEST OF CENTRAL CHILE.
THROUGH 30-36 HRS THE TROUGH IS TO MOVE TOWARDS THE COAST...WHERE
IT IS TO THEN REMAIN THROUGH 120-132 HRS. AS IT MEANDERS TO THE
EAST...THIS IS TO SHEAR SHORT WAVE VORTICES ACROSS THE CENTRAL
ANDES INTO CENTRAL ARGENTINA...WITH STRONG VORTEX TO SPILL ACROSS
THE ANDES INTO CUYO BY 48-54 HRS. THIS IS TO THEN TRACK TO THE
EAST-NORTHEAST ACROSS URUGUAY BY 60-72 HRS. ALTHOUGH SECONDARY
VORTICES ARE TO FOLLOW...THEY ARE TO LACK THE INTENSITY OF THE
FORMER. AT 250 HPA...A SUBTROPICAL JET MAXIMA IS TO MOVE ACROSS
CENTRAL CHILE TO ARGENTINA BY 36-48 HRS...TO FAVOR A DIVERGENT
PATTERN ALOFT THROUGH 72-84 HRS. AT LOW LEVELS...A NORTHERLY JET
ACROSS PARAGUAY-NORTHERN ARGENTINA IS TO SUSTAIN A MOIST ADVECTIVE
PATTERN. MOISTURE IS TO CONVERGE ACROSS CENTRAL ARGENTINA AND
ALONG A MEANDERING FRONT TO THE SOUTH. OVER THE NEXT FEW
DAYS...THE PRECIPITABLE WATER CONTENT IS FORECAST TO PEAK AT 50MM.
ACROSS CENTRAL ARGENTINA THE RISK OF SEVERE CONVECTION IS HIGH ON
DAYS 02-03...WITH HEAVY CONVECTION ON DAY 02 FORECAST TO PEAK AT
35-70MM...WHILE ON DAY 03 THIS INCREASES TO 75-150MM. NOTE THAT
DURING THAT PERIOD SOME OF THE NCEP ENSEMBLE MEMBERS SHOW MAXIMA
IN EXCESS OF 200MM.

ON THE SOUTHERN STREAM...A TIGHT BELT OF MID LEVEL WESTERLIES IS
TO THEN DOMINATE THE FAR EASTERN PACIFIC-SOUTHERN CONE/ANTARCTIC
PENINSULA-SOUTH ATLANTIC OVER THE NEXT THREE DAYS. THROUGH 72-96
HRS...HOWEVER...THE PATTERN IS FORECAST TO EVOLVE AS A LONG WAVE
TROUGH MOVES ACROSS 100W/110W TO 30S. IN RESPONSE TO THIS
TROUGH...A HIGH AMPLITUDE LONG WAVE RIDGE IS TO THEN ESTABLISH
ALONG 80W TO THE BELLINGSHAUSEN SEA. THE GLOBAL MODELS ARE IN
CLOSED AGREEMENT WITH THIS EVOLUTION...WITH MEAN OF THE GLOBAL
ENSEMBLES SUPPORTING THE DETERMINISTIC MODELS.

AS THE RIDGE BUILDS TO THE WEST...A HIGH AMPLITUDE/LONG WAVE
TROUGH IS TO GRADUALLY AMPLIFY ACROSS THE WESTERN SOUTH ATLANTIC.
TROUGH IS TO EXTEND BETWEEN 60W-20W AND TO THE SOUTH OF 25S BY
96-132 HRS. THE HIGH AMPLITUDE LONG WAVE TROUGH PATTERN IS TO THEN
PERSIST THROUGH DAY 07. UNDER INFLUENCE OF THE RIDGE TO THE
WEST...SHORT WAVE PERTURBATIONS ARE TO LIFT ACROSS THE
MALVINAS/FALKLAND ISLANDS AND THEN REVOLVE AROUND THE LONG WAVE
TROUGH. TIMING AND INTENSITY OF THESE PERTURBATIONS IS AN AREA OF
DISAGREEMENT AMONG THE DETERMINISTIC MODELS...WITH THE GLOBAL
ENSEMBLE MEMBERS IN HIGH  DIVERGENCE AS IT PERTAINS TO THIS
FEATURE. ALTHOUGH SOME OF THE EUROPEAN MEMBERS SHOW RISK OF
INTENSE CYCLOGENESIS OFF THE SOUTHERN COAST OF BRASIL LATER IN THE
CYCLE...THE VARIABILITY AMONG THE MEMBERS IS TOO HIGH TO ESTABLISH
CONFIDENCE IN THE FORECAST.

NORTH OVER THE CONTINENT...AT 200 HPA...THE SUBTROPICAL RIDGE
DOMINATES AREA TO THE NORTH OF 30S AND WEST OF 50W...WHILE
ANCHORING ON A HIGH OVER NORTHERN CHILE/SOUTHERN PERU. THIS RIDGE
FAVORS AN EASTERLY FLOW ACROSS THE CENTRAL/SOUTHERN ANDES OF CHILE
AND A SOUTHEASTERLY FLOW ACROSS THE JUNGLE TO EASTERN ECUADOR.
THIS FLOW IS TO STEER AN AREA OF ORGANIZED CONVECTION NORTHWARD
FROM NORTHERN BOLIVIA/SOUTHERN PERU EARLY IN THE CYCLE TO NORTHERN
PERU-ECUADOR/ SOUTHERN COLOMBIA BY 48-60 HRS. IN POSSIBLE
MCS...THIS IS TO FAVOR RAINFALL MAXIMA OF 30-60MM/DAY...AND
GRADUALLY DECREASES TO 20-40MM LATER IN THE CYCLE. EVOLVING UPPER
RIDGE PATTERN IS TO ALSO FAVOR HEAVY CONVECTION ACROSS
WESTERN/ANDEAN REGION OF ECUADOR WITH MAXIMA OF 30-60MM...WITH
ACTIVITY FORECAST TO SHIFT TO THE JUNGLE BY 48-60 HRS. ALSO AT
UPPER LEVELS...A TUTT LOW CENTERS OVER TOCANTINS IN
BRASIL...ANCHORING A SHORT WAVE TROUGH ACROSS THE NORTHEAST
STATES. THROUGH 60-72 HRS THE LOW FILLS TO AN OPEN TROUGH...AND
THIS IS TO WEAKEN THROUGH 96-120 HRS. EARLY IN THE CYCLE THIS IS
TO ENHANCE CONVECTIVE DEVELOPMENT ALONG THE NEAR EQUATORIAL TROUGH
AND THE NORTH COAST OF BRASIL...WITH MAXIMA OF 30-60MM. BUT AS THE
LOW/TROUGH WEAKENS THIS DECREASES TO 20-35MM BY 48-72 HRS.
SCATTERED CONVECTION IS TO THEN CONTINUE BUILDING ALONG THE NORTH
COAST AS PART OF THE DIURNAL HEAT CYCLE.

ARREAGA...INAMHI (ECUADOR)
SCHNEIDER...INMET (BRASIL)
DAVISON...WPC (USA)