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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1556Z Aug 27, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1156 AM EDT WED AUG 27 2014

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

MODEL DISCUSSION (VALID FROM AUGUST 27 AT 00UTC): THE GFS AND
EUROPEAN MODELS CONTINUE TO AGREE ON SHORT WAVE PATTERN
EVOLUTION... BUT THEN DIVERGE DURING THE MEDIUM RANGE PERIOD.
THESE MODELS CONTINUES TO DISAGREE ON THE TIMING AND AMPLITUDE OF
SHORT WAVE PERTURBATIONS ENTERING THE DOMAIN. THIS IS ALSO
RESULTING IN CYCLE-TO-CYCLE INCONSISTENCIES AS MODELS MAKE PATTERN
CORRECTIONS. AS A RESULT...CONFIDENCE IN THE FORECAST BEYOND 72/84
HRS REMAINS LOW.

AT MID LEVELS...A NORTH-TO-SOUTH RIDGE EXTENDS ACROSS ARGENTINA TO
TIERRA DEL FUEGO. EAST OF THIS AXIS...A BROAD TROUGH DOMINATES THE
FLOW ACROSS THE WESTERN ATLANTIC...AS IT EXTENDS BETWEEN 60W-20W
AND TO THE SOUTH OF 20S. THROUGH 24 HRS MEAN TROUGH AXIS IS TO
PULL ACROSS 40W. IT IS TO THEN START SHEARING TO THE
SOUTHEAST...WITH AXIS TO EVOLVE INTO A NARROW/ELONGATED TROUGH
ACROSS THE CENTRAL SOUTH ATLANTIC. AT LOW LEVELS THIS IS TO
SUSTAIN AN ELONGATED FRONT ACROSS THE ATLANTIC TO RIO DE
JANEIRO/ESPIRITO SANTO IN BRASIL... WITH TRAILING END WEST ACROSS
MATO GROSSO TO BOLIVIA. THROUGH 48 HRS THE TRAILING END OF THE
FRONT WILL START TO RETROGRESS TOWARDS PARAGUAY...AND BY 84-96 HRS
IT WILL MOVE ACROSS SOUTHERN BRASIL TO NORTHERN PROVINCES IN
ARGENTINA. AS IT MEANDERS OVER SOUTHERN BRASIL-NORTHERN
ARGENTINA...A SHORT WAVE TROUGH ON THE NORTHERN STREAM IS TO
INTERACT WITH THIS FRONT...INDUCING A FRONTAL WAVE ALONG THIS
BOUNDARY. AS THE FRONT MEANDERS OVER RIO DE JANEIRO/ESPIRITO SANTO
IN BRASIL IT IS TO TRIGGER SCATTERED CONVECTION WITH ACCUMULATION
OF 05-10MM. THIS IS TO PERSIST OVER THE NEXT 72-84 HRS.

ANOTHER TROUGH LIES OVER THE EASTERN PACIFIC...WITH AXIS AT 500
HPA ENVELOPING AREA BETWEEN 100W-70W AND TO THE SOUTH OF 17S.
SHORT WAVE PERTURBATIONS REVOLVE AROUND THIS AXIS...AND UNDER
INFLUENCE OF THE RIDGE OVER ARGENTINA THESE ARE TO SHEAR SOUTH
ACROSS THE SOUTHERN CONE/DRAKE PASSAGE. AS SUCCESSIVE SHORT WAVE
PERTURBATIONS STREAM ACROSS THE EASTERN PACIFIC AND CLASH WITH THE
RIDGE TO THE EAST...THIS WILL THEN SUSTAIN GRADUAL EROSION OF MID
LEVEL RIDGE PATTERN BY 96-120 HRS. AS THE RIDGE WEAKENS FOCUS OF
THE SHORT WAVE ENERGY IS TO GRADUALLY SHIFT NORTH ACROSS CENTRAL
CHILE. PATTERN IS TO THEN COLLAPSE WITH SHORT WAVE PERTURBATIONS
TO THEN POPULATE THE SOUTH AMERICAN DOMAIN. AT LOW LEVELS...A
RIDGE OVER ARGENTINA RELOCATES TO THE WESTERN ATLANTIC THROUGH
48-60 HRS...WHERE IT IS TO THEN HOLD THROUGH 96-120 HRS. AS THE
RIDGE PULLS AWAY...BROAD POLAR TROUGH IS TO EXPAND ACROSS THE
EASTERN PACIFIC...WITH AXIS EAST INTO SOUTHERN/CENTRAL CHILE BY 48
HRS. IT THEN MEANDERS INLAND ACROSS CHILE TO ARGENTINA LATER IN
THE CYCLE. THE TROUGH IS TO SUSTAIN A SURFACE FRONT ACROSS
SOUTHERN/CENTRAL CHILE BY 60-72 HRS...AND ACROSS PATAGONIA-LA
PAMPA-CUYO IN ARGENTINA BY 84-96 HRS.  ACROSS CENTRAL-SOUTHERN
CHILE THIS IS TO ENHANCE MOISTURE CONVERGENCE... WITH MAXIMA OVER
CENTRAL CHILE TO PEAK AT 15-25MM BY 60-84 HRS... WHILE OVER
SOUTHERN CHILE MAXIMA IS TO PEAK AT 10-15MM LATER IN THE CYCLE.
FURTHERMORE...AS THE MID/UPPER LEVEL TROUGH MEANDERS OVER THE
EASTERN PACIFIC...THIS IS TO FOCUS A SUBTROPICAL JET MAXIMA ACROSS
NORTHERN CHILE AND ARGENTINA. DIVERGENCE ON ITS LEFT ENTRANCE
REGION IS TO VENT SCATTERED CONVECTION ACROSS THE SOUTHERN SIERRA
OF PERU...TO FAVOR RAIN/SNOW SHOWERS ON DAYS 03-04.

AT 200 HPA...A BROAD CELL OF THE SUBTROPICAL RIDGE DOMINATES
NORTHERN SOUTH AMERICA...WHILE A CELL OF THE SUBEQUATORIAL RIDGE
DOMINATES THE TROPICAL ATLANTIC. THROUGH 48-72 HRS THE SUBTROPICAL
RIDGE IS TO WEAKEN/NEARLY COLLAPSE WHILE THE SUBEQUATORIAL RIDGE
RELOCATES TO EQUATORIAL SOUTH AMERICA. EVOLVING UPPER RIDGE
PATTERN IS TO RESULT IN DECREASE IN VENTILATION ALOFT AND
CONVECTION OVER NORTHWEST SOUTH AMERICA. EARLY IN THE CYCLE THIS
IS TO RESULT IN MAXIMA OF 15-340MM BETWEEN SOUTHERN
COLOMBIA-EASTERN ECUADOR AND NORTHERN PERU. THROUGH 48 HRS THE
DAILY MAXIMA DECREASES TO 15-20MM...AND BY 72-96 HRS IT PEAKS AT
10-15MM.

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