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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1640Z Dec 17, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1140 AM EST WED DEC 17 2014

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

MODEL DISCUSSION (VALID FROM DECEMBER 17 AT 00UTC): GLOBAL MODELS
CONTINUE TO AGREE ON PATTERN EVOLUTION ACROSS THE SOUTH AMERICAN
DOMAIN...FOLLOWING SIMILAR EVOLUTION THROUGH 132-144 HRS.
CONFIDENCE IN THE FORECAST IS HIGH.

AT MID LEVELS...SHORT WAVE TROUGH ON THE NORTHERN STREAM MOVES
ACROSS ARGENTINA-URUGUAY EARLY IN THE CYCLE...AND OFF THE COAST
INTO THE WESTERN ATLANTIC LATER THIS EVENING. TROUGH IS TO THEN
MEANDER EAST ACROSS 45W BY 48 HRS...NEARING 30W BY 96 HRS. AS IT
PULLS ACROSS THE CONTINENT...A SUBTROPICAL JET MAXIMA IS TO
ACCOMPANY THIS TROUGH. IN A PERPENDICULAR FLOW TO THE NORTHERN
ANDES OF CHILE THIS IS TO FAVOR GENERATION OF LEESIDE
PERTURBATIONS ACROSS MID SECTIONS OF SOUTH AMERICA. AT LOW
LEVELS...UPPER LEVEL JET AND MID LEVEL TROUGH ARE TO INTERACT WITH
A SURFACE TROUGH OVER PARAGUAY/SOUTHERN BRASIL...TRIGGERING
MODERATE TO HEAVY CONVECTION EARLY IN THE CYCLE. THROUGH 36 HRS
THIS IS TO PEAK AT 20-45MM. BUT AS THE MID LEVEL TROUGH MOVES AWAY
AND JET STARTS TO WEAKEN THIS DECREASES TO 15-25MM BY 48-60 HRS.

AMPLIFYING MID/UPPER LEVEL RIDGE OVER THE EASTERN SOUTH PACIFIC IS
TO THEN FAVOR RAPID EVOLUTION TO A LONG WAVE/HIGH AMPLITUDE
PATTERN ACROSS THE SOUTH AMERICAN DOMAIN. RIDGE IS TO BUILD
UPSTREAM ALONG 100W/110W TO THE POLAR ICE CAP.
THIS...IN-TURN...DIVERTS SHORT WAVE ENERGY NORTHWARD ACROSS THE
BELLINGSHAUSEN SEA TO THE SOUTHERN CONE. INFLOW OF SHORT WAVE
ENERGY WILL THEN FEED A TROUGH TO THE EAST. THE LATTER IS TO ENTER
SOUTHERN CHILE BY 48 HRS...THEN SLOWLY MEANDERS EAST WHILE RAPIDLY
AMPLIFYING NORTH ACROSS CHILE AND ARGENTINA. THROUGH 72 HRS THE
TROUGH IS TO NEAR 35S...AND BY 96-108 HRS IT ENTERS THE NORTHERN
PROVINCES IN ARGENTINA-SOUTHEAST PARAGUAY. EARLY IN THE CYCLE THIS
IS TO SUSTAIN AN ELONGATED FRONT ACROSS THE SOUTH PACIFIC INTO
SOUTHERN PORTIONS OF CHILE AND ARGENTINA. THE DEEP TROUGH PATTERN
IS TO FAVOR ADVECTION OF SUBTROPICAL MOISTURE INTO SOUTHERN
PORTIONS OF CHILE. AS A RESULT...EXPECTING ORGANIZED CONVECTION
BETWEEN ISLA DE CHILOE AND PUNTA ARENAS IN SOUTHERN CHILE OVER THE
NEXT TWO DAYS...WITH DAILY MAXIMA IN THIS AREA OF 20-40MM. BUT AS
THE TROUGH DEEPENS AND COLD AIR ADVECTS ACROSS THE DRAKE PASSAGE
MODELS SHOWING RISK OF SNOW/RAIN SHOWERS AFFECTING TIERRA DEL
FUEGO AND THE SOUTHERN ANDES. FURTHERMORE...THE DEEPENING TROUGH
WILL DRIVE THE FRONT NORTH ACROSS PATAGONIA/CENTRAL PORTIONS OF
CHILE BY 72 HRS. A PAMPERO JET IS TO THEN FORCE THIS BOUNDARY
FARTHER NORTH INTO URUGUAY-NORTHERN ARGENTINA BY 96 HRS...AND
ACROSS SOUTHERN BRASIL-PARAGUAY BY 120 HRS. AS THE FRONT SURGES
ACROSS ARGENTINA IT IS TO TRIGGER A SQUALL LINE AND SEVERE
CONVECTION ACROSS URUGUAY-SOUTHERN BRASIL-NORTHERN PROVINCES IN
ARGENTINA AND PORTIONS OF PARAGUAY...WITH RAINFALL AMOUNTS OF
50-100MM. BY 120-132 HRS THE FOCUS OF THE HEAVY CONVECTION SHIFTS
TO EASTERN/NORTHERN BOLIVIA-RONDONIA/MATO GROSSO IN BRASIL...WITH
RAINFALL MAXIMA OF 40-80MM.

AT 200 HPA...A RIDGE TO THE NORTH OF 30S IS TO REMAIN DOMINANT
SYNOPTIC SCALE FEATURE DURING THE NEXT FEW DAYS...WITH AXIS TO
CENTER ON A CLOSED HIGH THAT MEANDERS BETWEEN BOLIVIA AND MATO
GROSSO IN BRASIL. A TROUGH LIES TO THE EAST OF THIS RIDGE...WITH
AXIS NORTH ALONG THE NORTHEAST COAST OF BRASIL TO THE TROPICAL
ATLANTIC. AN AREA OF DIVERGENCE ALOFT IS TO DEVELOP BETWEEN THIS
TROUGH AND THE RIDGE TO THE WEST. THIS IS TO VENT DEEP CONVECTION
ACROSS NORTHEAST AND CENTRAL STATES OF BRASIL...TO SUSTAIN
SCATTERED CONVECTION WITH DAILY MAXIMA OF 20-40MM. LOCALLY HIGHER
AMOUNTS ARE HIGHLY LIKELY. FURTHERMORE...THE RIDGE ALOFT IS TO
ALSO FAVOR ORGANIZED CONVECTION ACROSS CENTRAL-SOUTHERN JUNGLE OF
PERU-CENTRAL BOLIVIA OVER THE NEXT COUPLE OF DAYS...WITH MAXIMA OF
20-45MM. BUT AS POLAR TROUGH DEEPENS TO THE SOUTH THIS WILL
DECREASE TO 15-25MM.

ARREAGA...INAMHI (ECUADOR)
DURAN...SENAHMI (PERU)
DAVISON...WPC (USA)