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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1701Z Dec 18, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1201 PM EST THU DEC 18 2014

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

MODEL DISCUSSION (VALID FROM DECEMBER 17 AT 00UTC): UPSTREAM OVER
THE PACIFIC...MODELS CONTINUE TO FORECAST THE SOUTHWARD
AMPLIFICATION OF A LONG WAVE RIDGE BETWEEN 120W-90W TO 70S. THIS
REACHES MAXIMUM AMPLITUDE BY 48-60 HRS. THE UNUSUALLY
STRONG/HIGHLY AMPLIFIED RIDGE PATTERN IS EXPECTED TO PERSIST
THROUGH 96-108 HRS. AT LOW LEVELS IT IS TO SUSTAIN A BROAD RIDGE
OVER THE EASTERN SOUTH PACIFIC...FORECAST TO ANCHOR ON A 1031-1033
HPA HIGH.

AS THE MID/UPPER LEVEL RIDGE BUILDS...THIS IS TO THEN SUSTAIN THE
NORTHWARD AMPLIFICATION OF A TROUGH ACROSS THE DRAKE PASSAGE TO
THE SOUTHERN CONE OF SOUTH AMERICA EARLY IN THE CYCLE. THIS IS AN
INTENSE PERTURBATION THAT IS TO SUSTAIN HEIGHT FALLS IN EXCESS OF
300GPM ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA THROUGH 36
HRS. BUILDING RIDGE TO THE WEST IS TO THEN INDUCE THE NORTHWARD
AMPLIFICATION OF THE TROUGH ACROSS ARGENTINA TO 30S BY 48 HRS. BY
84-96 HRS THE TROUGH IS TO EXTEND INTO SOUTHERN
BOLIVIA/PARAGUAY... UNUSUALLY DEEP FOR THIS TIME OF THE YEAR. AS
IT ENTERS THE CONTINENT...THE DEEPENING TROUGH IS TO FAVOR
ADVECTION OF SUBTROPICAL MOISTURE INTO SOUTHERN PORTIONS OF CHILE.
THIS IS TO SUSTAIN ORGANIZED CONVECTION WITH ACCUMULATION OF
20-40MM TO FOCUS BETWEEN TEMUCO-ISLA DE CHILOE IN SOUTHERN CHILE
THROUGH 36 HRS. FURTHERMORE...THE DEEPENING TROUGH WILL SUSTAIN A
COLD ADVECTIVE PATTERN ACROSS THE DRAKE PASSAGE TO THE SOUTHERN
ANDES OF CHILE. THIS...IN-TURN...IS LIKELY TO TRIGGER SNOW SHOWERS
ACROSS THE SOUTHERN ANDES THROUGH 30-36 HRS. AS THE TROUGH DEEPENS
ACROSS ARGENTINA IT IS TO THEN DRIVE A SURFACE FRONT NORTH ACROSS
THE CONTINENT. BY 42-48 HRS THE FRONT MOVES NORTH ACROSS
PATAGONIA/ CENTRAL PORTIONS OF CHILE...FAVORING SCATTERED
CONVECTION WITH ACCUMULATION OF 15-20MM. BUILDING SURFACE RIDGE IS
TO THEN SUSTAIN A PAMPERO JET...WITH SURFACE FRONT TO TRIGGER A
PREFRONTAL SQUALL LINE AS IT ACCELERATES ACROSS URUGUAY-CORDOBA IN
ARGENTINA THROUGH 66-72 HRS. CONDITIONS THEN APPEAR FAVORABLE FOR
SEVERE CONVECTION AND HEAVY RAINFALL AMOUNTS ACROSS SOUTHERN
BRASIL-URUGUAY-NORTHERN PROVINCES IN ARGENTINA AND PORTIONS OF
PARAGUAY. IN THIS AREA EXPECTING RAINFALL MAXIMA OF
75-150MM...WITH SOME OF THE MODELS SHOWING LOCALLY HIGHER AMOUNTS
TO PEAK AT 200-250MM.THROUGH 96-108 HRS THE FRONT MOVES NORTH
ACROSS PARANA IN BRASIL-PARAGUAY. AS IT ENTERS SOUTHEAST BRASIL
LATER IN THE CYCLE THE FRONT IS TO SUSTAIN ORGANIZED CONVECTION
WITH MAXIMA OF 30-60MM...WHILE OVER BOLIVIA-ACRE/RONDONIA IN
BRASIL AND SOUTHERN/CENTRAL JUNGLE OF PERU MAXIMA IS TO PEAK AT
75-125MM IN PREFRONTAL CONVECTION.

AT 200 HPA...A RIDGE OVER THE CONTINENT IS TO DOMINATE AREA TO THE
NORTH OF 30S DURING THE NEXT THREE DAYS...WITH AXIS FORECAST TO
CENTER ON A CLOSED HIGH THAT MEANDERS BETWEEN BOLIVIA AND MATO
GROSSO IN BRASIL. AS DEEP POLAR TROUGH ENTERS THE CONTINENT...AND
COLD/DRY AIR ADVECTS ACROSS MID SECTIONS OF SOUTH AMERICA...RIDGE
TO THE NORTH IS TO WEAKEN AND THE CLOSED HIGH REPOSITIONS TO
SOUTHEAST BRASIL LATER ON DAY 04. ALSO AT UPPER LEVELS...A TROUGH
LIES TO THE EAST OF THIS RIDGE...WITH AXIS NORTH ALONG THE
NORTHEAST COAST OF BRASIL TO THE TROPICAL ATLANTIC. AS RIDGE TO
THE SOUTH WEAKENS...THE LOW/TROUGH IS TO RETROGRESS TO NORTHEAST
BRASIL. A BROAD 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 OVER MOUNTAINOUS TERRAIN.
FURTHERMORE...THE RIDGE ALOFT IS TO ALSO FAVOR ORGANIZED
CONVECTION ACROSS CENTRAL-SOUTHERN JUNGLE OF PERU-CENTRAL BOLIVIA
EARLY IN THE CYCLE...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)