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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1636Z Dec 10, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1136 AM EST WED DEC 10 2014

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

MODEL DISCUSSION (VALID FROM DECEMBER 10 AT 00UTC): MINOR
CORRECTIONS TO PREVIOUS FORECAST AS THE GLOBAL MODELS CONTINUE TO
AGREE ON PATTERN EVOLUTION DURING THE NEXT SEVEN DAYS. OUR
CONFIDENCE IN THE FORECAST IS HIGH.

AT 500 HPA...A PROGRESSIVE SHORT WAVE TROUGH ON THE SOUTHERN
STREAM IS TO MOVE ACROSS 40W AND TO THE SOUTH OF 30S EARLY IN THE
CYCLE...CROSSING 20W BY 48 HRS...AND NEAR 00W BY 72 HRS. AT LOW
LEVELS...THE ASSOCIATED FRONT MOVES NORTH TO SOUTHERN
BRASIL-NORTHERN ARGENTINA LATER THIS AFTERNOON...WHERE IT IS TO
REMAIN OVER THE NEXT FEW DAYS. UNDER INFLUENCE OF THIS FRONT...AND
A POLAR RIDGE TO THE SOUTH...MOISTURE IS TO CONVERGE ACROSS THE
NORTHWEST PROVINCES IN ARGENTINA...TO FAVOR ORGANIZED CONVECTION
WITH MAXIMA OF 20-45MM OVER THE NEXT TWO DAYS. THROUGH 96-108 HRS
THIS IS TO DECREASE TO 05-10MM. ACROSS SOUTHEAST BRASIL THIS IS TO
FAVOR SCATTERED CONVECTION WITH MAXIMA OF 15-25MM OVER THE NEXT
TWO DAYS.

AS THE SURFACE FRONT MEANDERS OVER SOUTHERN BRASIL/NORTHERN
ARGENTINA...A WARM CORE SURFACE LOW IS TO FORM OFF THE COAST OF
SAO PAULO BY 54-60 HRS...WHERE IT IS TO REMAIN THROUGH 96-108 HRS.
AS IT MEANDERS OFF THE SOUTHEAST COAST OF BRASIL THIS IS TO
ENHANCE MOISTURE CONVERGENCE BETWEEN SANTA CATARINA AND NORTHERN
SAO PAULO...FAVORING ORGANIZED CONVECTION ALONG THE SERRA DO MAR.
IN THIS AREA EXPECTING MODERATE TO HEAVY RAINS ON DAYS 02-04...TO
RESULT IN RAINFALL MAXIMA OF 30-60MM/DAY. STRONG TOPOGRAPHICAL
FORCING IS LIKELY TO RESULT IN LOCALIZED MAXIMA IN EXCESS OF 100MM.

FARTHER UPSTREAM OVER THE PACIFIC...A MID/UPPER LEVEL RIDGE IS TO
THEN BUILD SOUTH ALONG 80W/90W TO 70S BY 48-60 HRS. IT IS TO THEN
SETTLE ALONG 80W BY 72-96 HRS. AS THE RIDGE BUILDS...THIS IS TO
THEN SUSTAIN THE NORTHWARD AMPLIFICATION OF A TROUGH TO THE EAST.
THE TROUGH IS TO EXTEND BETWEEN 75W-30W WHILE ANCHORING ON A
CLOSED LOW TO THE WEST OF THE SOUTH GEORGIA ISLANDS. SHORT WAVE
VORTICES ARE TO REVOLVE AROUND THIS AXIS...TO STREAM ACROSS THE
DRAKE PASSAGE/TIERRA DEL FUEGO. AT LOW LEVELS A BROAD RIDGE IS TO
THEN ESTABLISH OVER THE EASTERN PACIFIC WHILE A BROAD TROUGH IS TO
EXTEND NORTH ACROSS THE WEDDELL SEA TO THE SOUTHWEST ATLANTIC.
POLAR FRONTS...AS THEY REVOLVE AROUND THIS TROUGH...ARE TO CONFINE
TO EXTREME SOUTHERN PORTIONS OF CHILE. IN THIS AREA EXPECTING
LIGHT RAINFALL OVER THE NEXT FEW DAYS WITH ACCUMULATION OF
00-05MM/DAY.

NORTH OVER THE CONTINENT...AT 200 HPA...THE UPPER FLOW PATTERN IS
FORECAST TO GRADUALLY EVOLVE THROUGH 96-120 HRS. EARLY IN THE
CYCLE THE SUBTROPICAL RIDGE IS TO CENTER OVER THE EASTERN TROPICAL
PACIFIC JUST SOUTH OF THE GALAPAGOS ISLANDS. THIS HIGH IS TO
EXTEND A RIDGE TO THE EAST-SOUTHEAST ACROSS SOUTHERN PERU/NORTHERN
CHILE-BOLIVIA-SOUTHERN STATES OF BRASIL TO THE WESTERN ATLANTIC.
AS THE CYCLE PROGRESSES THE CLOSED HIGH IS FORECAST TO RELOCATE TO
THE EAST-SOUTHEAST...NEARING THE SOUTH COAST OF PERU BY 24-36 HRS.
BY 60-72 HRS IT IS TO MOVE INLAND ACROSS LAKE TITICACA...AND
THROUGH 120 HRS IT IS TO SETTLE OVER SANTA CRUZ IN EASTERN
BOLIVIA/MATO GROSSO DO SUL IN BRASIL. EARLY IN THE CYCLE THIS
RIDGE IS TO FAVOR A SOUTH-SOUTHEASTERLY FLOW ACROSS AMAZONAS IN
BRASIL/JUNGLE OF PERU...FORCING CONVECTIVE CELLS TO SPREAD
NORTHWARD ALONG THE FOOT OF THE ANDES. BUT AS THE HIGH
RELOCATES...WINDS WILL GRADUALLY SHIFT TO THE EAST ACROSS PERU.
EARLY IN THE CYCLE THIS IS TO ENHANCE VERTICAL WIND SHEAR TO FAVOR
ORGANIZED CONVECTION ACROSS EASTERN PERU-ACRE IN BRASIL-NORTHERN
BOLIVIA...WITH RAINFALL AMOUNTS OF 20-35MM. THROUGH 48-72 HRS THIS
DECREASES TO 15-30MM/DAY. ALSO...AS THE RIDGE BUILDS
INLAND...ORGANIZED CONVECTION IS TO BUILD ACROSS PARA TO
TOCANTINS/GOIAS-BAHIA/MINAS GERAIS IN BRASIL...WITH MAXIMA IN THIS
AREA TO PEAK AT 20-40MM/DAY. THIS STARTS TO FLARE UP EARLY ON DAY
01 AND PERSIST THROUGH DAY 03.

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