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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1605Z Dec 11, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1105 AM EST THU DEC 11 2014

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

MODEL DISCUSSION (VALID FROM DECEMBER 11 AT 00UTC): MODELS
CONTINUE AGREEING ON FORECAST EVOLUTION THROUGH THE NEXT FIVE-SIX
DAYS. OUR CONFIDENCE IN THE FORECAST IS HIGH.

AT 500 HPA...A PROGRESSIVE SHORT WAVE TROUGH ON THE SOUTHERN
STREAM IS RAPIDLY EXITING INTO THE SOUTH ATLANTIC CROSSING 20W
THURSDAY EVENING...AND NEAR 00W ON FRIDAY EVENING. AT LOW
LEVELS...THE ASSOCIATED FRONT IS STALLED ACROSS SOUTHERN
BRASIL-NORTHERN ARGENTINA WHERE IT IS TO REMAIN OVER THE NEXT FEW
DAYS. MOISTURE IS TO CONVERGE TO THE NORTH AND WEST OF THIS
FRONT...PRIMARILY ALONG THE ANDEAN FOOTHILLS. EXPECTING MAXIMA OF
20-40MM/DAY ACROSS NORTHWESTERN ARGENTINA THROUGH EARLY
FRIDA...AND MAXIMA OF 15-25MM/DAY ACROSS CUYO. BY FRIDAY INTO
EARLY SATURDAY EXPECTING MAXIMA OF 20-30MM/DAY IN SCATTERED
DIURNAL THUNDERSTORMS FROM MENDOZA NORTH INTO TUCUMAN.

ACROSS SOUTHEASTERN BRASIL/EASTERN PARAGUAY...ACTIVE DIURNAL
CONVECTION IS EXPECTED EARLY IN THE CYCLE UNDER FAVORABLE
VENTILATION IN THE SOUTHERN PERIPHERY OF UPPER RIDGE...AND IN A
REGION OF ENHANCED LOW-LEVEL CONVERGENCE AND INSTABILITY AHEAD OF
THE FRONT. MODELS ARE SHOWING SURFACE TROUGHS DEEPENING ACROSS
SOUTHEASTERN BRASIL...WHICH WILL SUSTAIN ACTIVE PATTERN OF
CONVECTION. INITIALLY...MAXIMA WILL REACH 25-50MM/DAY THROUGH
EARLY FRIDAY INCREASING TO 30-60MM/DAY BY FRIDAY INTO EARLY SUNDAY
WITH CONTINUED RISK OF MCS FORMATION. STRONG TOPOGRAPHICAL IN
ISOLATED LOCATIONS ALONG THE SERRA DO MAR COULD RESULT IN
LOCALIZED MAXIMA IN EXCESS OF 100MM.

FARTHER UPSTREAM OVER THE SOUTHERN PACIFIC...A MID/UPPER LEVEL
RIDGE IS TO BUILD SOUTH ALONG 80W/90W TO 70S BY FRIDAY EVENING. IT
IS TO THEN SETTLE ALONG 80W BY SATURDAY-SUNDAY HRS. AS THE RIDGE
BUILDS...THIS WILL 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 EARLY NEXT WEEK. EARLY IN THE
CYCLE THE SUBTROPICAL RIDGE IS TO CENTER OFF THE COAST OF CENTRAL
PERU...EXTENDING 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 MEANDER TO THE EAST-SOUTHEAST TO SETTLE OVER SANTA
CRUZ IN EASTERN BOLIVIA/MATO GROSSO DO SUL IN BRASIL BY EARLY NEXT
WEEK. 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)