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South American Forecast Discussion
 
(Latest Discussion - Issued 1741Z May 22, 2019)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
141 PM EDT WED MAY 22 2019

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

MODEL DISCUSSION FROM MAY 22/00UTC: GLOBAL MODELS AND ENSEMBLE
MEMBERS ARE IN GENERAL GOOD AGREEMENT THROUGH 72-84 HRS. FORECAST
CONFIDENCE DECREASES GRADUALLY AFTER...AND RAPIDLY AFTER 96 HRS.
THE LARGEST UNCERTAINTY APPEARS IN THE EASTERN PACIFIC AS EARLY AS
72-84 HRS...REGARDING THE EVOLUTION TO THE WEST OF A LONG WAVE
RIDGE THAT BUILDS NEAR 100W BY 96 HRS. ALSO...THE EUROPEAN MODELS
ARE CONSISTENTLY SLOWER THAN THE GFS ON EASTWARD THE PROGRESSION
OF THE MID-LEVEL TROUGH PROPAGATING ACROSS ARGENTINA ON WEDNESDAY.
THUS...WE FAVOR A SLOWER SOLUTION...WHICH EXTENDS THE PERIOD OF
LOCALLY HEAVY RAINS IN RIO GRANDE DO SUL/SANTA CATARINA THROUGH
LATE FRIDAY/EARLY SATURDAY.

A ROBUST UPPER TROUGH IS SLOWLY PROPAGATING EASTWARD ACROSS
CENTRAL ARGENTINA. THIS IS SUSTAINING CYCLOGENESIS IN NORTHEASTERN
ARGENTINA AND PUSHING A SURFACE COLD FRONT NORTHWARD. MODELS
CONTINUE SUGGESTING A CHANCE FOR ISOLATED SEVERE CONVECTION IN
PARANA AND SANTA CATARINA ON WEDNESDAY...IN THE WARM SECTOR OF THE
SYSTEM...WHERE EXPECTING MAXIMA OF 25-50MM/DAY. AS OCCLUDED LOW
FORMS...EXPECTING MAXIMA OF 15-30MM/DAY IN ENTRE RIOS/SOUTHERN
CORDOBA AND MOST OF NORTHERN URUGUAY. ACROSS SOUTHEAST/EASTERN
PARAGUAY AND PARANA BRASIL EXPECTING MAXIMA OF 15-20MM/DAY. ALSO
ON WEDNESDAY...FRONTAL CONVERGENCE AND INTERACTION WITH THE ANDES
WILL SUSTAIN ISOLATED MAXIMA OF 25-50MM/DAY IN SOUTHERN PERU AND
EXTREME NORTHWEST BOLIVIA. BY THURSDAY...LARGEST ACCUMULATIONS
WILL CLUSTER IN ASSOCIATION WITH CYCLOGENESIS IN RIO GRANDE DO
SUL. ACCUMULATIONS WILL BE REINFORCED BY ENHANCED MOISTURE
CONVERGENCE AS SOUTHERLY FLOW INTERACTS WITH THE SERRA
GAUCHA...AND BY UPPER JET DYNAMICS AS THE REGION WILL LIE IN THE
DIVERGENT EXIT OF AN UPPER JET. MOIST FLOW FROM THE ATLANTIC WILL
PROVIDE SUFFICIENT MOISTURE TO SUSTAIN MAXIMA OF 40-80MM/DAY IN
SOUTHERN SANTA CATARINA/CENTRAL AND NORTHERN RIO GRANDE DO
SUL...AND PARTS OF CORRIENTES AND NORTHERN URUGUAY. ALSO ON
THURSDAY...FRONTAL CONVECTION WILL BE STIMULATED BY JET DYNAMICS
IN SAO PAULO AND NORTHEAST PARANA TO SUSTAIN MAXIMA OF 25-50MM. A
SHEAR LINE WILL DEVELOP ACROSS THE AMAZON BASIN ON THURSDAY TO
ENHANCE ACCUMULATIONS ESPECIALLY ON FRIDAY. BY THURSDAY EVENING
THE SHEAR LINE WILL EXTEND ACROSS NORTHWEST MATO GROSSO INTO
EXTREME SOUTHEASTERN COLOMBIA...TO SUSTAIN MAXIMA OF
15-20MM/DAY.BY FRIDAY...WANING EVENT IN EASTERN SANTA CATARINA AND
NORTHEASTERN RIO GRANDE DO SUL WILL STILL PRODUCE LOCALLY HEAVY
PRECIPITATION...SUSTAINING MAXIMA OF 50-100MM/DAY. IN INTERIOR
PORTIONS OF THE SERRA GAUCHA EXPECTING MAXIMA OF 15-30MM/DAY. ALSO
ON THURSDAY...FRONTAL CONVECTION WILL INTERACT WITH UPPER JET
DYNAMICS AND COOL AIR FROM THE MID-UPPER TROUGH...AND SUSTAIN A
POTENTIAL FOR ISOLATED SEVERE CONVECTION IN CENTRAL MINAS GERAIS.
EXPECTING RAINFALL ACCUMULATIONS OF 10-15MM/DAY. SHEAR LINE
CONVECTION WILL PEAK IN NORTHWEST AMAZONAS...NORTHERN PERU AND
SOUTHERN COLOMBIA TO SUSTAIN MAXIMA OF 25-50MM. BY
SATURDAY...FRONTAL CONVECTION IN ESPIRITO SANTO WILL SUSTAIN
MAXIMA OF 05-10MM/DAY. BY SUNDAY...MODELS AGREE ON THE FLOW
BECOMING ONSHORE IN SOUTHERN BAHIA...WHICH WILL SUSTAIN AN
INCREASE IN MAXIMA TO AT LAST 20-40MM/DAY.

IN THE SOUTHERN CONE...UNDER THE INFLUENCE OF A BROAD TROUGH
PROPAGATING EASTWARD NEAR 100-90W...A SUBTROPICAL MOISTURE
CONNECTION IS INTERACTING WITH THE ANDES OF EXTREME SOUTHERN
CHILE. THIS WILL SUSTAIN MAXIMA OF 25-50MM IN SOUTHERN AYSEN INTO
NORTHERN MAGALLANES THROUGH EARLY THURSDAY...DECREASING TO MAXIMA
OF 15-20MM/DAY BY THURSDAY-FRIDAY. THE NEXT SYSTEM OF INTEREST IS
ANOTHER MID-UPPER TROUGH APPROACHING CENTRAL/SOUTHERN CHILE ON
SATURDAY-SUNDAY. MOISTURE WILL BE LIMITED...BUT THE TROUGH WILL BE
SUFFICIENTLY STRONG TO DEVELOP AN UPPER JET ACROSS THE ANDES OF
THE COQUIMBO AND ATACAMA REGIONS. THIS WILL FAVOR THE DEVELOPMENT
OF SNOWFALL IN HIGHER ELEVATIONS WITH ACCUMULATIONS OF 10-15CM BY
SUNDAY-MONDAY. ALSO DURING THIS PERIOD...FRONTAL CONVERGENCE IN
BIO BIO/SOUTHERN MAULE AND INTO ARAUCANIA WILL SUSTAIN MAXIMA OF
05-10MM/DAY.

NORTH ACROSS THE CONTINENT...THE SUBTROPICAL UPPER RIDGE/BOLIVIAN
HIGH CONTINUES STRONGER THAN NORMAL. AND IS MEANDERING EASTWARD TO
EXIT INTO THE ATLANTIC OCEAN BY LATE THURSDAY. STRONGEST
CONVECTION IS ORGANIZING ALONG THE NET...WHICH HAS MEANDERED NORTH
INTO THE NORTHERN GUIANAS...WHERE IT WILL REMAIN THROUGH THE
FORECAST CYCLE. CONVECTION WILL ALSO ORGANIZE WITH WAVES IN THE
TRADES...IN PARTICULAR WITH A WAVE PROPAGATING ALONG THE AMAZON
DELTA ON WEDNESDAY...SURINAME/CENTRAL PARA ON THURSDAY...AND
RORAIMA/EASTERN AMAZONAS ON FRIDAY. THIS WILL SUSTAIN MAXIMA OF
20-35 THROUGH FRIDAY...INCREASING TO MAXIMA OF 25-50MM ON
FRIDAY-SATURDAY AS IT INTERACTS WITH THE SHEAR LINE.

ALSO...UNDER THE INFLUENCE OF WARM SSTS AND ENHANCED PRECIPITABLE
WATER...HEAVY RAIN-PRODUCING THUNDERSTORMS ARE STILL EXPECTED IN
NORTHWEST ECUADOR INTO SOUTHWEST COLOMBIA TO SUSTAIN MAXIMA IN THE
ORDER OF 20-40MM. EARLY IN THE CYCLE...DECREASING TO MAXIMA OF
20-30MM BY THE WEEKEND.

DIAZ... DMC (CHILE)
MATHIAS...CHM (BRASIL)
GALVEZ...WPC (USA)