SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1144 AM EDT MON MAY 20 2013

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

MODEL COMPARISON (VALID FROM 00 UTC MAY 20): GLOBAL MODELS FOLLOW
SIMILAR PATTERN EVOLUTION THROUGH 96-108 HRS. THEY THEN START TO
DIVERGE LATER ON THE CYCLE HOW AN AMPLIFYING MID/UPPER TROUGH OVER
THE WESTERN ATLANTIC IS GOING TO EVOLVE. THE GFS IS CLOSER TO THE
MEAN OF THE GLOBAL ENSEMBLES...WHILE THE ECMWF TENDS TO DIVERGE
THE MOST. THE UKMET APPEARS AS THE COMPROMISE SOLUTION AMONG THESE
MODELS. BUT VARIABILITY IS TO HIGH TO ESTABLISH CONFIDENCE INTO
MEDIUM RANGE PERIOD. AS A RESULT...OUR CONFIDENCE IN THE FORECAST
IS LOW.

A BROAD MID/UPPER LEVEL TROUGH ON THE SOUTHERN STREAM IS
INITIALIZED OVER THE BELLINGSHAUSEN SEA TO THE EASTERN
PACIFIC/SOUTHERN CONE. THE TROUGH IS TO SLOWLY PULL ACROSS THE
ANTARCTIC PENINSULA THROUGH 84 HRS...INTO THE WEDDELL SEA BY 96
HRS. AS IT PULLS ACROSS THE ANTARCTIC PENINSULA IT WILL DRAW A
TROUGH ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA...WHERE IT
IS TO FOCUS ITS SHORT WAVE ENERGY. LATER IN THE CYCLE...AND UNDER
INFLUENCE OF A BUILDING RIDGE TO THE WEST...THE TROUGH IS FORECAST
TO THEN RAPIDLY AMPLIFY ALONG THE COAST OF ARGENTINA INTO
URUGUAY/SOUTHERN STATES OF BRASIL. BY 120-132 HRS A CLOSED LOW IS
TO FORM ALONG THIS AXIS TO THE NORTH-NORTHEAST OF THE FALKLAND
ISLANDS/MALVINAS. AT LOW LEVELS IT WILL SUSTAIN A BROAD TROUGH
OVER THE BELLINGSHAUSEN SEA/ANTARCTIC PENINSULA-SOUTHERN CONE OF
SOUTH AMERICA. POLAR FRONTS WILL REVOLVE AROUND THIS AXIS AS IT
MEANDERS ACROSS THE SOUTHERN CONE...WITH SUCCESSIVE FRONTS TO
RAPIDLY MOVE ACROSS SOUTHERN CHILE/PATAGONIA IN ARGENTINA. THIS
PATTERN PERSISTS OVER THE NEXT FOUR DAY...FAVORING STRONG BOUNDARY
LAYER WINDS OF 35-50KT AND TOPOGRAPHICALLY INDUCED CONVECTION.
OVER CHILE...BETWEEN TEMUCO-ISLA DE CHILOE...THE DAILY MAXIMA WILL
PEAK AT 15-0MM OVER THE NEXT THREE DAYS...WHILE OVER TIERRA DEL
FUEGO IT IS TO GENERALLY PEAK AT 05-10MM. LATER IN THE CYCLE...AS
THE SURFACE TROUGH PATTERN OVER THE WESTERN ATLANTIC STRENGTHENS
AND A POLAR RIDGE BUILDS OVER ARGENTINA...THE SURFACE FRONT WILL
SURGE ACROSS RIO DE LA PLATA TO NORTHERN URUGUAY/SOUTHERN
BRASIL-PARAGUAY. THIS IS TO FAVOR A COLD ADVECTIVE PATTERN INTO
MID SECTIONS OF SOUTH AMERICA.

FARTHER EAST...OVER THE SOUTH ATLANTIC...A SHORT WAVE TROUGH ON
THE NORTHERN STREAM MEANDERS BETWEEN 40W-15W AND TO THE SOUTH OF
25S. MID LEVEL TROUGH IS TO REMAIN NEARLY STATIONARY DURING THE
NEXT THREE DAYS...AND LATER IN THE CYCLE IT WILL START TO MIGRATE
TO THE EAST-SOUTHEAST. AS IT HOLDS...IT WILL SUSTAIN A BROAD
SURFACE TROUGH OVER THE CENTRAL ATLANTIC...AND A POLAR FRONT THAT
TRAILS WEST INTO THE SOUTHERN STATES OF BRASIL. AN OLD BOUNDARY
LIES TO THE NORTH... WITH AXIS WEST ALONG 30S 30W...SOUTHEAST
BRASIL TO CHACO PARAGUAYO/CENTRAL BOLIVIA. UNDER INFLUENCE OF A
SUBTROPICAL JET MAXIMA THIS BOUNDARY IS FORECAST TO HOLD OVER THE
NEXT FEW DAYS... BLOCKING MOISTURE TRANSPORT TO THE NORTHERN
PORTIONS OF SOUTH AMERICA. AS THE SURFACE FRONTS MEANDER ACROSS
SOUTHERN BRASIL-SAO PAULO...THESE IS TO FAVOR SCATTERED
CONVECTION. INITIALLY EXPECTING RAINFALL AMOUNTS OF 05-10MM...AND
THROUGH 72-96 HRS THIS WILL INCREASE TO 15-20MM. OTHER ACTIVITY IS
EXPECTED ON THE TAIL END OF THIS FRONT OVER CENTRAL BOLIVIA...WITH
RAINFALL AMOUNTS OF 05-10MM.

AT 200 HPA...WEAK CELL OF THE SUBTROPICAL RIDGE EXTENDS ACROSS
NORTHERN SOUTH AMERICA...WITH AXIS CONFINING TO CONTINENTAL AREA
NORTH OF 13S/14S. ALONG THIS AXIS...MODEL GUIDANCE SHOWS CLOSED
HIGH FORMING OVER AMAZONAS IN BRASIL-NORTHERN JUNGLE OF PERU BY
72-96 HRS...WITH AXIS TO THEN BUILD WHILE GRADUALLY MIGRATING EAST
ACROSS NORTHERN STATES OF BRASIL. AS THE HIGH/RIDGE
INTENSIFIES...IT WILL INDUCE A SHORT WAVE TROUGH OVER NORTHEAST
BRASIL. RIDGE PATTERN ALOFT WILL VENT ORGANIZED DEEP CONVECTION
ACROSS COLOMBIA-NORTHERN AMAZONAS IN BRASIL TO THE NORTHERN JUNGLE
OF PERU/EASTERN ECUADOR. EARLY IN THE CYCLE EXPECTING MAXIMA IN
THIS AREA TO PEAK AT 20-45MM. AS THE MID/UPPER LEVEL TROUGH FORMS
TO THE EAST...FOCUS OF ORGANIZED CONVECTION WILL SHIFT EAST TO
PARA IN BRASIL...WITH MAXIMA IN THIS AREA TO PEAK AT 20-45MM.
OTHER CONVECTION IS EXPECTED ACROSS AMAPA TO THE AMAZON RIVER
DELTA REGION...WITH RAINFALL AMOUNTS OF 15-20MM/DAY.

MAQUEDA...DINAC (PARAGUAY)
SILVA DE SOUZA...INMET (BRASIL)
DAVISON...WPC (USA)
GALVEZ...WPC (USA)