SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1216 PM EDT TUE MAY 21 2013

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

MODEL COMPARISON (VALID FROM 00 UTC MAY 21): CONTINUITY WITH
PREVIOUS RUN IS GOOD THROUGH 72-84 HRS...THEN MODELS MAKE PATTERN
ADJUSTMENTS TO EVOLVING LONG WAVE TROUGH OVER THE WESTERN
ATLANTIC. ALTHOUGH THEY CONTINUE TO FAVOR A HIGHLY AMPLIFIED LONG
WAVE PATTERN...THEY DIVERGE ON THE FOCUS OF THE SHORT WAVE
ENERGY...WITH LATEST RUNS OF THE MODELS SHIFTING TOWARDS A
SOUTHERN SOLUTION. IN THIS AREA THE GLOBAL ENSEMBLES SHOW HIGH
VARIABILITY AMONG THE MEMBERS...AND AS A RESULT EXPECTING FURTHER
CHANGES IN THE NEXT FEW RUNS.

ONCE AGAIN THE MODELS INITIALIZE A BROAD MID/UPPER LEVEL TROUGH ON
THE SOUTHERN STREAM FLOW...WITH AXIS EXPANDING ACROSS THE
BELLINGSHAUSEN SEA TO THE EASTERN PACIFIC/SOUTHERN CONE. THE
TROUGH IS TO SLOWLY PULL ACROSS THE ANTARCTIC PENINSULA THROUGH
60-72 HRS...INTO THE WEDDELL SEA BY 72 HRS. AS IT PULLS ACROSS THE
ANTARCTIC PENINSULA/SOUTHERN CONE...THE BROAD TROUGH WILL FOCUS
SHORT WAVE PERTURBATIONS ACROSS SOUTHERN PORTIONS OF CHILE INTO
PATAGONIA IN ARGENTINA. AT LOW LEVELS THIS IS TO REFLECT AS A
BROAD TROUGH ON THE SOUTHERN STREAM...WITH POLAR FRONTS TO REVOLVE
AROUND THIS AXIS AS THEY STREAM ACROSS SOUTHERN CHILE-PATAGONIA IN
ARGENTINA. THIS IS TO ALSO SUSTAIN A TIGHT PRESSURE GRADIENT AND
STRONG BOUNDARY LAYER WINDS...WITH TOPOGRAPHICALLY INDUCED
CONVECTION TO AFFECT CHILE BETWEEN PUNTA ARENAS-ISLA DE CHILOE. IN
THIS AREA IT IS TO FAVOR RAINFALL AMOUNTS OF 10-15MM/DAY THROUGH
60-72 HRS.

AT 72-96 HRS...AS A LONG WAVE RIDGE BUILDS TO THE WEST...TROUGH ON
THE SOUTHERN STREAM IS TO THEN SURGE ACROSS THE WESTERN ATLANTIC
INTO URUGUAY/SOUTHERN STATES OF BRASIL. THE HIGHLY AMPLIFIED LONG
WAVE TROUGH IS TO THEN PERSIST THROUGH 120-132 HRS. A CLOSED LOW
NEAR 57S 50W IS TO ANCHOR THE MID LEVEL TROUGH. AT LOW
LEVELS...THIS WILL SUSTAIN A DEEP POLAR TROUGH OVER THE WESTERN
ATLANTIC WITH ASSOCIATED FRONT TO RACE ACROSS RIO DE LA PLATA TO
URUGUAY-CORDOBA IN ARGENTINA BY 96 HRS. THROUGH 120 HRS THE FRONT
WILL MOVE FARTHER NORTH ACROSS THE SOUTHERN STATES OF BRASIL TO
SAO PAULO-PARAGUAY. AS THE FRONT SURGES NORTH...A POLAR RIDGE IS
TO THEN BUILD ACROSS ARGENTINA WHILE CENTERING ON A 1025 HPA HIGH.
THIS IS TO SUSTAIN A COLD ADVECTIVE PATTERN INTO MID SECTIONS OF
SOUTH AMERICA.

FARTHER EAST...OVER THE SOUTH ATLANTIC...A MID LEVEL SHORT WAVE
TROUGH ON THE NORTHERN STREAM MEANDERS BETWEEN 40W-15W AND TO THE
SOUTH OF 25S. THE MODELS FORECAST THIS TROUGH TO REMAIN NEARLY
STATIONARY DURING THE NEXT TWO DAYS...AND LATER IN THE CYCLE IT
WILL START TO MIGRATE TO THE EAST-SOUTHEAST. MEANWHILE...IT WILL
SUSTAIN A BROAD SURFACE TROUGH OVER THE CENTRAL ATLANTIC...AND A
POLAR FRONT THAT TRAILS WEST INTO THE SOUTHERN STATES OF BRASIL.
FURTHERMORE... 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 96 HRS THIS WILL INCREASE TO
15-20MM AS REINFORCED BY SURGING POLAR FRONT TO THE SOUTH. OTHER
ACTIVITY IS EXPECTED ON THE TAIL END OF THIS FRONT OVER CENTRAL
BOLIVIA...WITH RAINFALL AMOUNTS OF 05-10MM IN ISOLATED TO
SCATTERED CONVECTION.

AT 200 HPA...A WEAK CELL OF THE SUBTROPICAL RIDGE EXTENDS ACROSS
NORTHERN SOUTH AMERICA...WITH AXIS CONFINING TO CONTINENTAL AREA
NORTH OF 13S/14S. ALONG THIS AXIS...THE MODELS SHOW A CLOSED HIGH
FORMING OVER AMAZONAS IN BRASIL-NORTHERN JUNGLE OF PERU BY 48-72
HRS...WITH AXIS TO THEN BUILD WHILE GRADUALLY MIGRATING EAST
ACROSS NORTHERN STATES OF BRASIL LATER IN THE CYCLE. AS THE
HIGH/RIDGE INTENSIFIES...IT WILL INDUCE A SHORT WAVE TROUGH OVER
NORTHEAST BRASIL. RIDGE PATTERN ALOFT WILL VENT DEEP CONVECTION
ACROSS COLOMBIA-NORTHERN AMAZONAS IN BRASIL TO THE NORTHERN JUNGLE
OF PERU/EASTERN ECUADOR. IN THIS AREA DAILY MAXIMA IS TO PEAK AT
15-20MM/DAY. HIGHER AMOUNTS ARE EXPECTED TO THE EAST ACROSS
NORTHERN PARA TO RORAIMA IN BRASIL...WITH MAXIMA IN THIS AREA TO
PEAK AT 20-45MM OVER THE NEXT COUPLE OF DAYS...AND 15-20MM/DAY
THEREAFTER.

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