SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1136 AM EDT TUE MAY 14 2013

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

MODEL COMPARISON (VALID FROM 00 UTC MAY 14): THE GFS AGREES WITH
THE UKMET-ECMWF GLOBAL MODELS THROUGH 72-84 HRS. IT THEN DIVERGES
DURING THE SECOND HALF OF THE FORECAST...WITH MODEL TRENDING
TOWARDS A SOMEWHAT PERTURBED WESTERLY FLOW. THE EUROPEAN
MODELS...IN CONTRAST...TEND TO FAVOR A MORE AMPLIFIED LONG WAVE
PATTERN. THIS IS A SOLUTION WELL SUPPORTED BY THE MEAN OF THE
GLOBAL ENSEMBLES. AS A RESULT...OUR FORECAST AND CHARTS ARE
REFLECTIVE OF THIS SOLUTION.

THE GLOBAL MODELS ONCE AGAIN INITIALIZE A DEEP MID/UPPER LEVEL
TROUGH OVER THE WESTERN ATLANTIC TO THE SOUTHERN CONE OF SOUTH
AMERICA. A LONG WAVE RIDGE TO THE WEST...WITH AXIS NORTH TO SOUTH
ALONG 90W/95W...IS DIVERTING COLD POLAR ENERGY NORTHWARD INTO
BACKSIDE OF THIS TROUGH. THIS...IN-TURN...SUSTAINS THE NORTHWARD
AMPLIFICATION OF THIS TROUGH INTO MID SECTIONS OF SOUTH AMERICA
THROUGH 48-60 HRS...FOCUSING ITS SHORT WAVE ENERGY ACROSS RIO DE
LA PLATA BASIN-URUGUAY-SOUTHERN BRASIL. INITIALLY SLOW...THE
TROUGH IS TO ACCELERATE BY 48-72 HRS...WHEN IT IS TO START PULLING
TO THE EAST-SOUTHEAST ACROSS THE SOUTH ATLANTIC. BY 84-96 HRS IT
IS TO THEN ACCELERATE ACROSS 20W. AS THE LONG WAVE TROUGH MOVES
ACROSS THE SOUTHERN CONE...IT WILL DRAW A SUBTROPICAL/POLAR JET
PAIR ACROSS MID SECTIONS OF SOUTH AMERICA BY 42-72 HRS. AS THE
JETS PULL ACROSS THE CONTINENT...THEY ARE EXPECTED TO
INTERACT...ENHANCING UPPER DIVERGENCE ACROSS PARAGUAY-SOUTHERN
STATES OF BRASIL AND PORTIONS OF BOLIVIA.

AT LOW LEVELS AN ELONGATED FRONT IS TO MEANDER ACROSS THE SOUTH
ATLANTIC INTO SOUTHERN BRASIL-PARAGUAY/CHACO ARGENTINO. THIS
BOUNDARY IS GOING TO SERVE AS FOCUS OF MOISTURE CONVERGENCE OVER
THE NEXT 24-36 HRS...WITH MODELS FORECASTING PRECIPITABLE WATER
CONTENT OF 40-50MM ALONG THIS BOUNDARY. AS IT PERSISTS...AND UNDER
FAVORABLE JET DYNAMICS...EXPECTING SCATTERED CONVECTION ALONG THIS
BOUNDARY TO RESULT IN RAINFALL AMOUNTS OF 25-50MM/DAY.
SIMULTANEOUSLY...UNDER INFLUENCE OF AMPLIFYING POLAR TROUGH AND A
BUILDING POLAR RIDGE OVER ARGENTINA...A SURFACE FRONT IS FORECAST
TO SURGE ACROSS ARGENTINA DURING THE NEXT 36-42 HRS. THROUGH 48
HRS IT WILL MOVE NORTH ACROSS URUGUAY TO SOUTHERN
BRASIL-PARAGUAY...WHERE IT IS TO MERGE WITH THE OLD BOUNDARY TO
THE NORTH. BY 72-84 HRS THE FRONT WILL STALL OVER SAO PAULO-MATO
GROSSO DO SUL-CHACO PARAGUAYO. AHEAD OF THE FRONT...A NORTHERLY
JET OF 25-35KT OVER BOLIVIA/PARAGUAY IS TO SUSTAIN A WARM/MOIST
ADVECTIVE PATTERN...WHILE A 30-35KT SOUTHERLY JET OVER ARGENTINA
IS TO SUSTAIN A COLD ADVECTIVE PATTERN. AS THE AIR MASSES CLASH
OVER PARAGUAY-SOUTHERN BRASIL THIS IS LIKELY TO TRIGGER SEVERE
CONVECTION AND HEAVY RAINFALL AMOUNTS. OVER SOUTHERN
BRASIL-MESOPOTAMIA VALLEY AND SOUTHEAST PARAGUAY THE MAXIMA WILL
PEAK AT 35-70MM...WHILE OVER CENTRAL PARAGUAY/CHACO AND SANTA CRUZ
IN EASTERN BOLIVIA EXPECTING MAXIMA OF 25-50MM. BY 48-72 HRS
ACTIVITY IS TO RAPIDLY BUILD NORTH AND WEST ACROSS BOLIVIA TO
ACRE/SOUTHERN AMAZONAS IN BRASIL TO SOUTHERN JUNGLE OF PERU...WITH
MAXIMA IN THIS AREA TO PEAK AT 30-60MM. LATER IN THE CYCLE FOCUS
OF THE CONVECTION SHIFTS NORTH ACROSS NORTHERN PERU/NORTHERN
AMAZONAS TO SOUTHERN COLOMBIA...WITH MAXIMA OF 20-40MM.

A SHORT WAVE ON THE NORTHERN STREAM IS TO FOLLOW. THE MID/UPPER
LEVEL TROUGH IS TO MEANDER OFF THE CENTRAL/NORTHERN COAST OF CHILE
THROUGH 96-108 HRS. BY 120-132 HRS...AS A LONG WAVE TROUGH MOVES
TO THE EASTERN PACIFIC...THE SHORT WAVE IS TO THEN ACCELERATE
INLAND...SPILLING ACROSS THE ANDES INTO THE NORTHERN PROVINCES IN
ARGENTINA. OVER CENTRAL CHILE THIS IS TO FAVOR SCATTERED
CONVECTION...WITH MOST ACTIVE TO FOCUS ON THE ANDEAN
REGION...WHERE IT WILL RESULT IN RAINFALL AMOUNTS OF 15-20MM. AS
THE SHORT WAVE TROUGH PULLS ACROSS NORTHERN ARGENTINA...IT IS TO
THEN INTERACT WITH MEANDERING SURFACE FRONT OVER SOUTHERN
BRASIL-PARAGUAY. THIS IS TO STRENGTHEN THE SURFACE FRONT WHILE
TRIGGERING SCATTERED CONVECTION WITH MAXIMA OF 20-35MM.

AT UPPER LEVELS...NORTH OVER THE CONTINENT...LOW AMPLITUDE RIDGE
IS TO MEANDER ACROSS BRASIL DURING THE NEXT FEW DAYS...WITH AXIS
TO BECOME BETTER ORGANIZED THROUGH 72-96 HRS AS ORGANIZED DEEP
CONVECTION BUILDS ACROSS BRASIL TO BOLIVIA-PERU. MEANWHILE...AS
THE RIDGE HOLDS...IT WILL SUSTAIN AN INDUCED TROUGH ACROSS
NORTHEAST BRASIL. THIS TROUGH WILL INTERACT WITH THE ATLANTIC
ITCZ...TRIGGERING SCATTERED CONVECTION ACROSS AMAPA IN
BRASIL-AMAZON RIVER DELTA REGION. IN THIS AREA EXPECTING RAINFALL
AMOUNTS OF 20-35MM/DAY THROUGH 60-72 HRS...AND 05-10MM/DAY LATER
IN THE FORECAST CYCLE. OTHER ACTIVITY IS EXPECTED ALONG THE NEAR
EQUATORIAL TROUGH OVER NORTHERN AMAZONAS TO SOUTHERN
COLOMBIA-EASTERN ECUADOR...WITH RAINFALL AMOUNTS OF 20-30MM/DAY.

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