SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
236 PM EDT WED MAY 15 2013

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

MODEL COMPARISON (VALID FROM 00 UTC MAR 28): FORECAST CONFIDENCE
HAS IMPROVED WITH RESPECT TO PREVIOUS MODEL RUNS. GLOBAL MODELS
AND ENSEMBLE MEMBERS START DISAGREEING ON EVOLUTION OF
PERTURBATIONS IN THE SOUTHERN STREAM BY 96-108 HRSINCLUDING THE
SOUTHERN CONE.

MID-UPPER TROUGH IN THE NORTHERN STREAM IS PULLING SOUTHEASTWARD
ACROSS THE ATLANTIC OF SOUTHEASTERN BRASIL. TROUGH IS SUSTAINING
AN UPPER SUBTROPICAL JET. FAVORABLE UPPER DIVERGENCE IN LEFT
ENTRANCE OF UPPER JET WILL VENTILATE CONVECTION ACROSS MINAS
GERAIS/SOUTHERN ESPIRITO SANTO/EASTERN RIO DO JANEIRO EARLY IN THE
CYCLE. AT LOW LEVELSA MEANDERING SURFACE FRONT WILL EXTEND ACROSS
EASTERN RIO DO JANEIRO/SOUTHERN ESPIRITO SANTO THROUGH THE CYCLE
LEADING TO BEST MOISTURE CONVERGENCE. FURTHER INLANDTHE SOUTH
AMERICAN CONVERGENCE ZONE
(SACZ) HAS ORGANIZED AND WILL BE THE PRIMARY FOCUS FOR CONVERGENGE
THROUGH 48 HRSTO THEN LOSE ORGANIZATION. THROUGH 36 HRSEXPECTING
MOST PERSISTENT DEEP CONVECTION ACROSS MINAS GERAIS/EASTERN RIO DO
JANEIRO/SOUTHERN ESPIRITO SANTO TO PRODUCE ACCUMULATIONS OF
20-45MM. BY 36-60 HRSACCUMULATIONS WILL DECREASE TO 20-40MM/DAY.
ISOLATED HIGHER AMOUNTS ARE EXPECTED ACROSS COASTAL
AREAS/MOUNTAINS OF SOUTHERN ESPIRITO SANTOAS A LOW-LEVEL EASTERLY
JET DEVELOPS IN DEEP MOISTUER POOL. BY 60-84 HRS AMOUNTS WILL
RAPIDLY DECREASE INLAND WHILE LOCALIZED HEAVY RAIN PERSISTS ACROSS
SOUTHERN ESPIRITO SANTO. EXPECTING LOCALIZED MAXIMA OF
20-45MM/DAY.

THE SACZ WILL BE FOCUS FOR CONVECTION ACROSS NORTHERNMATO
GROSSO-RONDONIA-EASTERN ACRE EARLY IN THE CYCLE. SCAZ WILL LOSE
DEFINITION BY 48-60 HRS EVOLVING INTO A SURFACE TROUGH ACROSS SAME
REGIONS. AT UPPER LEVELSVENTILATION IN THE NORTHERN PERIPHERY OF
SUBTROPICAL HIGH WILL BE FAVORABLE NEAR AND TO THE NORTH OF THE
SACZ/SURFACE TROUGH. THIS WILL ALLOW FOR SCATTERED DIURNAL
CONVECTION BUILDING FROM THE BOUNDARY NORTHWARD INTO NORTHERN
AMAZONAS IN BRASIL PRODUCING ACCUMULATIONS OF 15-30 MM/DAY AND
ISOLATED HIGHER AMOUNTS THROUGH 84 HRS. LATER IN THE
CYCLESUBTROPICAL HIGH WILL ESTABLISH ACROSS RONDONIAWITH BEST
VENTILATION ACROSS ITS NORTHERN PERIPHERY. THIS WILL ACTIVATE
DIURNAL CONVECTION ACROSS NORTHWESTERN AMAZONIA ONCE AGAIN.
EXPECTING ACCUMULATIONS TO INCREASE TO 20-35MM/DAY BY LATE CYCLE
AFFECTING NORTHERN PERU/WESTERN AMAZONAS/SOUTHEASTERN COLOMBIA.
ACROSS ECUADORACCUMULATIONS WILL INCREASE TO 20-45MM/DAY BY
108-132 HRS. ACROSS SOUTHERN PERU/NORTHERN BOLIVIAALSO EXPECTING
AN INCREASE IN ACCUMULATIONS BY LATE CYCLE WITH MAXIMA AVERAGING
15-25MM/DAY. SEASONAL CONVECTION ACROSS THE PERUVIAN ANDES WILL
PRODUCE ACCUMULATIONS IN THE 05-10MM/DAY RANGE.

TO THE NORTHEASTUPPER TROUGHS HAVE ESTABLISHED ACROSS THE GUIANAS
AND THE NORDESTE IN BRASIL. ACTIVE CONVECTION ACROSS CENTRAL
AMAZONIA WILL BUILD THE SUBTROPICAL HIGH TO THE EAST INGO SOUTHERN
PARA/GOIAS. THIS WILL PRESS INTO TROUGH OVER THE NORDESTE
IMPROVING VENTILATION ACROSS MARANHAO/PIAUI AND THE ORTHER COAST
OF BRASIL BY 36-84 HRS. THIS WILL ENHANCE DIURNAL CONVECTION
PRODUCING ACCUMULATIONS OF 15-30MM/DAY ACROSS PIAUI AND TO THE
WESTAND 15-20MM/DAY TO THE EAST INTO RIO GRANDE DO NORTE BY 36-60
HRS. ACCUMULATIONS ARE TO PEAK BY 60-84 HRS REACHING 20-40MM/DAY
WITH ISOLATED HIGHER AMOUNTS. AS DEEP-LAYER EASTERLY FLOW
STRENGTHENS ACROSS THE NORDESTEA MID-LEVEL DRY LAYER WILL BE
ADVECTED WESTWARD FROM THE SUBTROPICAL ATLANTIC. THIS WILL INHIBIT
DEEP CONVECTION FROM EAST TO WEST THROUGH THE CYCLE LEADING TO A
SHARP DECREASE IN ACCUMULATIONS. BY 108-132 HRSEXPECTING
ACCUMULATIONS OF 20-35MM/DAY TO CONCENTRATE ACROSS THE AMAZON
DELTAAND TO THE WEST OF CENTRAL PARA-EASTERN MATO GROSSO.

NEXT SYSTEM OF INTEREST IS A MID-UPPER TROUGH THAT WILL ORGANIZE
INTO A CLOSED UPPER LOW OFF THE COAST OF CENTRAL CHILESTARTING ON
DAY 02. THE LOW WILL SLOWLY DEEPEN WHILE BLOCKED TO THE WEST OF
THE ANDESSTARTING TO SPILL ITS ENERGY ACROSS THE ANDES ON DAY 03.
A SUBTROPICAL JET WILL FORM IN THE NORTHERN PERIPHERY OF THIS LOW.
THE RIGHT EXIT OF THIS
JET WILL PROVIDE EXCELLENT VENTILATION ACROSS INLAND CENTRAL
ARGENTINA. THIS WILL INITIALLY VENT A FEW DIURNAL THUNDERSTORMS
PRODUCING LOCALLY HEAVY RAINFALLTO THEN LEAD TO CYCLOGENESIS AND
ORGANIZED CONVECTION. TO THE EAST OF THE ANDES AND AT LOW
LEVELSMOISTURE WILL BE INITIALLY LIMITED. EASTERLY FLOW WILL
DEVELOP ACROSS CENTRAL ARGENTINA AS EARLY AS 60-72 HRS TO ENHANCE
LOW-LEVEL MOISTURE CONVERGENCE ACROSS CUYO. AS TROUGH ACROSS
MID-SECTIONS OF THE CONTINENT LOSES DEFINITIONLONG FETCH
NORTHEASTERLIES WILL GRADUALLY TRANSPORT MOISTURE FROM
MID-SECTIONS OF THE CONTINENT INTO NORTHERN AND CENTRAL ARGENTINA.
WINDS ARE TO INCREASE IN SPEED TO WIDESPREAD 10-15KT
NORTHEASTERLIES ON DAY 04WHILE ON DAY 05 EXPECTING A 30-35KT
NORTHERLY JET TO DEVELOP ACROSS THE CHACO. THIS WILL LEAD TO A
RAPID INCREASE IN BOUNDARY LAYER MOISTURE DURING DAYS 04 AND 05.
INTERACTION BETWEEN SPILLING UPPER TROUGH AND MOIST AND UNSTABLE
AIRMASS TO THE EAST WILL PRODUCE VERY HEAVY RAINFALL AND THE
CHANCE FOR SEVERE CONVECTION DURING DAYS 04 AND 05. EXPECTING
AMOUNTS TO INCREASE TO 35-70MM/DAY ACROSS CUYO INTO SAN
LUIS-CORDOBA BY 84-108 HRS. BY 108-132 HRSEXPECTING STRONG
CONVECTION TO PERSIST ACROSS CUYO-SAN LUIS-NORTHERN LA
PAMPASPREADING INTO THE NORTHWESTERN BUENOS AIRES
PROVINCEPRODUCING ACCUMULATIONS OF 40-80MM/DAY. OTHER REGION
FAVORABLE FOR DEEP CONVECTION IS NORTHCENTRAL ARGENTINA/SOUTHERN
PARAGUAY INTO NORTHERN URUGUAY/SOUTHERN BRASIL WHERE EXPECTING
ACCUMULATIONS OF 50-100MM/DAY IN SEVERE CONVECTION. NOTE THAT
LOCALLY HIGHER AMOUNTS ARE POSSIBLE.

CARDOZA...FAP (PERU)
MAQUEDA...DINAC (PARAGUAY)
GALVEZ...WPC (USA)