Prognostic Meteorological Discussion
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FXSA20 KWBC 281625
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1224 PM EDT FRI APR 28 2017

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

MODEL DISCUSSION (VALID FROM APRIL 28 AT 0000 UTC): THE GLOBAL
MODELS AREA IN GOOD AGREEMENT THROUGH 108-120 HRS...FOLLOWING A
SIMILAR EVOLUTION OVER THE SOUTH AMERICAN DOMAIN. THIS IS
SUPPORTED BY THE GLOBAL ENSEMBLE MEMBERS...WITH CONFIDENCE IN THE
FORECAST REMAINING HIGH THROUGH DAY 05.

OVER THE EASTERN PACIFIC...AT MID/UPPER LEVELS...A HIGH AMPLITUDE
LONG WAVE TROUGH IS TO GRADUALLY BECOME THE DOMINANT FEATURE. THE
TROUGH REACHES MAXIMUM AMPLITUDE ON SATURDAY-SUNDAY...WHEN IT IS
FORECAST TO EXTEND BETWEEN 110W-75W TO 20S. AS IT AMPLIFIES...THE
HIGH AMPLITUDE TROUGH WILL THEN ESTABLISH A SUBTROPICAL
CONNECTION...FOCUSING MOISTURE AND SHORT WAVE ENERGY OVER CHILE TO
THE SOUTH OF CONCEPCION. AT LOW LEVELS THIS WILL SUSTAIN A DEEP
TROUGH OVER THE SOUTHEAST PACIFIC THROUGH THE MEDIUM RANGE
PERIOD...WITH POLAR FRONTS TO REVOLVE AROUND THIS AXIS. IN A TIGHT
PRESSURE GRADIENT...THIS IS TO ALSO FAVOR STRONG LOW LEVEL WINDS
OF 25-35KT...THAT ARE TO CONVERGE ACROSS SOUTHERN CHILE. THE
STRONG WINDS ARE TO THEN LIFT OVER THE SOUTHERN ANDES. JET ALOFT
IS TO THEN FAVOR AN UPPER DIVERGENT PATTERN THAT IS TO VENT DEEP
CONVECTION. STRONG LIFTING AND VENTILATION ALOFT WOULD BE
CONDUCIVE TO GENERATION OF THUNDERSTORMS ON MONDAY-TUESDAY.
STARTING TODAY... EXPECTING RAINFALL MAXIMA OF 20-30MM. THROUGH
SUNDAY THIS INCREASES TO 30-60MM...WITH THE MOST INTENSE
CLUSTERING BETWEEN ISLA DE CHILOE AND PUERTO MONTT. THIS IS TO
THEN WEAKEN LATER IN THE CYCLE.

THE DEEP/HIGH AMPLITUDE TROUGH TO THE WEST WILL THEN INDUCE THE
SOUTHWARD AMPLIFICATION OF A RIDGE ACROSS CHILE/ARGENTINA TO THE
DRAKE PASSAGE/ANTARCTIC PENINSULA. THE RIDGE REACHES MAXIMUM
AMPLITUDE ON SUNDAY-MONDAY...AND IT IS TO THEN MEANDER EAST INTO
THE WESTERN ATLANTIC EARLY NEXT WEEK. DOWNSTREAM FROM THIS
RIDGE...OVER THE SOUTH ATLANTIC...A BROAD TROUGH EXTENDS NORTH
BETWEEN 50W-10W TO 20S. THIS IS A SLOW TO EVOLVE SYSTEM...TO
MEANDER OVER THE CENTRAL-WESTERN ATLANTIC THROUGH SATURDAY
EVENING...IT THEN STARTS TO PULL TO THE EAST-SOUTHEAST LATER
DURING THE WEEKEND. AT LOW LEVELS...THE TROUGH SUSTAINS AN
ELONGATED FRONT ACROSS THE SOUTH ATLANTIC TO RIO DE
JANEIRO/ESPIRITO SANTO IN BRASIL...THEN INLAND ACROSS MATO
GROSSO-NORTHERN BOLIVIA TO SOUTHERN PERU. AS IT LOSES UPPER LEVEL
SUPPORT...BOUNDARY OVER THE CONTINENT IS TO FRONTOLIZE LATER ON
SUNDAY/EARLY MONDAY MORNING. THE MEANDERING FRONT SUSTAINS A
PREFRONTAL SHEAR LINE THAT IS TO EXTEND ACROSS MATO
GROSSO-AMAZONAS IN BRASIL. ACROSS NORTHERN BRASIL THIS IS TO FAVOR
HEAVY CONVECTION WITH ACCUMULATION OF 75-125MM EXPECTED THROUGH
SATURDAY MORNING. THIS DECREASES TO 35-70MM ON
SATURDAY-SUNDAY...WITH MOST ACTIVE ALONG THE EQUATOR TO SOUTHERN
VENEZUELA/SOUTHEAST COLOMBIA. THIS IS TO ALSO FAVOR ORGANIZED
CONVECTION BETWEEN NORTHERN PERU-SOUTHWEST COLOMBIA...WITH MAXIMA
OF 30-60MM. THROUGH SUNDAY IT DECREASES TO 20-40MM. ALONG THE
FRONT...MODERATE CONVECTION IS EXPECTED ACROSS ESPIRITO
SANTO-MINAS GERAIS TO TOCANTINS/GOIAS IN BRASIL...WITH MAXIMA OF
15-20MM OVER THE NEXT COUPLE OF DAYS.

AT 200 HPA...A RIDGE EXTENDS ACROSS PERU-BOLIVIA-BRASIL WHILE
CENTERING ON A CLOSED HIGH OVER MATO GROSSO. THE RIDGE UNDERCUTS
AN UPPER LEVEL TROUGH THAT EXTENDS TO THE EAST. THE TROUGH
MEANDERS OVER NORTHEAST BRASIL/AMAZON RIVER DELTA REGION. THE
TROUGH IS FORECAST TO REMAIN CUT OFF FROM THE FLOW...AND AS SUCH
IT IS EXPECTED TO GRADUALLY WEAKEN. MEANWHILE...THIS IS TO ENHANCE
COASTAL CONVECTION ALONG THE NORTH COAST OF BRASIL...WHERE WE
EXPECT MAXIMA OF 15-20MM/DAY. ACROSS NORTHERN PARA-AMAZONAS THE
DAILY MAXIMA WILL RANGE BETWEEN 20-45MM EARLY IN THE CYCLE TO
20-35MM/DAY LATER ON SUNDAY. OVER ECUADOR...MOST ACTIVE IS
EXPECTED ALONG THE ANDES...WITH ACCUMULATION OF 15-20MM/DAY OVER
THE NEXT TWO DAYS. THROUGH SUNDAY IT WILL INCREASE TO 20-30MM AS
ENHANCED BY A LOW LEVEL WESTERLY FLOW.

ABDEMUR...FAA (ARGENTINA)
ESPINOLA...DNM (PARAGUAY)
DAVISON...WPC (USA)
$$





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