Prognostic Meteorological Discussion
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106
FXSA20 KWBC 191618
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1217 PM EDT THU APR 19 2018

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

MODEL DISCUSSION (VALID FROM APR 19 AT 0000 UTC): AS IT WAS
EXPECTED...THE GFS MADE PATTERN CORRECTIONS DURING THE MEDIUM
RANGE PERIOD. IT NO LONGER SHOWS EXPLOSIVE CYCLOGENESIS OVER THE
SOUTH ATLANTIC...INSTEAD FORECASTING A WEAKER/MORE PROGRESSIVE MID
LEVEL TROUGH TO MAKE LANDFALL OVER CENTRAL CHILE EARLY ON SUNDAY.
THE LATEST CORRECTION IS WELL SUPPORTED BY THE MEAN OF THE GLOBAL
ENSEMBLES AND DETERMINISTIC GLOBAL MODELS. AS A RESULT...OVERALL
CONFIDENCE IN THE FORECAST IS HIGH THROUGH 108-120 HRS.

A MEANDERING FRONT LIES OVER THE SOUTH ATLANTIC TO THE CENTRAL
PROVINCES IN ARGENTINA. A MID LEVEL SHORT WAVE TROUGH STREAMING
ACROSS CENTRAL ARGENTINA IS TO INTERACT WITH THIS BOUNDARY. THIS
WILL TRIGGER ORGANIZED DEEP CONVECTION ACROSS THE BUENOS AIRES
PROVINCE/ENTRE RIOS IN ARGENTINA-URUGUAY DURING THE NEXT COUPLE OF
DAYS. IN THIS AREA THE MAXIMA IS TO PEAK AT 20-45MM THROUGH FRIDAY
MORNING...WITH MODELS SHOWING POTENTIAL FOR SEVERE CONVECTION. ON
FRIDAY TO SATURDAY EXPECTING ADDITIONAL RAINFALL AMOUNT OF
20-40MM. ON SATURDAY THIS GRADUALLY DECREASES TO 10-15MM.

CONSISTENT WITH PREVIOUS FORECAST A BROAD MID/UPPER LEVEL TROUGH
CONTINUES TO DOMINATE THE FLOW OVER THE SOUTHEAST PACIFIC. THE
TROUGH ENVELOPS AREA TO THE SOUTH OF 30S AND EAST OF 130W. THIS
TROUGH IS FORECAST TO MEANDER EAST ACROSS THE ANTARCTIC
PENINSULA/SOUTHERN CONE OF SOUTH AMERICA LATER ON SATURDAY.
MEANWHILE...THE BROAD TROUGH IS TO SUSTAIN A MOIST ADVECTIVE
PATTERN ACROSS THE SOUTHERN REGIONS OF CHILE...WITH A PLUME OF
DEEP SUBTROPICAL MOISTURE EXPECTED DURING THE NEXT DAY OR SO. AT
LOW LEVELS THIS SUSTAINS A BROAD TROUGH AND A TIGHT PRESSURE
GRADIENT ACROSS THE SOUTHERN CONE OF SOUTH AMERICA.
THIS...IN-TURN...FAVORS STRONG BOUNDARY LAYER WINDS. THESE ARE
FORECAST TO PEAK AT 30-50KT DURING THE NEXT FEW DAYS. AS IT
MEANDERS TO THE SOUTH...POLAR FRONTS ARE TO REVOLVE AROUND THIS
AXIS...WITH ONE TO MEANDER ACROSS SOUTHERN CHILE-PATAGONIA IN
ARGENTINA DURING THE NEXT THREE DAYS. THIS IS TO ENHANCE LOW LEVEL
MOISTURE CONVERGENCE BETWEEN TEMUCO-ISLA DE CHILOE IN SOUTHERN
CHILE. IN THIS AREA EXPECTING RAINFALL MAXIMA OF 20-30MM THROUGH
FRIDAY MORNING...AND 10-15MM ON FRIDAY TO SATURDAY. OVER TIERRA
DEL FUEGO TO THE SOUTH ENVIRONMENTAL CONDITIONS ARE TO FAVOR
SNOWFALL ON THE SOUTHERN ANDES...WHILE OVER LOWER ELEVATIONS
EXPECTING MIXED PRECIPITATION TO TRIGGER RAINFALL AMOUNTS OF
05-10MM/DAY DAYS.

ALSO AT MID LEVELS...A PROGRESSIVE SHORT WAVE TROUGH RACES ACROSS
THE EASTERN PACIFIC TO SOUTHERN REGIONS OF CHILE LATER ON
SATURDAY/EARLY SUNDAY MORNING. AS IT STREAMS ACROSS SOUTHERN CHILE
TO PATAGONIA/LA PAMPA IN ARGENTINA THIS IS TO INDUCE HEIGHT FALLS
OF 100-125GPM. LATER ON SUNDAY THE TROUGH WILL MOVE ACROSS
PATAGONIA TO THE WESTERN SOUTH ATLANTIC...WITH AXIS THEN FORECAST
TO ACCELERATE TO THE EAST-SOUTHEAST. AT LOW LEVELS THIS WILL
SUSTAIN A FRONTAL LOW OVER SOUTHERN CHILE LATER ON SATURDAY. THIS
MOVES ACROSS PATAGONIA TO THE WESTERN ATLANTIC EARLY ON SUNDAY
MORNING. BUILDING POLAR RIDGE TO THE SOUTH WILL THEN FAVOR A
PAMPERO LOW LEVEL JET THAT DRIVES THE FRONT NORTH ACROSS LA PAMPA
TO THE BUENOS AIRES PROVINCE LATER IN THE DAY...AND NORTH ACROSS
RIO DE LA PLATA-MENDOZA IN ARGENTINA ON MONDAY. OVER SOUTHERN
CHILE THE FRONTAL LOW WILL SUSTAIN MODERATE CONVECTION WITH MAXIMA
OF 15-25MM ON SATURDAY TO SUNDAY. OVER CENTRAL ARGENTINA THIS
INCREASES TO 40-80MM ON MONDAY IN POTENTIALLY SEVERE CONVECTION.
DURING THIS PERIOD THE GFS MODEL IS SHOWING RISK OF LARGE HAIL. AS
IT PUSHES NORTH ACROSS URUGUAY ON MONDAY THE MAXIMA IS TO PEAK AT
20-40MM.

OVER THE ATLANTIC...MEANWHILE...A MID/UPPER LEVEL TROUGH EXTENDS
BETWEEN 20W-40W AND TO THE SOUTH OF 20S. THIS IS TO SLOWLY MIGRATE
TO THE EAST-SOUTHEAST WHILE MERGING WITH A TROUGH TO THE SOUTH
LATER ON SATURDAY. AT LOW LEVELS THIS PROVIDES SUPPORT TO A
MEANDERING FRONT AND A PREFRONTAL SHEAR LINE ACROSS THE NORTHEAST
STATES OF BRASIL. THE FRONT AND THE SHEAR LINE ARE FORECAST TO
GRADUALLY WEAKEN LATER IN THE WEEK. THESE WILL BE ACCOMPANIED BY
STRONG BOUNDARY LAYER WINDS THAT ARE TO ENHANCE COASTAL
CONVERGENCE BETWEEN ESPIRITO SANTO AND EASTERN BAHIA IN BRASIL. IN
THIS AREA THE DAILY MAXIMA IS TO PEAK AT 20-40MM...WITH POTENTIAL
FOR HIGHER AMOUNTS DUE TO STRONG TOPOGRAPHICAL FORCING.

NORTH OVER THE CONTINENT...AT 200 HPA...DEEP LAYER MID/UPPER LEVEL
TROUGH EXTENDS OVER THE FAR EASTERN PACIFIC TO ECUADOR/NORTHERN
PERU. THIS IS TO REMAIN NEARLY STATIONARY WHILE GRADUALLY
WEAKENING. AS IT MEANDERS OVER THE CONTINENT THE DEEP MID LEVEL
TROUGH IS TO SUSTAIN AN INDUCED SURFACE TROUGH OVER THE NORTHERN
JUNGLE OF PERU-ECUADOR TO SOUTHERN COLOMBIA. THIS...IN-TURN...WILL
FAVOR A MOIST EASTERLY FLOW THAT IS TO CONVERGE/ADIABATICALLY LIFT
ALONG THE LEE OF THE ANDES. IN THIS AREA EXPECTING RAINFALL
AMOUNTS OF 30-60MM/DAY DURING THE NEXT FEW DAYS. LATER DURING THE
WEEKEND THIS DECREASES TO 20-30MM/DAY. ANOTHER TROUGH TO THE EAST
EXTENDS FROM A CLOSED LOW OVER EASTERN BOLIVIA TO MATO GROSSO/
SOUTHERN AMAZONAS IN BRASIL. THIS FAVOR DEEP LAYER CONVECTIVE
INSTABILITY THAT IS ENHANCING CONVECTIVE DEVELOPMENT ACROSS
CENTRAL BRASIL. IN THIS AREA EXPECTING RAINFALL AMOUNTS OF 20-30MM
IN SCATTERED DEEP CONVECTION. THROUGH FRIDAY THIS DECREASES TO
15-20MM/DAY. DURING THE WEEKEND THE TROUGH WILL LIFT ACROSS
TOCANTINS/GOIAS IN BRASIL TO BAHIA...MEANWHILE FAVORING SCATTERED
CONVECTION WITH MAXIMA OF 20-30MM. THIS IS TO ALSO VENT COASTAL
CONVECTION ALONG THE NORTH COAST OF BRASIL. IN THIS AREA ACTIVITY
INCREASES FROM 10-15MM/DAY EARLY IN THE CYCLE TO 20-30MM ON
SATURDAY-SUNDAY.

BALBINO...INMET (BRASIL)
DIAZ...INUMET (URUGUAY)
DAVISON...WPC (USA)
$$





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