Prognostic Meteorological Discussion
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000
FXSA20 KWBC 251631
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1231 PM EDT THU MAY 25 2017

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

MODEL DISCUSSION (VALID FROM MAY 25 AT 0000 UTC): GLOBAL MODELS
ARE IN CLOSE AGREEMENT ON FORECAST PATTERN EVOLUTION DURING THE
NEXT SIX DAYS. OVERALL FORECAST CONFIDENCE IS HIGH.

A MEANDERING MID LEVEL TROUGH OFF THE COAST OF CHILE REMAINS THE
SYSTEM OF CONCERN. THE DEEP TROUGH ANCHORS ON A CLOSED LOW NEAR
ARCHIPELAGO DE JUAN FERNANDEZ. AS IT MEANDERS OFF THE COAST OF
CHILE THE MID LEVEL VORTEX IS TO SHEAR SHORT WAVE PERTURBATIONS
ACROSS THE CENTRAL/NORTHERN ANDES INTO NORTHERN
ARGENTINA/PARAGUAY-SOUTHERN STATES OF BRASIL/URUGUAY. THIS IS TO
PROVIDE DYNAMICAL SUPPORT TO ORGANIZED DEEP CONVECTION OVER THE
NEXT 48-72 HRS. AT 250 HPA...THE TROUGH ALOFT IS TO FOCUS A
SUBTROPICAL JET ACROSS THE NORTHERN ANDES OF CHILE/ARGENTINA INTO
MID SECTIONS OF SOUTH AMERICA. THE JET...IN-TURN...WILL SUSTAIN AN
UPPER DIVERGENT PATTERN THAT IS TO VENT ORGANIZED DEEP CONVECTION.
THE TROUGH ALOFT IS TO ALSO FOCUS SUBTROPICAL MOISTURE ACROSS
CENTRAL REGIONS OF CHILE. THIS WILL THEN FEED CONVECTIVE
DEVELOPMENT ACROSS THE CENTRAL ANDES...WITH SCATTERED SNOW SHOWERS
EXPECTED ON ELEVATIONS ABOVE 3000 METERS. OVER THE NEXT DAY OR SO
THIS IS TO RESULT IN ADDITIONAL SNOWFALL AMOUNTS OF 05-10CM.

AT LOW LEVELS...MOISTURE IS CONVERGING ALONG A MEANDERING FRONT
OVER SOUTHERN BRASIL-NORTHERN ARGENTINA. BOUNDARY LAYER DEW POINT
TEMPERATURES TO THE NORTH OF THIS BOUNDARY RANGE BETWEEN 18-21C.
THIS IS HIGH ENOUGH TO SUSTAIN HEAVY RAINFALL AMOUNTS...AND AS IT
COMBINES WITH INFLOW OF MID LEVEL ENERGY WILL SUSTAIN
ENVIRONMENTAL CONDITIONS FAVORABLE AND CONDUCIVE TO GENERATION OF
MESO SCALE CONVECTIVE SYSTEMS DURING THE NEXT 36-48 HRS...WITH
POTENTIAL FOR SEVERE CONVECTION DURING THE SHORT RANGE PERIOD.
ACROSS URUGUAY-MESOPOTAMIA VALLEY IN ARGENTINA AND SOUTHEAST
PARAGUAY THIS IS TO RESULT IN RAINFALL MAXIMA OF 50-100MM THROUGH
FRIDAY MORNING. ON FRIDAY TO SATURDAY FOCUS OF THE HEAVY
CONVECTION SHIFTS TO THE SOUTHERN STATES OF BRASIL...DECREASING
FROM 35-70MM ON FRIDAY TO 30-60MM ON SATURDAY. FURTHERMORE...AS
THE TROUGH ALOFT MOVES ACROSS THE NORTHERN ANDES OF CHILE/SOUTHERN
BOLIVIA IT IS TO ENHANCE CONVECTIVE DEVELOPMENT TO THE
NORTH...WITH ACTIVITY TO FOCUS BETWEEN SOUTHERN JUNGLE OF PERU AND
CENTRAL BOLIVIA ON FRIDAY TO SATURDAY. IN THIS AREA EXPECTING
RAINFALL AMOUNTS OF 15-30MM IN SCATTERED CONVECTION.

ON THE SOUTHERN STREAM FLOW...DEEPENING MID LEVEL TROUGH IS TO
DOMINATE THE FLOW ACROSS THE SOUTHWEST ATLANTIC DURING THE NEXT
COUPLE OF DAYS. THROUGH FRIDAY EVENING THIS IS TO EVOLVE INTO A
BROAD/LONG WAVE TROUGH THAT IS TO EXTEND BETWEEN 60W-20W TO 30S.
AT LOW LEVELS IT WILL SUSTAIN A POLAR FRONT THAT SURGES ACROSS
CENTRAL ARGENTINA LATER TODAY/EARLY FRIDAY MORNING. THE SURGING
FRONT WILL THEN REINFORCE THE OLD FRONTAL BOUNDARY TO THE
NORTH...HELPING SUSTAIN A PERSISTENT BAROCLINIC BOUNDARY ACROSS
MID SECTIONS OF SOUTH AMERICA.

GLOBAL MODELS ARE NOW IN GOOD AGREEMENT ON THEIR FORECAST OF THE
NEXT MID LEVEL VORTEX. THE TROUGH PULLS ACROSS 90W TO 30S LATER ON
FRIDAY...NEARING SOUTHERN/CENTRAL CHILE DURING THE EARLY MORNING
HOURS ON SATURDAY. AXIS IS TO THEN SPLIT AS IT ENTERS THE SOUTHERN
CONE...WITH SOUTHERN HALF TO STREAM ACROSS PATAGONIA IN ARGENTINA
WHILE THE NORTHERN HALF EVOLVES INTO A CLOSED LOW OFF THE CENTRAL
COAST OF CHILE EARLY ON SUNDAY. THIS IS TO THEN SLOWLY SHEAR SHORT
WAVE ENERGY ACROSS CENTRAL/NORTHERN CHILE IN ARGENTINA. VORTICES
WILL THEN INTERACT WITH MEANDERING FRONT OVER SOUTHERN
BRASIL-NORTHERN ARGENTINA/PARAGUAY TO SUSTAIN A SURGE IN
CONVECTION ON MONDAY TO TUESDAY NEXT WEEK.

THE INFLOW OF MID LEVEL VORTICES ACROSS CHILE/ARGENTINA ARE TO
THEN HAVE A DETRIMENTAL EFFECT ON SUBTROPICAL RIDGE PATTERN TO THE
NORTH. AT 200 HPA THE MODELS CONSISTENTLY FORECAST THE RIDGE TO
FLATTEN EARLY ON FRIDAY...WITH AXIS TO THEN CONFINE TO CONTINENTAL
AREA TO THE NORTH OF 20S WHILE CENTERING ON A CLOSED HIGH NEAR
RONDONIA IN BRASIL. AS THE RIDGE PATTERN WEAKENS...DIURNAL
CONVECTION ACROSS BRASIL WILL BECOME MORE SPARSE/LESS ORGANIZED.
MEANWHILE...ACROSS PORTIONS OF PARA AND AMAZONAS EXPECTING DIURNAL
CONVECTION OVER THE NEXT COUPLE OF DAYS TO TRIGGER RAINFALL
AMOUNTS OF 10-15MM/DAY. ON SATURDAY AND ONWARD THIS WILL THEN BE
FOLLOWED BY AN INCREASE IN CONVECTION ALONG THE NORTH COAST. MOST
ACTIVE...HOWEVER...IS EXPECTED OVER NORTHEAST BRASIL...WHERE
MEANDERING FRONT IS TO SUSTAIN MODERATE TO HEAVY CONVECTION DURING
THE NEXT 48-72 HRS. IN THIS AREA EXPECTING RAINFALL AMOUNTS OF
15-30MM/DAY WITH MAXIMA OF 20-45MM.

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




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