Prognostic Meteorological Discussion
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FXSA20 KWBC 151752
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1252 PM EST FRI DEC 15 2017

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

MODEL DISCUSSION (VALID FROM DEC 15 AT 0000 UTC): THE GFS AND
ECMWF MAINTAIN GOOD CONTINUITY WITH THE PREVIOUS FORECAST THROUGH
72-84 HRS. THEY THEN MAKE PATTERN CORRECTIONS TO A DEEP TROUGH
REACHING THE SOUTHEAST PACIFIC LATER IN THE CYCLE. IN THIS RUN
THESE MODELS NOW FAVOR A DEEPER/HIGHER AMPLITUDE TROUGH TO ENTER
SOUTHERN CHILE BY 108-120 HRS. THE UKMET IS SLOWLY SHIFTING TO A
MORE AMPLIFIED PATTERN AS INDICATED BY THE OTHER MODELS... THUS
LEADING TO A HIGHER CONFIDENCE IN THE FORECAST THROUGH THE MEDIUM
RANGE PERIOD.

DEEPENING MID LEVEL TROUGH REACHES SOUTHERN/CENTRAL REGIONS OF
CHILE LATER THIS EVENING...MEANWHILE FOCUSING ITS SHORT WAVE
ENERGY OVER THE CONTINENT TO THE SOUTH OF 34S. ON SATURDAY THE
TROUGH MOVES ACROSS PATAGONIA TO THE CENTRAL PROVINCES IN
ARGENTINA...FOCUSING ITS SHORT WAVE ENERGY ACROSS CENTRAL-NORTHERN
ARGENTINA/RIO DE LA PLATA BASIN. ON SUNDAY THE TROUGH IS TO
CONTINUE ACROSS 40W...AND ACCELERATES ACROSS 15W ON MONDAY. AT LOW
LEVELS THIS WILL SUSTAIN A DEEPENING TROUGH ACROSS PATAGONIA IN
ARGENTINA/SOUTHERN CHILE LATER TODAY...WITH A SURFACE FRONT ACROSS
PATAGONIA IN ARGENTINA-SOUTHERN REGIONS OF CHILE. AS THE TROUGH
DEEPENS...AND A POLAR RIDGE BUILDS ACROSS PATAGONIA IN
ARGENTINA...IT IS TO FAVOR A SOUTHERLY PAMPERO LOW LEVEL JET. THIS
DISPLACES THE FRONT NORTH ACROSS THE BUENOS AIRES
PROVINCE-CORDOBA/MENDOZA IN ARGENTINA LATER ON
SATURDAY...MEANWHILE TRIGGERING SQUALLY WEATHER/SEVERE CONVECTION.
IN THIS PERIOD FORECAST HELICITY VALUES SHOW POTENTIAL FOR TORNADO
INDUCING SUPER CELL DEVELOPMENT ACROSS THE BUENOS AIRES
PROVINCE-ENTRE RIOS...WITH LARGE HAIL HIGHLY PROBABLE. IN THIS
AREA THE RAINFALL MAXIMA WILL PEAK AT 40-80MM...WITH LOCALLY
HIGHER HIGHLY POSSIBLE. ON SUNDAY THE SOUTHERLY JET DRIVES THE
FRONT NORTH TO SOUTHERN BRASIL-PARAGUAY. THIS DAY THE
ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR AN MCS TO DEVELOP OVER
CHACO ARGENTINO/PARAGUAYO TO SOUTHERN BOLIVIA...WITH MAXIMA IN
THIS AREA TO PEAK AT 35-70MM. ON MONDAY TO TUESDAY...AS THE FRONT
LOSES ITS UPPER LEVEL SUPPORT...IT IS TO MEANDER SOUTH TO NORTHERN
ARGENTINA.

THE MODELS THEN FORECAST A SHORT WAVE TROUGH ON THE NORTHERN
STREAM TO MOVE ACROSS THE EASTERN PACIFIC TO CENTRAL REGIONS OF
CHILE LATER ON SUNDAY. THE MID LEVEL PERTURBATION WILL GRADUALLY
SPILL ACROSS THE CENTRAL ANDES OF CHILE WHILE SHEARING SHORT WAVE
ENERGY ACROSS CENTRAL PROVINCES IN ARGENTINA. SOME INTERACTION IS
POSSIBLE BETWEEN THIS FEATURE AND THE AFOREMENTIONED FRONT
MEANDERING OVER SOUTHERN BRASIL-NORTHERN ARGENTINA. ON
MONDAY-TUESDAY THIS IS LIKELY TO TRIGGER ORGANIZED CONVECTION
ACROSS THE MESOPOTAMIA VALLEY IN ARGENTINA-NORTHERN URUGUAY TO RIO
GRANDE DO SUL...WITH RAINFALL MAXIMA OF 30-60MM.

LATER IN THE CYCLE...AS PREVIOUSLY INDICATED...PROGRESSIVE MID
LEVEL TROUGH IS TO AMPLIFY BETWEEN 120W-90W TO 35S LATER ON
MONDAY...WITH AXIS PULLING ACROSS 85W/90W TO 30S ON TUESDAY. THE
TROUGH IS TO FOCUS ITS SHORT WAVE ENERGY OVER CHILE/ARGENTINA TO
THE SOUTH OF CONCEPCION...WHILE ALSO SUSTAINING A SUBTROPICAL
CONNECTION AS IT APPROACHES THE SOUTHERN CONE. AT LOW LEVELS THIS
IS TO SUSTAIN A DEEP OCCLUDED LOW OVER THE BELLINGSHAUSEN
SEA/SOUTHEAST PACIFIC ON MONDAY-TUESDAY...WHILE THE ASSOCIATED
FRONT REACHES SOUTHERN CHILE LATER ON MONDAY/EARLY TUESDAY
MORNING. THIS IS TO TRIGGER MODERATE CONVECTION ACROSS SOUTHERN
REGIONS OF CHILE WITH MAXIMA OF 20-30MM.

AT 200 HPA...A CLOSED HIGH OVER NORTHERN BOLIVIA ANCHORS A CELL OF
THE SUBTROPICAL RIDGE ACROSS NORTHWEST SOUTH AMERICA. MODELS
CONTINUE TO FORECAST THE GRADUAL INTENSIFICATION OF THIS RIDGE ON
SUNDAY-MONDAY...WHEN IT BUILDS ACROSS BOLIVIA-MATO GROSSO/MATO
GROSSO DO SUL TO THE SOUTHERN STATES OF BRASIL. EARLY IN THE CYCLE
THE UPPER LEVEL RIDGE IS TO INDUCE THE NORTHWARD AMPLIFICATION OF
A TROUGH ACROSS THE SOUTHEAST STATES OF BRASIL. BUT AS THE RIDGE
BUILDS TO THE SOUTH-SOUTHEAST LATER DURING THE WEEKEND...THE
TROUGH IS TO THEN AMPLIFY ACROSS NORTHEAST BRASIL TO PARA/AMAPA
LATER ON MONDAY-TUESDAY. AS THE UPPER PATTERN EVOLVES...FOCUS OF
THE ORGANIZED CONVECTION SHIFTS FROM NORTHEAST TO CENTRAL BRASIL
ON SATURDAY-SUNDAY...AND TO NORTHWEST BRASIL-PERUVIAN
JUNGLE/SOUTHERN COLOMBIA ON MONDAY-TUESDAY. OVER
TOCANTINS-GOIAS-PARA IN BRASIL INITIALLY EXPECTING MAXIMA OF
35-70MM. THROUGH SUNDAY THIS DECREASES TO 20-30MM...WHILE ACROSS
PARA AND AMAZONAS IT INCREASES FROM 20-35MM EARLY IN THE CYCLE TO
40-80MM ON SATURDAY...RANGING BETWEEN 30-60MM ON SUNDAY TO MONDAY.
ON TUESDAY THIS WANES...DECREASING TO 20-35MM. ACROSS THE
CENTRAL/SOUTHERN JUNGLE OF PERU...A SURGE IN CONVECTION IS
EXPECTED ON TUESDAY...WITH MAXIMA OF 30-60MM.

OVIEDO...SHN (ARGENTINA)
POMIER...SENAMHI (BOLIVIA)
RIVAS...SENAMHI (PERU)
DAVISON/GALVEZ...WPC (USA)
$$




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