Prognostic Meteorological Discussion
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FXSA20 KWBC 281626
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1226 PM EDT WED SEP 28 2016

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

MODEL DISCUSSION (VALID FROM SEPTEMBER 28 AT 0000 UTC): THE MODELS
MAINTAIN GOOD CONTINUITY WITH PREVIOUS FORECASTS...ONLY MAKING
MINOR CORRECTIONS TO PREVIOUS FORECAST PACKAGE. CONFIDENCE IN THE
FORECAST IS HIGH THROUGH DAY 05.

A DEEPENING MID LEVEL SHORT WAVE TROUGH IS TO RAPIDLY AMPLIFY OVER
THE WESTERN ATLANTIC EARLY IN THE CYCLE. THROUGH TUESDAY AFTERNOON
THE BROAD TROUGH WILL THEN BOUND BETWEEN 55W-20W WHILE REMAINING
TO THE SOUTH OF 20S. AT LOW LEVELS...THIS WILL SUSTAIN A POLAR
FRONT ACROSS URUGUAY-CENTRAL ARGENTINA LATER THIS AFTERNOON. A
1031-1033 HPA POLAR RIDGE TO THE SOUTH WILL THEN DRIVE THIS
BOUNDARY NORTH ACROSS THE SOUTHERN STATES OF BRASIL TO NORTHERN
ARGENTINA/SOUTHEAST PARAGUAY LATER ON THURSDAY. ON FRIDAY TO
SATURDAY THE FRONT WILL COME TO STALL ACROSS RIO DE
JANEIRO/NORTHERN SAO PAULO...WHILE TRAILING OVER PARAGUAY. AS THE
FRONT SURGES ACROSS SOUTHERN TO SOUTHEAST BRASIL...AND UNDER
INFLUENCE OF A SOUTHERLY LOW LEVEL JET OVER THE WESTERN
ATLANTIC...EXPECTING SCATTERED COASTAL CONVECTION BETWEEN NORTHERN
SAO PAULO AND RIO DE JANEIRO TO RESULT IN RAINFALL AMOUNTS OF
05-10MM. OTHER CONVECTION IS EXPECTED ALONG THE FRONT AS IT TRAILS
ACROSS PARAGUAY TO NORTHERN ARGENTINA. ON FRIDAY TO SATURDAY
COUPLED JETS WILL FAVOR A BROAD AREA OF DIVERGENCE THAT IS TO VENT
DEEP CONVECTION. AT LOW LEVELS THIS WILL COMBINE WITH A MOIST
NORTHERLY FLOW THAT IS TO CONVERGE ALONG THE MEANDERING FRONT.
THIS WILL FAVOR MODERATE CONVECTION WITH ACCUMULATION OF 15-30MM.
ON SATURDAY TO SUNDAY...AS A SHORT WAVE TROUGH IN THE NORTHERN
STREAM MOVES ACROSS NORTHERN ARGENTINA...CONDITIONS WILL THEN
BECOME FAVORABLE FOR A MESO SCALE CONVECTIVE SYSTEM (MCS) TO
FORM...WITH POTENTIAL FOR SEVERE THUNDERSTORMS IN HEAVY CONVECTION
TO SUSTAIN MAXIMA OF 30-60MM.

A MID LEVEL SHORT WAVE TROUGH ON THE NORTHERN STREAM FOLLOWS. THIS
IS TO MOVE BETWEEN 80W-90W AND TO THE SOUTH OF 20S LATER
TODAY...REACHING CENTRAL CHILE ON THURSDAY EVENING. THE TROUGH IS
TO THEN MEANDER INLAND WHILE PRESSING AGAINST A RIDGE TO THE EAST.
AS THEY COLLIDE THE TROUGH IS FORECAST TO GRADUALLY
WEAKEN...MOVING OFF THE COAST OF PATAGONIA LATER ON FRIDAY AND TO
NEARLY DISSIPATE AS IT MEANDERS EAST DURING THE DAY ON SATURDAY.
AT LOW LEVELS THIS WILL SUSTAIN A BROAD TROUGH OFF THE CENTRAL
COAST OF CHILE LATER ON WEDNESDAY...WITH AXIS TO MOVE INLAND EARLY
ON THURSDAY. ACROSS CENTRAL AND SOUTHERN CHILE THIS WILL FAVOR
LIGHT RAINFALL AMOUNTS WITH ACCUMULATION OF 05-10MM ON THURSDAY TO
FRIDAY. ACROSS CENTRAL ARGENTINA THE MAXIMA WILL PEAK AT 15-20MM
ON THURSDAY AS THE ASSOCIATED FRONT MEANDERS EAST ACROSS
PATAGONIA-LA PAMPA...WHILE ON FRIDAY THE MAXIMA IS TO PEAK AT
10-15MM OVER SOUTHERN-CENTRAL PORTIONS OF PATAGONIA IN ARGENTINA.

NORTH OVER THE CONTINENT...AT 200 HPA...AN ILL ORGANIZED CELL OF
THE SUBTROPICAL RIDGE DOMINATES AREA TO THE NORTH OF 10S. THIS
PATTERN WILL GENERALLY HOLD NEARLY UNCHANGED DURING THE NEXT FOUR
DAYS...WITH SOME INTENSIFICATION POSSIBLE ON DAY 05...WHEN MODELS
SHOW RIDGE BUILDING ACROSS AMAZONAS IN BRASIL TO NORTHERN BOLIVIA.
THE RIDGE ALOFT IS TO VENT DIURNAL CONVECTION ACROSS NORTHERN
BRASIL-CENTRAL/NORTHERN PERU AND EASTERN ECUADOR. IN THIS AREA
INITIALLY EXPECTING MAXIMA OF 15-30MM/DAY...DECREASING TO
10-15MM/DAY THROUGH FRIDAY. HOWEVER...ON SATURDAY...AS TROPICAL
STORM MATTHEWS MOVES TO THE CENTRAL CARIBBEAN...THIS IS TO ENHANCE
LOW LEVEL MOISTURE CONVERGENCE ACROSS NORTHWEST SOUTH AMERICA.
OVER EASTERN ECUADOR-NORTHERN PERU AND SOUTHERN COLOMBIA MAXIMA
WILL THEN INCREASE TO 40-80MM...WITH LOCALLY HIGHER AMOUNTS LIKELY
ON THE HIGH JUNGLE OF ECUADOR-NORTHERN PERU. OTHER CONVECTION IS
TO CLUSTER ALONG THE COAST OF ECUADOR/WESTERN SLOPES OF THE ANDES
FROM TIME-TO-TIME...AS ENHANCED BY PERIODS OF WESTERLY LOW TO MID
LEVEL FLOW. THIS WILL ENHANCE MOISTURE ADVECTION/CONVERGENCE TO
FAVOR ACCUMULATION OF 15-20MM.

OSORIO...DMC (CHILE)
ALVA...SENAMHI (PERU)
DAVISON...WPC (USA)
$$





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