Prognostic Meteorological Discussion
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000
FXSA20 KWBC 021544
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1144 AM EDT WED SEP 02 2015

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

MODEL DISCUSSION (VALID FROM SEP 02 AT 0000 UTC): THE GLOBAL
MODELS FOLLOW SIMILAR EVOLUTION THROUGH 108 HRS...THEN DIVERGE ON
THE TIMING/AMPLITUDE OF SHORT WAVE PERTURBATIONS ENTERING THE
SOUTHERN CONE. THE HIGH VARIABILITY AMONG THE DETERMINISTIC MODELS
IS ALSO EVIDENT BY THE ENSEMBLE MEMBERS. AS A RESULT...LONG RANGE
PATTERN CONFIDENCE IS LOW.

CONSISTENT WITH PREVIOUS FORECAST A MID LEVEL RIDGE IS FORECAST TO
DOMINATE THE FLOW ACROSS MOST OF SOUTH AMERICA DURING THE NEXT
SEVERAL DAYS. THE RIDGE IS TO REMAIN WELL ENTRENCHED ACROSS
CENTRAL BRASIL/SOUTHERN BRASIL-EASTERN BOLIVIA-PARAGUAY TO CENTRAL
CHILE/ ARGENTINA. AS THE RIDGE HOLDS OVER THE
CONTINENT...PROGRESSIVE SHORT WAVE PERTURBATIONS IN THE SOUTHERN
STREAM FLOW ARE TO CONFINE TO
EXTREME SOUTHERN PORTIONS OF SOUTH AMERICA AND THE DRAKE PASSAGE.
AT LOW LEVELS A RIDGE WILL DOMINATE THE FLOW ACROSS THE SOUTHEAST
PACIFIC-SOUTHERN SOUTH AMERICA...WITH PROGRESSIVE FRONTS TO STREAM
ACROSS THE DRAKE PASSAGE TO TIERRA DEL FUEGO/EXTREME SOUTHERN
CHILE AND ARGENTINA DURING THE NEXT FEW DAYS. IN A TIGHT
GRADIENT... EXPECTING STRONG BOUNDARY LAYER WINDS AND SCATTERED
COASTAL CONVECTION TO THE SOUTH OF 50S. MOST INTENSE IS EXPECTED
ON DAYS 01 AND 03...WHEN STRONG MID LEVEL PERTURBATIONS ARE TO
PULL ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA.

RIDGE OVER THE CONTINENT CONTINUES TO BLOCK A NORTHERN STREAM
TROUGH OVER THE EASTERN PACIFIC. THE MID LEVEL TROUGH IS TO
MEANDER BETWEEN 100W-85W OVER THE NEXT THREE DAYS. LATER IN THE
CYCLE THE RIDGE OVER THE CONTINENT WEAKENS ENOUGH TO ALLOW THIS
TROUGH TO MIGRATE EAST TOWARDS THE CONTINENT...WITH MID LEVEL AXIS
TO PULL ACROSS CENTRAL/ NORTHERN CHILE TO THE CENTRAL/NORTHERN
PROVINCES IN ARGENTINA BY 108-132 HRS. ALTHOUGH FAVORABLE JET
DYNAMICS AND CYCLONIC VORTICITY ADVECTION ARE TO ACCOMPANY THIS
TROUGH...MOISTURE WILL REMAIN IN SHORT SUPPLY AS IT CROSSES THE
ANDES. AS A RESULT... ONLY EXPECTING LIGHT CONVECTION TO DEVELOP
ON THE CENTRAL ANDES OF CHILE LATER ON DAY 04...INCREASING TO
05-10MM ON DAY 05.

A BROAD TROUGH TO THE EAST DOMINATES THE MID/UPPER LEVELS ACROSS
THE SOUTH ATLANTIC. THE BROAD TROUGH PATTERN IS TO PERSIST DURING
THE NEXT THREE DAYS. AFTERWARDS...AS A SHORT WAVE TROUGH LIFTS
NORTH ALONG THE COAST OF ARGENTINA...A SECONDARY LOW IS TO
GRADUALLY FORM OFF THE SOUTHEAST COAST OF BRASIL BY 96-108 HRS.
GLOBAL MODELS CONTINUE TO DIVERGE ON HOW INTENSE THIS LOW IS GOING
TO BE...WITH THE UKMET FORECASTING A DEEPER/SLOWER TROUGH TO FORM
THAN WHAT THE GFS AND ECMWF SUGGEST. AT LOW LEVELS...THE
ASSOCIATED FRONT EXTENDS WEST ACROSS SOUTHEAST BRASIL TO PARAGUAY.
THIS BOUNDARY IS TO REMAIN NEARLY STATIONARY OVER THE NEXT FEW
DAYS. AS IT MEANDERS OVER SOUTHEAST BRASIL THIS IS TO FAVOR
COASTAL CONVECTION BETWEEN PARANA AND RIO DE JANEIRO...WITH
RAINFALL AMOUNTS OF 05-10MM/DAY. LOCALLY HIGHER
AMOUNTS...HOWEVER...ARE POSSIBLE DUE TO STRONG TOPOGRAPHICAL
FORCING ALONG THE SOUTHERN SIERRA. ACROSS SOUTHEAST PARAGUAY TO
MISIONES IN ARGENTINA INFLOW OF SHORT WAVE ENERGY IS TO GENERALLY
FAVOR SCATTERED CONVECTION. THIS IS NOW FORECAST TO PEAK AT
15-30MM THROUGH 36 HRS...AND BY 72-84 HRS IT IS TO PEAK AT
15-20MM.

AT 200 HPA...A BUILDING CELL OF THE SUBEQUATORIAL RIDGE IS TO
EXPAND ACROSS THE TROPICAL ATLANTIC TO NORTHERN SOUTH AMERICA
THROUGH 36-84 HRS...WHERE IT IS TO THEN HOLD. AS THE RIDGE BUILDS
FROM THE EAST A TROUGH OVER WESTERN BRASIL IS FORECAST TO
GRADUALLY WEAKEN AND DISSIPATE. DIURNAL CONVECTION IS TO GENERALLY
CLUSTER OVER NORTHWEST BRASIL-NORTHERN PERU TO SOUTHERN
COLOMBIA...WITH MAXIMA IN THIS AREA TO PEAK AT 10-15MM/DAY EARLY
IN THE CYCLE. THIS THEN DECREASES TO 05-10MM THROUGH 72-96 HRS.
OTHER CONVECTION TO THE SOUTH IS TO BUILD ALONG THE SOUTHERN
SIERRA OF PERU TO THE ALTIPLANO OF BOLIVIA. INITIALLY EXPECT
RAINFALL AMOUNTS OF 05-10MM IN ISOLATED CONVECTION. THROUGH 72 HRS
THIS INCREASES TO 15-20MM.

ROMERO...DMC (CHILE)
FARINA...DMH (PARAGUAY)
DAVISON...WPC (USA)
GALVEZ...WPC (USA)

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