Prognostic Meteorological Discussion
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FXSA20 KWBC 011534
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1134 AM EDT TUE SEP 01 2015

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

MODEL DISCUSSION (VALID FROM SEP 01 AT 0000 UTC): GLOBAL MODELS
ARE IN GENERAL AGREEMENT THROUGH 144 HRS...WITH DIFFERENCES ON THE
SOUTHERN STREAM CONFINING TO EXTREME SOUTHERN PORTIONS OF
ARGENTINA AND CHILE. OVERALL CONFIDENCE IN THE FORECAST IS HIGH.

MID LEVEL RIDGE IS TO DOMINATE THE FLOW ACROSS MOST OF SOUTH
AMERICA DURING THE NEXT SEVERAL DAYS...WITH AXIS TO REMAIN WELL
ENTRENCHED ACROSS CENTRAL BRASIL/SOUTHERN BRASIL-EASTERN
BOLIVIA-PARAGUAY TO CENTRAL CHILE/ARGENTINA. UNDER THE BLOCKING
INFLUENCE OF THIS RIDGE...SHORT WAVE PERTURBATIONS ON THE SOUTHERN
STREAM ARE TO CONFINE TO EXTREME SOUTHERN PORTIONS OF
CHILE-PATAGONIA IN ARGENTINA. VARIABILITY AMONG THE MODELS ON THE
INTENSITY OF THE ENSUING MID/UPPER LEVEL FLOW PATTERN LEADS TO
TIMING DIFFERENCES AMONG THE MODELS THAT GROW PROGRESSIVELY LARGE
THROUGH DAY 06. THIS IS THE ONLY AREA WHERE FORECAST CONFIDENCE IS
LOW BEYOND 84-96 HRS. AT LOW LEVELS EXPECTING A RIDGE TO DOMINATE
THE FLOW ACROSS THE SOUTHEAST PACIFIC...WITH PROGRESSIVE FRONTS TO
STREAM ACROSS THE DRAKE PASSAGE TO TIERRA DEL FUEGO/EXTREME
SOUTHERN CHILE AND ARGENTINA DURING THE NEXT SIX DAYS. IN A TIGHT
GRADIENT...EXPECTING STRONG BOUNDARY LAYER WINDS AND SCATTERED
COASTAL CONVECTION TO THE SOUTH OF 50S...WITH DAILY MAXIMA TO PEAK
AT 10-15MM EARLY IN THE CYCLE. THIS DECREASES TO 05-10MM/DAY
THROUGH DAY 03 AND THEREAFTER.

ON THE NORTHERN STREAM...MID LEVEL SHORT WAVE TROUGH IS TO MEANDER
OVER THE EASTERN PACIFIC BETWEEN 105W-80W AND TO THE SOUTH OF 15S.
THIS IS TO REMAIN QUASISTATIONARY THROUGH 96-108 HRS. BY 132-144
HRS HRS...UNDER THE STEERING FLOW OF THE RIDGE TO THE EAST...THE
TROUGH WILL MOVE INLAND ACROSS CENTRAL CHILE TO CUYO IN ARGENTINA.
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.

A BROAD TROUGH TO THE EAST IS TO DOMINATE THE MID/UPPER LEVELS
ACROSS THE SOUTH ATLANTIC. BROAD TROUGH PATTERN IS TO PERSIST
DURING THE NEXT FOUR DAYS. BUT LATER IN THE CYCLE...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
120-132 HRS. GLOBAL MODELS DIVERGE ON HOW INTENSE THIS LOW IS
GOING TO BE...WITH THE UKMET FORECASTING A DEEPER/SLOWER TROUGH TO
FORM. THE MEAN OF THE GLOBAL ENSEMBLES...HOWEVER...TEND TO FAVOR
THE FASTER/WEAKER GFS-ECMWF SOLUTION. AT LOW LEVELS THIS WILL
SUSTAIN AN ELONGATED FRONT THAT IS TO MEANDER 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...WITH DAILY MAXIMA TO PEAK AT 10-15MM.

AT 200 HPA...OVER NORTHERN SOUTH AMERICA...THE SUBTROPICAL RIDGE
EXTENDS ALONG THE COAST OF PERU TO NORTHERN CHILE...THEN EAST
ACROSS MID SECTIONS OF SOUTH AMERICA. AS IT BRANCHES TO THE EAST
THE RIDGE UNDERCUTS A TROUGH TO THE NORTH. THE TROUGH EXTENDS OVER
CENTRAL TO NORTHWEST BRASIL. THIS TROUGH IS TO REMAIN CUT OFF FROM
THE FLOW...AND AS SUCH IT IS FORECAST TO GRADUALLY
WEAKEN/DISSIPATE THROUGH 54-60 HRS. AS IT WEAKENS...A CELL OF THE
SUBEQUATORIAL RIDGE IS TO THEN BUILD WEST ACROSS EQUATORIAL SOUTH
AMERICA. EARLY IN THE CYCLE THE UPPER LEVEL TROUGH IS TO FAVOR
DEEP LAYER CONVECTIVE INSTABILITY ACROSS NORTHWEST SOUTH AMERICA.
CONVECTION ALONG THE CENTRAL-NORTHERN JUNGLE OF PERU TO EASTERN
ECUADOR/SOUTHERN COLOMBIA IS TO THEN PEAK AT 15-30MM. BUT AS THE
TROUGH WEAKENS THIS DECREASES TO 10-15MM BY 48-72 HRS...AND LATER
IN THE CYCLE IT IS TO PEAK AT 05-10MM IN ISOLATED TO SCATTERED
CONVECTION. OTHER CONVECTION IS EXPECTED TO THE SOUTH ACROSS THE
SOUTHERN SIERRA OF PERU TO THE ALTIPLANO OF BOLIVIA. INITIALLY
EXPECT RAINFALL AMOUNTS OF 00-05MM IN ISOLATED CONVECTION. THROUGH
72-96 HRS THIS INCREASES TO 10-15MM.

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

$$




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