Prognostic Meteorological Discussion
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1210 PM EDT WED APR 26 2017

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

MODEL DISCUSSION (VALID FROM APRIL 26 AT 0000 UTC): MINOR
CORRECTIONS TO PREVIOUS FORECAST OVER THE CONTINENT DURING THE
NEXT FOUR DAYS...WHILE OVER THE OCEANS THE MODELS START TO DIVERGE
BY 84-96 HRS. ACROSS THE EASTERN PACIFIC AND WESTERN ATLANTIC THE
MODELS  DIVERGE ON THE SPEED AND AMPLITUDE OF MID LEVEL SHORT WAVE
PERTURBATIONS. THE HIGH VARIABILITY AMONG THE GLOBAL MODELS IS
ALSO EVIDENT AMONG THE ENSEMBLE MEMBERS. THIS...IN-TURN...LEADS TO
LOWER CONFIDENCE IN THE FORECAST DURING THE MEDIUM RANGE PERIOD.

EARLY IN THE CYCLE THE MODELS AGREE ON THE RAPID INTENSIFICATION
OF A POLAR TROUGH OVER THE WESTERN ATLANTIC...WITH AXIS TO ANCHOR
ON A DEEPENING MID LEVEL LOW TO THE EAST-SOUTHEAST OF THE BUENOS
AIRES PROVINCE. THE DEEP TROUGH WILL DOMINATE THE FLOW OVER THE
CONTINENT DURING THE NEXT COUPLE OF DAYS. AT 250 HPA...COUPLED
JETS WILL SUSTAIN THE EXPLOSIVE INTENSIFICATION OF A SURFACE LOW
EAST OF BUENOS AIRES. THIS IS TO DEEPEN FROM A 998 HPA LOW EARLY
IN THE CYCLE TO A 958 HPA LOW LATER THIS EVENING...TO RESULT IN A
METEOROLOGICAL BOMB THAT IS TO SUSTAIN TROPICAL STORM INTENSITY
WINDS OVER THE WESTERN ATLANTIC. THIS WILL COMBINE WITH BUILDING
POLAR RIDGE OVER ARGENTINA TO SUSTAIN A SOUTHERLY/PAMPERO LOW
LEVEL JET ACROSS ARGENTINA INTO MID SECTIONS OF SOUTH AMERICA. THE
STRONG LOW LEVEL WINDS WILL THEN DRIVE A SURFACE FRONT NORTH
ACROSS SAO PAULO-MATO GROSSO IN BRASIL TO NORTHERN BOLIVIA LATER
TODAY. ON THURSDAY THE FRONT MOVES TO ESPIRITO SANTO-MINAS GERAIS
WHILE TRAILING ACROSS BRASIL TO NORTHERN BOLIVIA/SOUTHERN PERU. IT
IS TO THEN REMAIN NEARLY STATIONARY THROUGH SATURDAY...WHEN IT
WILL START TO FRONTOLIZE. ACROSS MATO GROSSO-SAO PAULO THIS WILL
SUSTAIN SCATTERED CONVECTION WITH ACCUMULATION OF 20-35MM THROUGH
THURSDAY MORNING. ON FRIDAY THIS INCREASES TO 20-40MM WHILE
CLUSTERING ACROSS GOIAS-MINAS GERAIS-ESPIRITO SANTO/RIO DE
JANEIRO. HIGHER AMOUNTS ARE EXPECTED TO THE WEST. OVER
BOLIVIA-ACRE/RONDONIA IN BRASIL TO THE SOUTHERN JUNGLE OF PERU THE
SOUTHERLY JET WILL SUSTAIN A SURGE IN CONVECTION DURING THE DAY
TODAY...TO MAXIMA OF 50-100MM. ON THURSDAY THIS SPREADS ACROSS THE
CENTRAL JUNGLE OF PERU WHERE CONDITIONS ARE TO REMAIN FAVORABLE
FOR GENERATION OF MESO SCALE CONVECTIVE SYSTEMS THAT ARE TO
TRIGGER MAXIMA OF 40-80MM. ACROSS NORTHERN PERU-EASTERN ECUADOR
AND SOUTHERN COLOMBIA THIS INCREASES TO 75-150MM ON FRIDAY.

UPSTREAM OVER THE PACIFIC...A BROAD/LOW AMPLITUDE RIDGE IS TO THEN
DOMINATE THE MID/UPPER LEVELS OVER THE FAR EASTERN PACIFIC DURING
THE NEXT THREE DAYS...RELOCATING TO THE SOUTHERN CONE LATER IN THE
CYCLE WHILE YIELDING TO A DEEPENING TROUGH TO THE WEST. THE MODELS
ARE IN AGREEMENT ON GENERAL TRANSITION THROUGH SATURDAY...THEN
DISAGREE ON AMPLITUDE/SPEED OF MID LEVEL PERTURBATIONS LIFTING
ACROSS THE EASTERN PACIFIC. THE GFS FAVORS A SLOW TO EVOLVE BROAD
TROUGH TO MOVE OFF THE COST OF CHILE LATER ON SUNDAY...WHILE THE
EUROPEAN MODELS FORESEE A FASTER EVOLUTION AND A HIGHER AMPLITUDE
TROUGH. ALTHOUGH THE MEAN OF THE GLOBAL ENSEMBLES TENDS TO FAVOR
THE GFS SOLUTION...VARIABILITY IS TOO HIGH TO ESTABLISH CONFIDENCE
IN THE FORECAST. AS THE PATTERN EVOLVES...POLAR FRONTS ARE TO
STREAM ACROSS SOUTHERN CHILE/PATAGONIA IN ARGENTINA DURING THE
NEXT FEW DAYS. THESE ARE TO SUSTAIN A SUBTROPICAL CONNECTION AS
THEY MAKE LANDFALL OVER SOUTHERN CHILE...TRIGGERING MODERATE
CONVECTION WITH MAXIMA OF 20-30MM. THROUGH SATURDAY THIS INCREASES
TO 20-40MM...WITH MOST INTENSE EXPECTED BETWEEN TEMUCO AND ISLA DE
CHILOE.

OVER NORTHERN SOUTH AMERICA...AT 200 HPA...BROAD CELL OF THE
SUBTROPICAL RIDGE WILL DOMINATE THE FLOW TO THE NORTH OF 20S. AS
THE RIDGE HOLDS OVER THE CONTINENT...IT IS TO INDUCE THE NORTHWARD
AMPLIFICATION OF A TROUGH OVER THE TROPICAL ATLANTIC TO THE NORTH
COAST OF BRASIL LATER TODAY. THROUGH FRIDAY THE TROUGH IS TO
MEANDER WEST ACROSS NORTHERN BRASIL TO THE GUIANAS. THE TROUGH
WILL THEN ENHANCE COASTAL CONVECTION OVER NORTHERN BRASIL TO FAVOR
DAILY MAXIMA OF 15-20MM...WHILE TO THE WEST EXPECTING RAINFALL
MAXIMA OF 20-45MM ALONG THE NEAR EQUATORIAL TROUGH OVER
PARA-AMAZONAS/RORAIMA IN BRASIL.

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





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