Prognostic Meteorological Discussion
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000
FXSA20 KWBC 221538
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1137 AM EDT WED MAR 22 2017

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

MODEL DISCUSSION (VALID FROM MARCH 22 AT 0000 UTC): PATTERN
EVOLUTION DURING MEDIUM RANGE PERIOD CONTINUES TO PUZZLE THE
GLOBAL MODEL GUIDANCE...WITH MODELS DISAGREEING ON HOW A SHORT
WAVE TROUGH ENTERING THE SOUTHERN CONE LATER IN THE PERIOD IS
GOING TO EVOLVE. THE ENSEMBLE MEMBERS ALSO DISAGREE ON THIS
EVOLUTION...AND AS A RESULT OUR CONFIDENCE IN THE FORECAST BEYOND
96 HRS IS LOW.

ON THE SOUTHERN STREAM FLOW...A BROAD/LOW AMPLITUDE TROUGH WILL
DOMINATE THE FLOW OVER THE PACIFIC TO THE SOUTH OF 40S.
PROGRESSIVE MID LEVEL PERTURBATIONS EMBED IN THIS FLOW...WITH ONE
MOVING ACROSS SOUTHERN CHILE TO PATAGONIA IN ARGENTINA LATER
TODAY. THIS SETS THE STAGE FOR A DEEPER/HIGHER AMPLITUDE TROUGH TO
ENTER THE SOUTHERN CONE LATER ON FRIDAY. AS IT PRESSES AGAINST AN
INDUCED RIDGE TO THE EAST...THE LATTER IS EXPECTED TO SPLIT IN
TWO. THE SOUTHERN HALF IS TO THEN RAPIDLY MOVE ACROSS THE DRAKE
PASSAGE TO THE SOUTHWEST ATLANTIC EARLY ON SATURDAY
MORNING...WHILE THE NORTHERN HALF OF THE TROUGH EVOLVES INTO A
CLOSED LOW JUST WEST OF PUERTO MONTT. AT LOW LEVELS...AN ELONGATED
FRONT IS TO STRETCH OVER THE SOUTH ATLANTIC TO PATAGONIA IN
ARGENTINA/SOUTHERN CHILE OVER THE NEXT TWO DAYS... FAVORING LIGHT
CONVECTION TO THE SOUTH OF ISLA DE CHILOE. ON FRIDAY...UNDER
INFLUENCE OF THE HIGHER AMPLITUDE MID LEVEL TROUGH...A
DEEPER/STRONGER BOUNDARY IS TO ENTER THE SOUTHERN CONE. THE TROUGH
IS TO ESTABLISH A SUBTROPICAL CONNECTION AS IT NEARS SOUTHERN
REGIONS OF CHILE. THIS WILL THEN FEED CONVECTIVE DEVELOPMENT ON
FRIDAY WITH MAXIMA OF 15-20MM...WHILE ON SATURDAY THE MAXIMA IS TO
PEAK AT 10-15MM.

A SHORT WAVE TROUGH ON THE NORTHERN STREAM TO THE NORTH MEANDERS
OFF THE CENTRAL COAST OF CHILE. THE MID LEVEL VORTEX IS TO REMAIN
NEARLY STATIONARY OVER THE NEXT COUPLE OF DAYS. AS IT MEANDERS OFF
THE COAST OF CHILE THIS IS TO GRADUALLY SHEAR SHORT WAVE ENERGY
ACROSS THE CENTRAL ANDES INTO THE NORTHERN-CENTRAL PROVINCES IN
ARGENTINA/RIO DE LA PLATA BASIN. THESE WILL THEN ENHANCE DIURNAL
CONVECTION...WITH RAINFALL MAXIMA OF 20-35MM EXPECTED OVER
URUGUAY/ RIO GRANDE DO SUL IN SOUTHERN BRASIL THROUGH THURSDAY
MORNING...AND 10-15MM ON THURSDAY TO FRIDAY. THROUGH SATURDAY
CONVECTION IS TO BECOME MORE ACTIVE/BETTER ORGANIZED...AS A
NORTHERLY LOW LEVEL JET OVER BOLIVIA/PARAGUAY FAVOR A MOIST
ADVECTIVE PATTERN. THE INFLOW OF MID LEVEL ENERGY AND LOW LEVEL
MOISTURE WILL THEN SUSTAIN SCATTERED CONVECTION BETWEEN CORDOBA IN
ARGENTINA AND PARAGUAY...WITH ACCUMULATION OF 20-35MM.

OVER NORTHERN SOUTH AMERICA...AT 200 HPA...BROAD CELL OF THE
SUBTROPICAL RIDGE IS TO MEANDER OVER THE CONTINENT TO THE NORTH OF
25S. DURING THE NEXT FEW DAYS THE RIDGE IS TO ANCHOR ON A CLOSED
HIGH OVER BOLIVIA. THE RIDGE ALOFT WILL VENT ITCZ/NEAR EQUATORIAL
TROUGH RELATED CONVECTION TO THE NORTH OF 10S...WITH MOST ACTIVE
CLUSTERING ACROSS PARA-AMAZONAS AND THE NORTH COAST OF BRASIL.
ACROSS PARA-AMAZONAS THE DAILY MAXIMA WILL RANGE BETWEEN
25-50MM...WHILE OVER THE NORTH COAST OF BRASIL IS TO GENERALLY
RANGE BETWEEN 20-35MM/DAY. HIGHER AMOUNTS...HOWEVER...ARE EXPECTED
NEAR THE AMAZON RIVER DELTA REGION/AMAPA IN BRASIL...WHERE THE
MAXIMA IS TO PEAK AT 30-60MM OVER THE NEXT COUPLE OF DAYS. THE
RIDGE ALOFT IS TO ALSO VENT DEEP CONVECTION ALONG THE NORTH COAST
OF PERU AND SOUTHWEST ECUADOR...WHERE UNDER INFLUENCE OF THE
EASTERN PACIFIC ITCZ EXPECTING HEAVY CONVECTION OVER THE NEXT
COUPLE OF DAYS TO TRIGGER RAINFALL MAXIMA OF 50-100MM/DAY. ON
FRIDAY IT DECREASES TO 35-70MM...WHILE ON SATURDAY THE MAXIMA IS
TO PEAK AT 25-50MM. OTHER CONVECTION IS EXPECTED ON THE SIERRA OF
PERU AND ALONG THE CENTRAL COAST NEAR LIMA...WITH MAXIMA OF
15-20MM/DAY OVER THE COURSE OF THE NEXT FEW DAYS.

SOUTH OF 10S...MEANWHILE...AN OLD FRONT OVER THE ATLANTIC IS TO
EXTEND ACROSS SOUTHEAST BRASIL TO MATO GROSSO DO SUL/PARAGUAY.
UNDER INFLUENCE OF A SUBTROPICAL JET ALOFT THE BOUNDARY IS TO
RETAIN ITS BAROCLINICITY DURING THE NEXT FOUR TO FIVE DAYS.
THIS...IN-TURN...IS TO BLOCK MOISTURE TRANSPORT TO MID SECTIONS OF
SOUTH AMERICA. ACROSS ESPIRITO SANTO/MINAS GERAIS AND MATO GROSSO
THIS IS TO FAVOR SCATTERED CONVECTION WITH ACCUMULATION OF
20-30MM. THROUGH SATURDAY THIS DECREASES TO 10-15MM. OTHER
CONVECTION IS TO CLUSTER ALONG THE TRAILING END OF THE FRONT OVER
PARAGUAY/NORTHWEST ARGENTINA-SOUTHERN BOLIVIA...WITH MAXIMA OF
20-40MM/DAY OVER THE NEXT TWO DAYS.

ABDEMUR...FAA (ARGENTINA)
TEPES...DMC (CHILE)
DAVISON...WPC (USA)
$$




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