Prognostic Meteorological Discussion
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FXSA20 KWBC 201559
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1158 AM EDT WED SEP 20 2017

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

MODEL DISCUSSION (VALID FROM SEP 20 AT 0000 UTC): GLOBAL MODELS
CONTINUE IN GOOD AGREEMENT THROUGHOUT THE SHORT RANGE PERIOD. THEY
STRUGGLE WITH THE EVOLUTION OF SHORT WAVE PERTURBATIONS AFTER
114-120 HRS. THIS IS ESPECIALLY TRUE WITH THE GFS SOLUTION OF AN
UPPER TROUGH ENTERING EXTREME SOUTHERN CHILE AND SOUTHERN
ARGENTINA BETWEEN 120 AND 144 HRS.

THE 500 HPA CROSSING THE ANDES OF CHILE AND ARGENTINA IS SPLITTING
IN TWO. THE SOUTHERN TROUGH WILL EXIT INTO THE ATLANTIC DURING
WEDNESDAY...WHILE IS PRESSES INTO A RIDGE THAT EXTENDS FROM
SOUTHERN PERU ACROSS NORTHERN ARGENTINA/PARAGUAY INTO SOUTHERN
BRASIL. THIS WILL ENHANCE THE UPPER DIVERGENT PATTERN ACROSS
CENTRAL ARGENTINA WHICH WILL RESULT IN STIMULATED PRECIPITATION.
AT LOW LEVELS...THE TROUGH SUSTAINS AN ELONGATED FRONT THAT IS
FORECAST TO EXTEND ACROSS CENTRAL URUGUAY-ENTRE RIOS-SANTIAGO DEL
ESTERO BY WEDNESDAY EVENING...ACCOMPANIED BY A SHORT-LIVED PAMPERO
JET THAT IS FORECAST TO WEAKEN EARLY ON THURSDAY. BY THURSDAY
EVENING...THE FRONT EXTENDS ACROSS RIO GRANDE DO
SUL-CORRIENTES-WESTERN PARAGUAY INTO SOUTHERN BOLIVIA...WHEN IT
WILL BECOME NEARLY STATIONARY. MOST OF THE PRECIPITATION WITH THIS
SYSTEM WILL OCCUR OVER THE COOL AIR MASS...TO THE SOUTH OF THE
FRONT...AS THE MAIN MECHANISM TRIGGERING CONVECTION WILL BE THE
INFLUENCE OF THE UPPER JETS. THROUGH EARLY THURSDAY...POST-FRONTAL
CONVECTION WILL PRODUCE ACCUMULATIONS OF 20-30MM/DAY THROUGH EARLY
THURSDAY. ACCUMULATIONS ARE TO DECREASE TO MAXIMA OF 15-20MM/DAY
FROM THURSDAY THROUGH EARLY FRIDAY...EXTENDING FROM CORDOBA/SANTA
FE INTO SOUTHERN URUGUAY. DURING FRIDAY...A SHORT WAVE TROUGH
FORECAST TO LAG WEST OF THE ANDES IS EXPECTED TO CROSS INTO
NORTHEASTERN ARGENTINA. THIS WILL PROVIDE ADDITIONAL FORCING FOR
AN INCREASE IN RAINFALL AMOUNTS. BY FRIDAY-EARLY
SATURDAY...EXPECTING ACCUMULATIONS OF 20-30MM/DAY. POST FRONTAL
ACTIVITY WILL LEAD TO MAXIMA OF 10-15MM/DAY IN SOUTHERN BOLIVIA
AND EXTREME WESTERN PARAGUAY. BY SATURDAY-SUNDAY...EXPECTING
LINGERING SHOWERS TO PRODUCE ISOLATED ACCUMULATIONS OF 10-15MM/DAY
ACROSS MOST OF PARAGUAY-MISIONES-SANTA CATARINA IN BRASIL. BY
SUNDAY-MONDAY...CONVECTION WILL FOCUS IN ACROSS SANTA
CATARINA-PARAGUAY-MATO GROSSO DO SUL...WHERE EXPECTING ISOLATED
ACCUMULATIONS OF 10-15MM/DAY.

ALSO IN SOUTHERN SOUTH AMERICA...A TRAIN OF FRONTS WILL ARRIVE
INTO SOUTHERN CHILE ON WEDNESDAY. THIS WILL ENHANCE MOISTURE
CONVERGENCE IN AYSEN AND MAGALLANES TO PRODUCE ACCUMULATIONS OF
05-10MM/DAY.  ACCUMULATIONS ARE TO PEAK ON THURSDAY-FRIDAY...WHEN
EXPECTING MAXIMA OF 10-15MM/DAY BETWEEN VALDIVIA AND ISLA
CHILOE...AIDED BY WEAK UPPER TROUGH CROSSING THE SOUTHERN ANDES.
TO THE SOUTH OF ISLA CHILOE EXPECTING MAXIMA OF 05-10MM/DAY. A NEW
PEAK IN PRECIPITATION IS EXPECTED ON SATURDAY-SUNDAY...AS A ROBUST
UPPER TROUGH PRODUCES HEIGHT FALLS EXCEEDING 200 GPM TO THE SOUTH
OF PUERTO MONTT...AND A WEAK SUBTROPICAL MOISTURE CONNECTION
ENHANCES MOISTURE CONVERGENCE. THIS WILL LEAD TO MAXIMA IN THE
ORDER OF 10-15MM/DAY FROM ISLA CHILOE SOUTH INTO NORTHERN
MAGALLANES/AYSEN...ALTHOUGH VERY ISOLATED HIGHER AMOUNTS ARE
POSSIBLE IN REGIONS OF OROGRAPHIC FORCING.

NORTH ACROSS THE CONTINENT...A CLOSED LOW HAS FORMED OVER MINAS
GERAIS/GOIAS/EASTERN MATO GROSSO IN BRASIL. THIS UPPER LOW HAS A
SIGNATURE IN THE MID-TROPOSPHERE...WHICH IS LEADING TO
DESTABILIZATION OF THE LOW-LEVELS. ALTHOUGH MOISTURE IS
LIMITED...IT WILL BE SUFFICIENT TO TRIGGER ISOLATED SHOWERS AND
THUNDERSTORMS ON A DAILY BASIS THROUGH FRIDAY...WHEN THE TROUGH IS
FORECAST TO EXIT EAST INTO THE ATLANTIC. MOST WIDESPREAD ACTIVITY
IS EXPECTED ON THURSDAY AND FRIDAY...WHEN ACCUMULATIONS WILL RANGE
FROM 00-05MM/DAY.

MORE ACTIVE CONVECTION IS EXPECTED IN NORTHWESTERN SOUTH
AMERICA...WHERE... VENTILATION FROM THE SUBTROPICAL
RIDGE...MOISTURE ADVECTION FROM THE ATLANTIC...AND GRADUAL
DESTABILIZATION THROUGH THE CYCLE...WILL LEAD TO AN INCREASE IN
THE COVERAGE OF CONVECTION AND IN ACCUMULATIONS AS THE CYCLE
PROGRESSES. INITIALLY...ACTIVITY WILL BE CONSTRAINED TO THE AMAZON
DELTA AND EXTREME NORTHERN BRASIL-COLOMBIA WITH MAXIMA UNDER
15MM/DAY. BY FRIDAY AND THROUGH MONDAY...DIURNAL CONVECTION IN THE
NORTHWESTERN AMAZON WILL LEAD TO MAXIMA OF 20-30MM/DAY ON A DAILY
BASIS. DIURNAL CONVECTION IS ALSO FORECAST TO INCREASE IN THE
PERUVIAN ANDES/SELVA ALTA INTO THE WEEKEND...WHERE EXPECTING
MAXIMA UNDER 15MM/DAY.

GONZALEZ...DMC (CHILE)
VANNUCCI...SMN (ARGENTINA)
DAVISON/GALVEZ...WPC (USA)
$$





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