Prognostic Meteorological Discussion
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FXSA20 KWBC 211618
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1217 PM EDT THU SEP 21 2017

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

MODEL DISCUSSION (VALID FROM SEP 21 AT 0000 UTC): GLOBAL MODELS
HAVE DECREASED THEIR CONFIDENCE ON THE FORECAST AFTER 96-108 HRS
TO THE SOUTH OF 40S. ALSO...THEY ARE STRUGGLING RESOLVING AN EVENT
OF STRONG CONVECTION EXPECTED FOR NORTHEASTERN ARGENTINA AND
URUGUAY BY 114-132 HRS. ALTHOUGH THERE SIGNAL FOR A PEAK IN
CONVECTION AND ISOLATED SEVERE POTENTIAL DURING THIS PERIOD...THE
CONFIDENCE ON THE DETAILS IS LOW.

AS AN UPPER TROUGH CROSSES THE ANDES OF CHILE AND ARGENTINA ON
THURSDAY...IT WILL CONTINUE ENHANCING PRECIPITATION IN THE RIO DE
LA PLATA BASIN. NOTE THAT THE ACTIVITY IS RESPONDING MORE TO THE
FORCING PROVIDED BY THE DIVERGENT ENTRANCE OF AN UPPER JET LOCATED
TO THE SOUTH OF THE RIO DE LA PLATA...OVER THE COLD SIDE OF THE
SURFACE FRONT THAT IS FORECAST TO EXTEND ACROSS RIO GRANDE DO
SUL...CENTRAL PARAGUAY...EXTREME SOUTHERN BOLIVIA BY THURSDAY
EVENING. THROUGH EARLY FRIDAY...NUMEROUS SHOWERS WILL PRODUCE
MAXIMA OF 15-20MM/DAY THAT STRETCHES FROM SOUTHERN
CORDOBA/SOUTHERN SANTA FE INTO SOUTHERN URUGUAY AND THE NORTHERN
BUENOS AIRES PROVINCE. A SECOND TROUGH IS FORECAST TO CROSS THE
ANDES EARLY ON FRIDAY. THIS WILL PROVIDE ADDITIONAL ENHANCEMENT TO
STIMULATE AN INCREASE IN ACCUMULATIONS ON FRIDAY-SATURDAY TO
MAXIMA OF 20-30MM/DAY BETWEEN CORRIENTES-NORTHERN URUGUAY-SOUTHERN
RIO GRANDE DO SUL. BY SATURDAY-SUNDAY...ACCUMULATIONS WILL
DECREASE TO MAXIMA OF 10-15MM/DAY...WHILE EXTENDING FROM PARAGUAY
TO MISIONES-ARGENTINA...TO SANTA CATARINA AND NORTHERN RIO GRANDE
DO SUL. BY SUNDAY-SUNDAY...ONSHORE FLOW ALONG THE SERRA DO MAR OF
PARANA AND SANTA CATARINA WILL LEAD TO ACCUMULATIONS OF
10-15MM/DAY. OTHERWISE EXPECTING MAXIMA OF 05-10MM/DAY.

IN THE MEANTIME...AS THE FRONT WEAKENS...EXPECTING WARMING AND
MOISTURE RETURN INTO NORTHERN ARGENTINA DURING SUNDAY/MONDAY. THE
WARMING AND MOISTENING WILL ENHANCE INSTABILITY BY MONDAY...WHICH
WILL RESULT IN A MARGINAL SEVERE POTENTIAL DURING MONDAY AFTERNOON
AS A TROUGH CROSSES THE ANDES. MODELS ARE SUGGESTING LIFTED INDEX
VALUES BELOW -6C...AND OTHER STABILITY INDICES LOOK FAVORABLE AS
WELL. THE CONFIDENCE ON THE DYNAMICS WITH THIS EVENT IS STILL
LOW...THUS WE ARE SUGGESTING ACCUMULATIONS OF 20-40MM/DAY WITH A
MARGINAL RISK OF SEVERE CONVECTION.

ALSO IN SOUTHERN SOUTH AMERICA...THE ARRIVAL OF AN UPPER TROUGH
EARLY ON SATURDAY INTO SOUTHERN CHILE...WILL INTERACT WITH A TRAIN
OF FRONTS TI STIMULATE PRECIPITATION TO THE SOUTH OF TEMUCO.
LARGEST ACCUMULATIONS ARE EXPECTED BETWEEN SOUTHERN CHILOE INTO
NORTHERN MAGALLANES...WHERE EXPECTING MAXIMA OF 10-15MM/DAY AND
VERY ISOLATED HIGHER AMOUNTS IN REGIONS OF OROGRAPHIC UPLIFT. TO
THE SOUTH EXPECTING MAXIMA OF 05-10MM/DAY. THE TROUGH WILL BE
ACCOMPANIED BY SIGNIFICANT COLD AIR ADVECTION IN ITS REAR FLANK.
THE COOLING ON SUNDAY INTO EARLY MONDAY WILL BE SUFFICIENT TO
SUSTAIN SNOWFALL AT LOWER ELEVATIONS IN SOUTHWESTERN MAGALLANES.
ON SUNDAY-EARLY MONDAY...EXPECTING SNOWFALL ACCUMULATIONS OF
03-06CM IN WESTERN AND SOUTHERN MAGALLANES-CHILE AND TIERRA DEL
FUEGO. BETWEEN COASTAL NORTHERN MAGALLANES AND THE TAITAO
PENINSULA IN CHILE...EXPECTING A MIX OF RAIN AND SHOW WITH LIQUID
ACCUMULATIONS OF 05-10MM. TO THE NORTH INTO
VALDIVIA/OSORNO...EXPECTING RAINFALL ACCUMULATIONS OF 05-10MM.

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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