Prognostic Meteorological Discussion
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FXSA20 KWBC 201618
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1217 PM EDT FRI APR 20 2018

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

MODEL DISCUSSION (VALID FROM APR 20 AT 0000 UTC): MODELS CONTINUE
IN GENERALLY GOOD AGREEMENT THROUGH THE CYCLE. FORECAST CONFIDENCE
DECAYS SLIGHTLY AFTER 84-96 HRS TO THE SOUTH OF 45S...ESPECIALLY
OVER THE PACIFIC AND ATLANTIC. AFTER THIS PERIOD...THE CONFIDENCE
DECAYS VERY GRADUALLY THROUGH 120 HRS.

UNDER PRESENCE OF AN EXTENSIVE LONG WAVE TROUGH APPROACHING THE
SOUTHERN CONE...ACTIVE WEATHER PATTERN IS EXPECTED IN SOUTHERN
CHILE THROUGH THE CYCLE. THROUGH EARLY SATURDAY...EXPECTING A MIX
OF RAIN AND SNOW TO AFFECT AREAS TO THE SOUTH OF THE TAITAO
PENINSULA ACCUMULATING MAXIMA OF 10-15MM/DAY. TO THE
NORTH...EXPECT SIMILAR AMOUNTS IN RAINFALL IN AREAS FROM
TEMUCO/VALDIVIA TO THE SOUTH. A PEAK IN ACCUMULATIONS IS EXPECTED
ON SATURDAY-SUNDAY FROM VALDIVIA INTO NORTHERN AYSEN...AS A
SUBTROPICAL MOISTURE CONNECTION INTERACTS WITH UPPER DYNAMICS TO
SUSTAIN MAXIMA OF 15-25MM/DAY.

ACTIVE PATTERN OD CONVECTION IS TO CONTINUE IN THE RIO DE LA PLATA
BASIN AS MOISTURE AND INSTABILITY REMAIN IN PLACE...AND A COUPLE
OF SHORT WAVE TROUGHS ACCOMPANIED BY UPPER JET DYNAMICS CROSS THE
REGION THROUGH THE CYCLE. ON FRIDAY...A SHORT WAVE TROUGH IS
CROSSING INTO THE ATLANTIC ALONG 56-58W. THIS WILL CONTINUE
SUSTAINING STRONG CONVECTION IN URUGUAY/RIO GRANDE DO SUL THROUGH
EARLY SATURDAY. THIS WILL SUSTAIN MAXIMA OF 20-40MM/DAY WITH THE
RISK OF SEVERE WEATHER/MCS...INCLUDING THE POSSIBILITY OF HAIL. BY
SATURDAY-SUNDAY...REMAINING INSTABILITY IN NORTHERN URUGUAY/ENTRE
RIOS/CORRIENTES AND RIO GRANDE DO SUL WILL SUSTAIN THE DEVELOPMENT
OF A FEW THUNDERSTORMS TO PRODUCE ACCUMULATIONS OF 10-15MM/DAY.
VERY ISOLATED MAXIMA NEAR 20MM IS POSSIBLE. A MORE SIGNIFICANT
WEATHER EVENT IS EXPECTED ON SUNDAY. MODELS CONTINUE AGREEING ON A
PEAK IN SEVERE CONVECTION...AS ENHANCED LOW-LEVEL MOISTURE
CONVERGENCE IN THE EXIT OF THE SOUTH AMERICA LOW-LEVEL JET
INTERACTS WITH AN UPPER TROUGH AND THE TROPICAL ENTRANCE OF AN
UPPER JET. LIFTED INDEX VALUES HAVE DECREASED SLIGHTLY FROM
PREVIOUS RUNS...YET THEY REMAIN CLOSE TO -8. THE FORECAST
CONTINUES TO SHOW MAXIMA OF 40-80MM/DAY WITH THE RISK FOR SEVERE
WEATHER AND MCS FORMATION. UPPER DYNAMIC FORCING WILL GRADUALLY
DECREASE AFTER...YET INSTABILITY AND LOW-LEVEL MOISTURE
CONVERGENCE WILL REMAIN ENHANCED THROUGH WEDNESDAY. BY
MONDAY-TUESDAY EXPECTING MAXIMA OF 30-60MM/DAY IN SOUTHERN SANTA
FE-ENTRE RIOS INTO URUGUAY...WHILE EXPECTING MAXIMA OF 20-40MM/DAY
IN SANTIAGO DEL ESTERO/NORTHERN CORDOBA. BY TUESDAY-WEDNESDAY
EXPECTING A DECREASE TO MAXIMA OF 20-40MM/DAY IN CORRIENTES/ENTRE
RIOS/URUGUAY...AND MAXIMA OF 15-30MM/DAY UPSTREAM.

SEASONALLY ACTIVE PATTERN IS TO ALSO CONTINUE ALONG THE EASTERN
COAST OF BRASIL...UNDER THE INFLUENCE OF A SURFACE FRONT...SHEAR
LINE...AND DEEPENING MID-LEVEL TROUGH. INITIALLY...FRONTAL
CONVERGENCE WILL SUSTAIN A FEW HEAVY THUNDERSTORMS BETWEEN
NORTHERN BAHIA AND ALAGOAS TO PRODUCE MAXIMA OF 30-60MM/DAY THOUGH
EARLY SATURDAY. NORTH...WITH THE SHEAR LINE...EXPECTING MAXIMA OF
20-30MM/DAY CLUSTERING BETWEEN SERGIPE AND RIO GRANDE DO NORTE. BY
SATURDAY-MONDAY...EXPECTING MAXIMA OF 20-40MM/DAY BETWEEN NORTHERN
BAHIA AND PERNAMBUCO...WHILE FRONT AND SHEAR LINE GRADUALLY
WEAKEN. SIMILAR AMOUNTS ARE EXPECTED BY MONDAY-TUESDAY ACROSS
CENTRAL BAHIA-ALAGOAS...AIDED BY TRADE WIND CONVERGENCE. AFTER
THIS PERIOD...UNDER INFLUENCE OF STRENGTHENING UPPER
TROUGH...EXPECTING AN INCREASE IN SOUTHERN BAHIA/ESPIRITO SANTO TO
MAXIMA OF 20-35MM/DAY. NORTH ACROSS THE NORDESTE EXPECTING A
DECREASE TO MAXIMA OF 15-20MM/DAY ALONG COASTAL LOCATIONS.

ALSO IN TROPICAL SOUTH AMERICA...ACTIVITY WILL REMAIN ENHANCED
ALONG THE NET. A SIGNIFICANT FEATURE...EARLY IN THE CYCLE...WILL
BE AN UPPER TROUGH TO EXTEND ACROSS NORTHERN PERU ON FRIDAY
THROUGH SUNDAY. THIS TROUGH IS ENHANCING UPPER DIVERGENCE IN THE
AMAZON BASIN OF NORTHERN PERU AND WESTERN BRASIL...WHERE IT WILL
SUSTAIN MAXIMA OF 30-60MM/DAY ON FRIDAY-SATURDAY...DECREASING
AFTER. IN ECUADOR EXPECTING MAXIMA OF 20-30MM/DAY INITIALLY...AS
UPPER CONVERGENT PATTERN LIMITS THE POTENTIAL FOR EXCESSIVE
AMOUNTS. THIS WILL PEAK ON SATURDAY-SUNDAY WHEN EXPECTING A PEAK
IN OROGRAPHIC MOISTURE CONVERGENCE. ACTIVE CONVECTION ALONG THE
NET WILL SUSTAIN MAXIMA IN THE ORDER OF 25-50MM/DAY THROUGH THE
CYCLE.

BALBINO...INMET (BRASIL)
DIAZ...INUMET (URUGUAY)
GALVEZ...WPC (USA)
$$





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