Prognostic Meteorological Discussion
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FXSA20 KWBC 241813
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
112 PM EST FRI FEB 24 2017

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

MODEL DISCUSSION (VALID FROM FEBRUARY 24 AT 0000 UTC): GLOBAL
MODELS AND ENSEMBLE MEMBERS ARE IN AGREEMENT THROUGH 84-96 HRS.
FORECAST CONFIDENCE DECREASES GRADUALLY AFTER TO THE SOUTH OF 40S.
YET...THEY ALL AGREE ON THE LONG WAVE PATTERN THAT RESOLVES A
LARGE TROUGH OVER THE BELLINGSHAUSEN SEA BY WEDNESDAY...AND A
RIDGE EXITING INTO THE ATLANTIC TO THE EAST.

IN CHILE...THE MOST REMARKABLE EVOLUTION THROUGH THE CYCLE WILL BE
THE ACTIVE CONVECTION ALONG THE ANDEAN MOUNTAIN RANGE. THERE ARE
MAINLY THREE FACTORS ENHANCING THE ACTIVITY.
FIRST...EAST/NORTHEASTERLY WINDS IN THE ARGENTINEAN SIDE OF THE
MOUNTAIN RANGE...WHICH ARE ADVECTING AN UNSTABLE AIR MASS AND
ENHANCING OROGRAPHIC LIFT ALONG THE EASTERN FLANKS OF THE ANDES.
ALSO...A SHORT WAVE UPPER LOW WILL BE MEANDERING OFF THE COAST OF
CENTRAL/NORTHERN CHILE THROUGH THE CYCLE TO ENHANCE EASTERLY
MOISTURE TRANSPORT AT MID-LEVELS. FINALLY...POSITIVE SST ANOMALIES
OFF THE COAST OF BIO BIO ARE LEADING TO HIGH SURFACE DEW POINTS IN
THE CHILEAN SIDE OF THE SOUTHERN ANDES AS WELL. ALTHOUGH STRONGEST
ACTIVITY WILL REMAIN IN THE HIGH CORDILLERA TO PRODUCE MAXIMA IN
THE 15-20MM/DAY RANGE...ESPECIALLY ON SATURDAY-MONDAY...LIGHT
ACCUMULATIONS ARE POSSIBLE IN THE VALLEYS...ESPECIALLY ON SUNDAY
AFTERNOON...WHEN EXPECTING MAXIMA OF 00-05MM/DAY IN MAULE AND
OHIGGINS. ALSO IN CHILE...AN UNSEASONABLY STRONG SUBTROPICAL
MOISTURE CONNECTION WILL ENHANCE ACTIVITY IN AYSEN ON
MONDAY-TUESDAY TO PRODUCE MAXIMA OF 25-50MM/DAY. HEAVY RAINFALL
WILL RELOCATE TO LOS RIOS-LOS LAGOS ON TUESDAY TO PRODUCE
LOCALIZED MAXIMA IN THE SAME RANGE. COLD AIR CUMULUS WILL LEAD TO
SHOWERY CONDITIONS IN REGIONS TO THE SOUTH TO SUSTAIN MAXIMA OF
10-15MM/DAY ON TUESDAY.

IN ARGENTINA...MODELS ARE NOT INITIALIZING THE SURFACE DEW POINTS
CORRECTLY. SURFACE OBSERVATIONS INDICATE DEW POINTS IN THE 24-25C
RANGE IN PARTS OF CENTRAL ARGENTINA...WHILE MODELS SUGGEST 21-22C.
THIS MEANS THAT THE INSTABILITY IS BEING UNDERESTIMATED IN THE
MODEL SOLUTIONS...THUS THE POTENTIAL FOR STRONGER THUNDERSTORMS
AND LOCALLY LARGER ACCUMULATIONS IS LARGER THAN SUGGESTED. THERE
ARE NO SIGNIFICANT FRONTS ENTERING THE REGION THROUGH THE FORECAST
CYCLE...SO MOST OF THE ACTIVITY WILL BE MODULATED BY WEAK FRONTS
THAT WILL MAKE IT INTO NORTHERN PATAGONIA...AND SHORT WAVE UPPER
TROUGHS. A REGION THAT IS FORECAST TO REMAIN ACTIVE THROUGH THE
CYCLE IS THE CORDILLERA/INTERIOR PORTIONS OF CENTRAL AND NORTHERN
ARGENTINA...WHERE WEAK NORTHEASTERLY WINDS WILL ENHANCE LOW-LEVEL
MOISTURE CONVERGENCE THROUGH THE CYCLE AND STIMULATE DIURNAL
THUNDERSTORMS. A PEAK IN CONVECTION IS EXPECTED ON FRIDAY WHERE
THUNDERSTORMS ARE TO PRODUCE MAXIMA OF 20-35MM/DAY BETWEEN LA
PAMPA AND THE SOUTHERN BUENOS AIRES PROVINCE. DIURNAL
THUNDERSTORMS ARE ALSO EXPECTED ON SATURDAY BUT THEY WILL PRODUCE
LESSER AMOUNTS. THE NEXT PEAK IN ACTIVITY WILL OCCUR ON
SUNDAY-MONDAY WHERE ENHANCED MOISTURE CONVERGENCE AHEAD OF A
LOW-LEVEL JET WILL LEAD TO MAXIMA OF 30-60MM/DAY IN NORTHERN
PATAGONIA-LA PAMPA-SOUTHERN BUENOS AIRES PROVINCE. BY MONDAY
EXPECTING MAXIMA OF 20-35MM/DAY IN THIS REGION. BY TUESDAY
EXPECTING MAXIMA OF 15-20MM/DAY STRETCHING FROM CORDOBA SOUTHEAST
INTO THE BUENOS AIRES PROVINCE/RIO DE LA PLATA. ANOTHER REGION
WHERE CONVECTION WILL BE ENHANCED...UNDER VENTILATION FROM THE
BOLIVIAN HIGH WILL BE NORTHWESTERN ARGENTINA. EXPECTING A PEAK IN
ACTIVITY BETWEEN SATURDAY AND MONDAY EVENING...WHERE DIURNAL
THUNDERSTORMS ARE TO PRODUCE LOCALIZED MAXIMA OF 20-35MM.

CONVECTION IN SOUTHEASTERN BRASIL IS TO GRADUALLY ACTIVATE DUE TO
THE INFLUENCE OF THE STRENGTHENING BOLIVIAN HIGH/SUBTROPICAL RIDGE
TO THE WEST. THIS WILL UNDULATE/DEEPEN AN UPPER TROUGH THAT WILL
EXTEND ACROSS SAO PAULO/PARANA BY SUNDAY-MONDAY...LYING IN THE
EASTERN PERIPHERY OF THE SUBTROPICAL RIDGE. IN INTERACTION WITH AN
UNSTABLE AIR MASS TO ITS EAST/NORTHEAST...EXPECTING A GRADUAL
INCREASE IN DIURNAL CONVECTION IN THE REGION. MODELS ARE
SUGGESTING MORE CLEARLY THE FORMATION OF THE SOUTH ATLANTIC
CONVERGENCE ZONE BY MONDAY. BY MONDAY EVENING...IT WILL EXTEND
ACROSS RIO DE JANEIRO-NORTHERN SAO PAULO-SOUTHERN MATO
GROSSO...AND BY TUESDAY IT WILL EXTEND ACROSS RIO DE
JANEIRO...CENTRAL MATO GROSSO...RONDONIA.  EXPECTING A GRADUAL
INCREASE IN MAXIMA FROM 15-30MM/DAY ON FRIDAY TO 35-70MM/DAY BY
SUNDAY. ACTIVITY WILL PEAK ON MONDAY AND TUESDAY TO START
DECREASING AFTER. DURING THIS PERIOD...EXPECTING MAXIMA IN THE
40-80MM/DAY RANGE WITH THE RISK OF MCS FORMATION.

NORTH ACROSS THE CONTINENT...VERY ACTIVE CONVECTION CONTINUES IN
THE WESTERN AMAZON. A SURFACE AND MID-LEVEL LOW CONTINUES SLOWLY
RETROGRESSING ACROSS ACRE/CENTRAL PERU. THIS LOW IS MODULATING THE
EASTERLY TRADES AND PRODUCING SEVERAL REGIONS OF LOW-LEVEL
MOISTURE CONVERGENCE IN THE REGION. AIDED BY UPPER VENTILATION IN
THE NORTHERN PERIPHERY OF THE SUBTROPICAL RIDGE...EXPECTING DAILY
RAINFALL TOTALS IN THE ORDER OF 30-70MM/DAY THROUGH SUNDAY.
LARGEST ACCUMULATIONS ARE TO RELOCATE TO THE NORTH INTO SOUTHERN
COLOMBIA/NORTHERN BRASIL FROM SUNDAY THROUGH TUESDAY.

ALSO IN TROPICAL SOUTH AMERICA...SEASONALLY ACTIVE CONVECTION WILL
CONTINUE ACROSS THE ALTIPLANO AND PERUVIAN/ECUADORIAN ANDES.
ACTIVITY IS FAVORED BY VERY MOIST MID-LEVELS AIDED BY ACTIVE
AMAZONIAN CONVECTION...BY PLENTIFUL MOISTURE IN THE PACIFIC BASIN
IN ASSOCIATION WITH ABOVE-AVERAGE SEA SURFACE TEMPERATURES...AND
ENHANCED UPPER VENTILATION IN ASSOCIATION WITH THE BOLIVIAN
HIGH/SUBTROPICAL RIDGE. EXPECTING RAINFALL ACCUMULATIONS IN THE
ORDER OF 15-30MM/DAY. ANOTHER REGION WITH ACTIVE CONVECTION WILL
CONTINUE BEING THE NORTHERN COAST OF PERU/SOUTHERN ECUADOR...WHERE
ENHANCED CONVECTIVE INSTABILITY IN ASSOCIATION WITH SEA SURFACE
TEMPERATURES EXCEEDING 28C...AND THE ACTIVE SECOND BAND OF THE
ITCZ WILL LEAD TO STRONG EVENING THUNDERSTORMS WITH ACCUMULATIONS
EXCEEDING 50MM ON A QUASI-DAILY BASIS.

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





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