THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


SOUTH AMERICAN MODEL DISCUSSION




SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
333 PM EST THU DEC 13 2018

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

MODEL DISCUSSION FROM DEC 13/00 UTC: GLOBAL MODELS AND ENSEMBLE
MEMBERS ARE IN GOOD AGREEMENT ON THE EVOLUTION OF THE FORECAST
THROUGH 108-120 HRS. CONFIDENCE STARTS DECREASING GRADUALLY AFTER
THIS PERIOD.

AS EXPECTED FOR DECEMBER...THE WESTERLIES IN SOUTHERN SOUTH
AMERICA ARE CONSTRAINING TO LATITUDES TO THE SOUTH OF 50S. THIS
WILL CONFINE PRECIPITATION TO AREAS TO THE SOUTH OF SOUTHERN AYSEN
IN CHILE. YET...INITIALLY...AS A POTENT UPPER TROUGH CROSSES THE
ANDES INTO ARGENTINA...IT WILL PROVIDE SUFFICIENT DYNAMIC FORCING
TO SUSTAIN PRECIPITATION BETWEEN ARAUCANIA AND LOS LAGOS. THIS
WILL BE LOCALLY REINFORCED BY A WEAKENING COLD FRONT. EXPECTING
MAXIMUM ACCUMULATIONS OF 10-15MM/DAY THROUGH EARLY FRIDAY.
OTHERWISE...WESTERLY WIND CONVERGENCE WILL SUSTAIN ISOLATED MAXIMA
OF 10-15MM/DAY IN EXTREME SOUTHERN CHILE. A SHORT WAVE TROUGH
ARRIVES ON TUESDAY...WHICH WILL SUSTAIN MAXIMA OF 10-15MM/DAY
EXTENDING INTO SOUTHERN AYSEN.

TO THE EAST OF THE ANDES...AS THE AFOREMENTIONED POTENT UPPER
TROUGH CROSSES ON THURSDAY...IT WILL INTERACT WITH A MOIST AND
UNSTABLE AIR MASS SITTING IN CENTRAL/NORTHEASTERN ARGENTINA. THIS
WILL TRIGGER ONCE AGAIN ANOTHER ROUND OF SEVERE CONVECTION AND
POTENTIAL OF MCS FORMATION ON THURSDAY. LARGEST ACCUMULATIONS ARE
EXPECTED IN NOCTURNAL CONVECTION BETWEEN CORDOBA/SANTA FE/ENTRE
RIOS INTO WESTERN URUGUAY. IN THESE LOCATIONS EXPECTING MAXIMA OF
100-150MM/DAY. YET...LOCALLY HEAVY RAINFALL IS EXPECTED IN THE
REST OF URUGUAY AND THE BUENOS AIRES PROVINCE TO SUSTAIN MAXIMA OF
25-50MM/DAY. FURTHER SOUTH...IN ASSOCIATION WITH THE FORMATION OF
THE OCCLUDED LOW...EXPECTING MAXIMA OF 20-40MM/DAY. BY
FRIDAY...THE SURFACE FRONT WILL EXTEND ACROSS NORTHERN
URUGUAY...CORRIENTES INTO SALTA. MODELS AGREE ON STRONG 10-15KT
EASTERLIES DEVELOPING AT 850 HPA. THESE WILL ENHANCE LOW-LEVEL
MOISTURE CONVERGENCE ALONG SALTA/JUJUY TO SUSTAIN MAXIMA OF
30-60MM/DAY. ELSEWHERE...CONVECTION WILL ORGANIZE IN ASSOCIATION
WITH THE SURFACE FRONT TO SUSTAIN MAXIMA OF 20-40MM. BY
SATURDAY...THE FRONT WILL START RETURNING AS A WARM FRONT ACROSS
EXTREME SOUTHERN BRASIL AND NORTHERN ARGENTINA. EXPECTING
ACCUMULATIONS OF 20-35MM/DAY IN MOST OF NORTHERN ARGENTINA. BY
SUNDAY...AS A POLAR TROUGH CROSSES THE SOUTHERN ANDES AND
ASSOCIATED FRONTS APPROACH CENTRAL ARGENTINA...EXPECTING ONCE
AGAIN AN INCREASE IN CONVECTION. THIS WILL LEAD TO ACCUMULATIONS
OF 25-50MM/DAY. A FURTHER INCREASE IS EXPECTED BY MONDAY-TUESDAY
WHEN EXPECTING MAXIMA OF 35-70MM/DAY ALONG THE FRONT...INCLUDING A
SLIGHT RISK FOR SEVERE CONVECTION.

IN TROPICAL SOUTH AMERICA...THE SUBTROPICAL RIDGE/BOLIVIAN HIGH
STARTS CENTERING OVER THE NORTHERN ALTIPLANO...WHILE A DEEP UPPER
LOW CENTER OVER NORTHWEST MINAS GERAIS IN BRASIL. MODELS ARE
SUGGESTING THAT THE UPPER PATTERN WILL CHANGE VERY LITTLE THROUGH
THE CYCLE. AT LOW-LEVELS...STRONGEST CONVECTION IS ORGANIZING WITH
THE NEAR EQUATORIAL TROUGH. INITIALLY...THE ENVIRONMENT MOST
FAVORABLE FOR LARGE ACCUMULATIONS WILL BE NORTHERN BRASIL...WHERE
EXPECTING MAXIMA IN THE ORDER OF 25-50MM/DAY THROUGH SATURDAY.
AFTER THIS PERIOD...AS THE DEEP LAYER MOIST POOL MEANDERS TO THE
WEST...EXPECTING LARGEST ACCUMULATIONS TO ALSO MEANDER WESTWARD.
FROM SATURDAY AND ON...EXPECTING MAXIMA OF 20-40MM/DAY GENERALLY
EXTENDING FROM RONDONIA/AMAZONAS IN BRASIL...WEST INTO MOST OF THE
AMAZON OF ECUADOR...NORTHERN BOLIVIA AND PERU.

ALSO IN TROPICAL SOUTH AMERICA...LOCALIZED ENHANCEMENT OF
CONVECTION  IS EXPECTED IN ASSOCIATION WITH THE UPPER LOW. THIS
WILL TRIGGER ISOLATED MAXIMA OF 15-30MM/DAY IN WESTERN MINAS
GERAIS/GOIAS/EASTERN MATO GROSSO IN BRASIL THROUGH EARLY FRIDAY.
AS THE UPPER LOW WEAKENS...EXPECTING A GRADUAL DECREASE IN THE
STRENGTH OF CONVECTION AND ASSOCIATED ACCUMULATIONS.

ANOTHER REGION OF INTEREST IS THE PACIFIC BASIN OF ECUADOR. THE
PACIFIC ITCZ CONTINUES LOCATED WELL TO THE SOUTH OF ITS
CLIMATOLOGICAL POSITION...AND IS FORECAST TO CONTINUE ENTERING THE
COAST NEAR THE BORDER OF COLOMBIA AND ECUADOR. THE ENVIRONMENT IS
FAVORABLE FOR ANOTHER ROUND OF LOCALLY HEAVY THUNDERSTORMS ON
THURSDAY EVENING...AS WESTERLY WINDS PEAK AT LOW-LEVELS IN A
PRECIPITABLE WATER POOL EXCEEDING 60MM. THIS WILL SUSTAIN
LOCALIZED MAXIMA OF 30-60MM/DAY IN INTERIOR PORTIONS OF NORTHWEST
ECUADOR AND SOUTHWEST COLOMBIA. EXPECTING A DECREASING TREND
AFTER...YET...DIURNAL CONVECTION IS EXPECTED ON A DAILY BASIS
THROUGH THE CYCLE.

IZA...INAMHI (ECUADOR)
MONCADA...DMC (CHILE)
GALVEZ/DAVISON...WPC (USA)




Last Updated: 333 PM EST THU DEC 13 2018