Prognostic Meteorological Discussion
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FXSA20 KWBC 241708
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
108 PM EDT WED AUG 24 2016

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

MODEL DISCUSSION (VALID FROM AUG 24 AT 0000 UTC): FORECAST
CONFIDENCE CONTINUES HIGH THROUGH THE CYCLE. SIGNIFICANT
DISCREPANCIES START TO APPEAR AFTER 120 HRS. MOSTLY OVER THE
SOUTHEASTERN PACIFIC/SOUTHERN CONE.

STATIONARY FRONT WILL EXTEND ACROSS THE COAST OF BAHIA-BRASIL
THROUGH FRIDAY...WHEN IT WILL LOSE DEFINITION. A SURGE IN THE
EAST-SOUTHEASTERLY TRADES OF 25-35KT AT 850 HPA IS ACCOMPANIED BY
A PRECIPITABLE WATER POOL APPROACHING 50MM ON TUESDAY. THIS WILL
CONTINUE FAVORING ENHANCED MOISTURE CONVERGENCE AND PERSISTENT
SHOWERS THAT ARE TO PRODUCE MODERATE RAINFALL AMOUNTS. LARGEST
ACCUMULATIONS ARE EXPECTED ON WEDNESDAY-EARLY THURSDAY WHEN
EXPECTING MAXIMA OF 25-50MM. AMOUNTS ARE TO START DECREASING ON
THURSDAY...WHEN ACTIVITY WILL PRODUCE MAXIMA OF 15-30MM. ON FRIDAY
EXPECTING MAXIMA OF 15-20MM. CONDITIONS WILL IMPROVE AFTER THIS
PERIOD...YET...SCATTERED TRADE WIND SHOWERS WILL PRODUCE MAXIMA IN
THE ORDER OF 10MM DURING THE WEEKEND.

ALSO IN NORTHERN SOUTH AMERICA...THE SOUTHERN TAIL OF A TROPICAL
WAVE IS PROPAGATING WESTWARD FROM 60W ON TUESDAY...TO 70-72W BY
FRIDAY...TO THEN LOSE DEFINITION. CONVECTION IS ENHANCED ALONG
THIS TROUGH...AND WILL BE FURTHER ENHANCED BY VENTILATION IN THE
BASE OF A SOUTHERN HEMISPHERE UPPER TROUGH THAT WILL RELOCATE FROM
THE AMAZON DELTA/PARA IN BRASIL ON TUESDAY...TO AMAZONAS-BRASIL ON
FRIDAY. EXPECTING MAXIMA OF 15-35MM/DAY IN THE THUNDERSTORMS THAT
DEVELOP ALONG THIS TROUGH AS THEY PROPAGATE WESTWARD ACROSS THE
NORTHERN AMAZON BASIN. A DECREASING TREND IN ACCUMULATIONS AND
COVERAGE IS EXPECTED THROUGH THE CYCLE.

A WEAK SUBTROPICAL MOISTURE CONNECTION IS ARRIVING ON SOUTHERN
CHILE DURING WEDNESDAY. LOW-LEVEL WINDS ARE NOT FAVORABLE FOR
HEAVY RAINFALL AS THEY ARE FORECAST TO REMAIN PARALLEL TO THE
COAST FOR THE MOST PART. A PERIOD OF ENHANCED WESTERLIES IS
EXPECTED AS THE OCCLUDED LOW ARRIVES ON THURSDAY EVENING.
ACCORDINGLY...THE FORECAST RAINFALL AMOUNTS HAVE BEEN DECREASED
FOR WEDNESDAY-THURSDAY TO MAXIMA OF 15-20MM/DAY...AND INCREASED
FOR THURSDAY FRIDAY TO MAXIMA OF 15-30MM/DAY. LARGEST
ACCUMULATIONS ARE TO CONCENTRATE BETWEEN THE LOS LAGOS AND AYSEN
REGIONS. NOTE THAT ISOLATED LARGER AMOUNTS ARE EXPECTED ALONG THE
PRECORDILLERA AND MINIMA WILL CONCENTRATE ALONG THE CENTRAL
VALLEY. THE UPPER TROUGH ASSOCIATED WITH THIS EVENT IS FORECAST TO
AMPLIFY OFF THE COAST OF CHILE ON THURSDAY THROUGH SATURDAY. THIS
WILL LEAD TO HEIGHT FALLS ACROSS CENTRAL CHILE ON FRIDAY...AND
ACROSS CENTRAL/NORTHWESTERN ARGENTINA AND NORTHERN CHILE ON
SATURDAY. THIS PERSISTENT AMPLIFYING TROUGH WILL ENHANCE UPPER
DIVERGENCE OVER THE ANDES...WHICH WILL STIMULATE PRECIPITATION IN
THE CORDILLERA BY THE WEEKEND. SNOWFALL AMOUNTS WILL BE LIMITED BY
THE LIMITED MOISTURE AVAILABLE. YET...MODELS AGREE ON A STRONG
TROUGH THAT WILL APPROACH THE ANDES ON SATURDAY...AND CROSS THEM
ON SUNDAY. THIS WILL HAVE A SIGNIFICANT IMPACT ON CONVECTION TO
THE EAST OF THE ANDES (SEE BELOW). NORTHERN TAIL FO THIS TROUGH IS
FORECAST TO AMPLIFY OFF THE COAST OF NORTHERN CHILE/SOUTHERN PERU
ON SUNDAY AND MONDAY. THIS WILL LEAD TO WINDS EXCEEDING 20KT IN
THE ALTIPLANO AND WESTERN CORDILLERA DURING THE WEEKEND...AND
ISOLATED SNOW SHOWERS IN THE CORDILLERA OF ANTOFAGASTA/ATACAMA IN
CHILE...AND SALTA IN ARGENTINA.


AS THE AFOREMENTIONED UPPER TROUGH CROSSES THE ANDES...EXPECTING
CYCLOGENESIS AND FRONTOGENESIS TO DEVELOP BY FRIDAY. A LOW-LEVEL
JET FROM BOLIVIA IS DEVELOPING ON WEDNESDAY AND WILL BUILD UP A
MOISTURE POOL OF NEAR 40MM OF PRECIPITABLE WATER BY FRIDAY AND
SATURDAY. MODELS ARE ALL AGREEING ON A STRONG UPPER DIVERGENT
PATTERN SETTING UP ON SATURDAY AS THE DIVERGENT ENTRANCE OF AN
UPPER JET SETS OVER THE REGION. ALTHOUGH INSTABILITY WILL BE
LIMITED...AS LIFTED INDEX VALUES ONLY DIP TO NEAR -3...THE
CONFIGURATION WILL BE CONDUCIVE FOR LOCALLY HEAVY RAINS BETWEEN
ENTRE RIOS AND URUGUAY ON SATURDAY...WHEN EXPECTING MAXIMA OF
30-60MM/DAY. THE EVENT IS FORECAST TO PEAK ON SUNDAY-EARLY MONDAY
WHEN EXPECTING MAXIMA IN THE ORDER OF 35-70MM/DAY FROM
SOUTHEASTERN PARAGUAY INTO RIO GRANDE DO SUL IN BRASIL. AS A
LOW-LEVEL JET FROM THE SOUTH DEVELOPS IN NORTHEASTERN
ARGENTINA...EXPECTING THUNDERSTORM DEVELOPMENT OVER THE CHACO.
STORMS WILL PRODUCE MAXIMA IN THE ORDER OF 20-45MM/DAY ON
SUNDAY-MONDAY. THE MODELS ARE THEN AGREEING ON SURFACE COLD FRONT
ARRIVING INTO CENTRAL PERU BY EARLY NEXT WEEK TO THEN RAPIDLY
DISSIPATE.

OSORIO...DMC (CHILE)
CASTILLO...UCR (COSTA RICA)
ALVA...SENAMHI (PERU)
GALVEZ...WPC (USA)
$$





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