Prognostic Meteorological Discussion
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FXSA20 KWBC 031548
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1147 AM EDT TUE MAY 03 2016

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

MODEL DISCUSSION (VALID FROM MAY 03 AT 0000 UTC): AS PREVIOUSLY
NOTED...MODELS ARE HAVING PROBLEMS REACHING A CONSENSUS ON HOW
PATTERN OVER THE EASTERN PACIFIC IS TO EVOLVE DURING THE SECOND
HALF OF THE FORECAST...FAILING TO AGREE ON THE INTENSITY/SPEED OF
SHORT WAVE PERTURBATIONS ENTERING CHILE-ARGENTINA. THESE
DIFFERENCES ARE QUITE EVIDENT ON THE GAMUT OF GLOBAL ENSEMBLE
MODELS...WITH HIGH VARIABILITY NOTED ON DAY 04 AND ONWARD. AS A
RESULT...CONFIDENCE IN THE FORECAST DURING THE MEDIUM RANGE PERIOD
IS LOW.

A MID/UPPER LEVEL RIDGE LIES ALONG THE COAST OF CHILE TO THE
BELLINGSHAUSEN SEA. SUCCESSIVE SHORT WAVE PERTURBATIONS ARE TO
PRESS AGAINST THIS AXIS....LEADING TO GRADUAL EROSION OF THE RIDGE
TO THE NORTH OF 40S WHILE TO THE SOUTH THE RIDGE IS EXPECTED TO
HOLD. AS THE RIDGE WEAKENS TO THE NORTH OF 40S THIS IS TO THEN
ALLOW THE SHORT WAVE PERTURBATIONS TO CONTINUE FARTHER EAST INTO
CHILE AND ARGENTINA. IN THIS PATTERN EXPECTING A SHORT WAVE TROUGH
TO ENTER CENTRAL CHILE BY 60-72 HRS...INTO CENTRAL ARGENTINA BY
78-84 HRS. ANOTHER FOLLOW...REACHING CENTRAL CHILE BY 96-108
HRS...TO THEN SPILL ACROSS THE ANDES INTO CENTRAL ARGENTINA LATER
IN THE CYCLE. AT LOW LEVELS...A WANING FRONT IS TO REACH CENTRAL
CHILE BY 72 HRS... AND ANOTHER BY 120-132 HRS. THESE ARE TO FAVOR
LIGHT RAINFALL AMOUNTS AS THEY ENTER CENTRAL CHILE...WITH RAINFALL
AMOUNTS OF 00-05MM IN ISOLATED TO SCATTERED CONVECTION. OVER
NORTHERN PATAGONIA-LA PAMPA IN ARGENTINA EXPECTING LIGHT TO
MODERATE RAINFALL AMOUNTS WITH ACCUMULATION OF 05-10MM...WITH MOST
INTENSE BY 72-84 HRS. HIGHER AMOUNTS ARE EXPECTED TO THE NORTH
OVER PARAGUAY/CORRIENTES IN ARGENTINA-NORTHERN MESOPOTAMIA
VALLEY...WHERE IN INTERACTION WITH AN OLD/QUASISTATIONARY FRONT
THIS IS TO RESULT IN MODERATE AMOUNTS.

EAST OF THE MID/UPPER LEVEL RIDGE...OVER THE SOUTH ATLANTIC...A
BROAD MID/UPPER LEVEL TROUGH DOMINATES THE FLOW. THE BROAD TROUGH
IS TO PERSIST OVER THE NEXT SEVERAL DAYS. AS IT HOLDS...SHORT WAVE
TROUGHS ARE TO REVOLVE AROUND THE LONG WAVE AXIS...WITH SHORT WAVE
PERTURBATIONS TO LIFT ACROSS THE MALVINAS/FALKLAND ISLANDS FROM
TIME-TO-TIME. AT LOW LEVELS MODELS SHOW A SIMILAR PATTERN
EVOLUTION DURING THE NEXT SEVERAL DAY...WITH A BROAD LOW LEVEL
TROUGH TO DOMINATE THE WESTERN-CENTRAL SOUTH ATLANTIC. POLAR
FRONTS ARE TO REVOLVE AROUND THIS TROUGH...LIFTING ALONG THE COAST
OF PATAGONIA INTO THE CENTRAL PROVINCES IN ARGENTINA/RIO DE LA
PLATA BASIN-URUGUAY. BUT LACKING UPPER LEVEL SUPPORT THESE ARE TO
ONLY FAVOR LIGHT RAINFALL AMOUNTS...WITH MOST ACTIVE ACROSS LA
PAMPA ON DAY 03 WHEN IT IS TO RESULT IN RAINFALL MAXIMA OF 05-10MM.

AN OLD FRONTAL BOUNDARY TO THE NORTH TRAILS ACROSS THE CENTRAL
SOUTH ATLANTIC TO ESPIRITO SANTO/BAHIA IN BRASIL. THE BOUNDARY IS
FORECAST TO GRADUALLY WEAKEN THROUGH 42-48 HRS. MEANWHILE...THIS
IS TO FAVOR A PREFRONTAL SHEAR LINE ACROSS EASTERN BAHIA. IN THIS
AREA A BRISK SOUTHEASTERLY FLOW IS TO CONVERGE ALONG THE
COAST...TO RESULT IN MODERATE RAINFALL AMOUNTS OF 20-35MM THROUGH
36 HRS...AND 05-10MM AT 36-60 HRS.

AT 200 HPA...A NEAR ZONAL WESTERLY FLOW IS TO ENVELOP CONTINENTAL
AREA TO THE NORTH OF 20S...WHILE A WEAK/ILL ORGANIZED RIDGE
CONFINES TO THE NORTHEAST COAST OF BRASIL. THIS PATTERN IS
FORECAST TO HOLD OVER THE NEXT FEW DAYS. EARLY IN THE CYCLE THIS
IS TO FAVOR MODERATE CONVECTION ALONG THE NORTH COAST OF BRASIL
INTO PARA...WHERE THE MAXIMA IS TO PEAK AT 20-35MM. THROUGH 72-96
HRS THIS DECREASES TO 10-15MM/DAY. OVER NORTHERN AMAZONAS-RONDONIA
IN BRASIL TO SOUTHERN COLOMBIA/VENEZUELA THIS IS TO FAVOR MODERATE
TO HEAVY CONVECTION WITH ACCUMULATION OF 20-45MM EXPECTED THROUGH
36 HRS. BY 72-96 HRS THIS IS TO DECREASE TO MAXIMA OF 20-30MM IN
ISOLATED TO SCATTERED CONVECTION.

ENRIQUEZ...DGAC (ECUADOR)
AYALA...DINAC (PARAGUAY)
GALVEZ/DAVISON...WPC (USA)

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