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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1530Z Sep 05, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1130 AM EDT FRI SEP 05 2014

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

MODEL DISCUSSION (VALID FROM SEPTEMBER 05 AT 00UTC): MINOR
CORRECTIONS TO PREVIOUS FORECAST...AS MODELS CONTINUE TO PREDICT
INTENSE CYCLOGENESIS OVER EASTERN ARGENTINA/WESTERN ATLANTIC OVER
THE NEXT COUPLE OF DAYS. THIS IS TO TRIGGER HEAVY RAINS AND SEVERE
CONVECTION ACROSS RIO DE LA PLATA BASIN-MESOPOTAMIA VALLEY/
CORRIENTES IN ARGENTINA...AND IT IS LIKELY TO FAVOR AN MCS OVER
PARAGUAY/MATO GROSSO DO SUL. DURING MEDIUM RANGE PERIOD MODELS
CONTINUE TO DIVERGE ON INTENSITY OF MID LEVEL TROUGH ENTERING THE
CONTINENT...WITH BOTH EUROPEAN MODELS FAVORING A DEEPER
PERTURBATION THAN WHAT THE GFS FORECASTS.

A DEEP MID/UPPER LEVEL TROUGH EXTENDS OVER THE EASTERN PACIFIC.
THIS IS PRESSING AGAINST A WANING RIDGE TO THE EAST. AS THE RIDGE
YIELDS TO THE POLAR TROUGH...SHORT WAVE ENERGY IS TO FOCUS ACROSS
THE CENTRAL PROVINCES IN ARGENTINA. THE TROUGH IS TO THEN RAPIDLY
SPILL ACROSS THE ANDES INTO CENTRAL ARGENTINA...WHERE IT IS TO
SUSTAIN HEIGHT FALLS OF 75-150GPM. AS IT MOVES EAST OF
PATAGONIA/BUENOS AIRES PROVINCE BY 36-48 HRS...UNDER FAVORABLE JET
DYNAMICS...THE TROUGH/LOW IS FORECAST TO DEEPEN. IT IS TO THEN
MEANDER TO THE EAST-SOUTHEAST...NEARING 50S 40W BY 96 HRS. AT LOW
LEVELS...IT SUSTAINS A POLAR FRONT ACROSS CENTRAL PROVINCES IN
ARGENTINA. INFLOW OF SHORT WAVE ENERGY ALOFT IS TO SUSTAIN
CYCLOGENESIS ALONG THIS BOUNDARY BY 24 HRS...WITH A LOW TO FORM
OVER NORTHERN PATAGONIA/LA PAMPA IN ARGENTINA. AS THE LOW MOVES TO
THE WESTERN ATLANTIC BY 30-36 HRS...IT IS TO RAPIDLY OCCLUDE. AS
THE LOW DEEPENS AND A POLAR RIDGE BUILDS ACROSS CENTRAL
ARGENTINA...THE SURFACE FRONT IS TO SURGE ACROSS BUENOS AIRES
PROVINCE-CORDOBA/MENDOZA IN ARGENTINA BY 24-36 HRS...MOVING NORTH
TO RIO GRANDE DO SUL-NORTHERN PROVINCES IN ARGENTINA BY 48-60 HRS.
BOUNDARY IS TO THEN FRONTOLIZE LATER ON DAY 04 AS IT LOSES UPPER
LEVEL SUPPORT. THE DEEPENING SURFACE LOW IS TO TRIGGER SCATTERED
CONVECTION ACROSS NORTHERN PATAGONIA-LA PAMPA IN ARGENTINA EARLY
IN THE CYCLE...WITH ACCUMULATION OF 15-20MM. MOST
INTENSE...HOWEVER...IS EXPECTED AS THE FRONT SURGES ACROSS THE
BUENOS AIRES PROVINCE-CENTRAL/NORTHERN ARGENTINA EARLY IN THE
CYCLE. IN THIS AREA EXPECTING RAINFALL MAXIMA OF 30-60MM IN
POTENTIALLY SEVERE CONVECTION...WITH THE LI PEAKING AROUND -8
DURING THIS PERIOD. IN THIS AREA EXPECTING MAXIMA OF 30-60MM. BY
36-60 HRS...AS THE FRONT MOVES NORTH TO PARAGUAY...CONDITIONS WILL
BECOME FAVORABLE FOR AN MCS TO DEVELOP ACROSS
PARAGUAY-CORRIENTES/MISIONES IN ARGENTINA. IN THIS AREA RAINFALL
MAXIMA IS TO PEAK AT 30-60MM THROUGH 60 HRS...AND 05-10MM AT 60-84
HRS.

FARTHER EAST...OVER THE SOUTH ATLANTIC...AN ELONGATED TROUGH
EXTENDS TO THE NORTHWEST...WITH AXIS ALONG 40S 10W...30S 30W...TO
24S 40W. AS RIDGE PATTERN TO THE WEST COLLAPSES...THE TROUGH IS
FORECAST TO GRADUALLY WEAKEN. THIS IS TO THEN SHEAR A SHORT WAVE
TROUGH ON THE NORTHERN STREAM THAT MEANDERS ACROSS 20W/25W TO 15S
EARLY IN THE CYCLE. THROUGH 72 HRS THE TROUGH PULLS ACROSS
10W...THEN ACCELERATES TO THE EAST-SOUTHEAST LATER IN THE CYCLE.
AT LOW LEVELS...BROAD RIDGE LIES OVER THE WESTERN SOUTH
ATLANTIC...WITH AN ELONGATED FRONT TO THE NORTH STRETCHING TO THE
NORTHEAST STATES OF BRASIL. SURFACE RIDGE WILL MIGRATE EAST AS IT
YIELDS TO ADVANCING POLAR FRONT OVER ARGENTINA. AS THE RIDGE
MIGRATES...THIS IS TO DISPLACE THE SURFACE FRONT NORTH ACROSS
EASTERN BAHIA TO PERNAMBUCO/RIO GRANDE DO NORTE. IN A BRISK
SOUTHEASTERLY FLOW...EXPECTING SCATTERED COASTAL CONVECTION ACROSS
EASTERN BAHIA-PERNAMBUCO IN BRASIL TO TRIGGER RAINFALL AMOUNTS OF
10-15MM/DAY. THROUGH 48-60 HRS THIS INCREASES TO 15-20MM...WITH
MODERATE RAINFALL AMOUNTS EXPECTED TO PERSIST OVER THE NEXT FEW
DAYS.

AT 200 HPA...A RIDGE IS TO ESTABLISH EARLY IN THE CYCLE ACROSS
WESTERN BRASIL-PERU TO MID SECTIONS OF SOUTH AMERICA. THE RIDGE IS
FORECAST TO CENTER ON A CLOSED HIGH OVER NORTHERN PARA/AMAZONAS IN
BRASIL. AS THE RIDGE BUILDS...A TROUGH IS TO AMPLIFY TO THE
EAST...WITH AXIS NORTH ACROSS MINAS GERAIS/BAHIA TO NORTHEAST
BRASIL. EVOLVING UPPER LEVEL PATTERN WILL SUSTAIN A GRADUAL DRYING
TREND OVER NORTHWEST BRASIL-SOUTHERN COLOMBIA-ECUADOR AND PORTIONS
OF PERU. HOWEVER...AS THE TROUGH DEEPENS ACROSS NORTHEAST BRASIL
AND COOL AIR ADVECTS ACROSS MINAS GERAIS/BAHIA TO
TOCANTINS/GOIAS...IT IS TO ENHANCE DIURNAL CONVECTIVE DEVELOPMENT
TO RESULT IN RAINFALL AMOUNTS OF 05-10MM. THIS INCREASES TO
15-20MM BY 48-84 HRS. LATER IN THE CYCLE THE RIDGE WILL RELOCATE
TO THE EASTERN TROPICAL PACIFIC...AND THE TROUGH IS TO RELOCATE TO
PARA/EASTERN AMAZONAS. FOCUS OF THE CONVECTION IS TO THEN SHIFT TO
NORTHWEST SOUTH AMERICA.

JALFIN...SMN (ARGENTINA)
CAROCA...DMC (CHILE)
DAVISON...WPC (USA)