SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1048 AM EST WED NOV 18 2009
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
 
NOTE: DUE TO PRESSING OPERATIONAL REQUIREMENTS...THIS PRODUCT
WILL BE SUSPENDED UNTIL DECEMBER 02 2009. GRAPHICAL PRODUCTS...
HOWEVER...WILL BE ISSUED.

MODEL COMPARISON (VALID FROM 00Z NOV 18). ON THE SOUTHERN
STREAM...AND CONTRARY TO WHAT THE GLOBAL ENSEMBLES PREVIOUSLY
SUGGESTED...THE GFS CORRECTED IN FAVOR OF A MORE PROGRESSIVE
PATTERN ACROSS THE DRAKE PASSAGE/SOUTHERN CONE. THIS IS NOW IN
AGREEMENT WITH THE EUROPEAN MODELS THROUGH AT LEAST 96-108 HRS.
BY 120-144 HRS THE GFS APPEARS MORE AMPLIFIED THAN THE EUROPEAN
MODELS OVER THE FAR EASTERN PACIFIC...WITH MEAN OF THE GLOBAL
ENSEMBLES TRENDING TOWARDS A LOWER AMPLITUDE/WEAKER PATTERN. BUT
THE SPREAD IN THIS AREA IS TO HIGH TO ASCERTAIN/ESTABLISH
CONFIDENCE IN THE MEDIUM RANGE FORECAST PATTERN. 

NORTH OVER THE CONTINENT...AT 200 HPA...THE SUBTROPICAL RIDGE
BOUNDS BETWEEN 08S AND 30S...AND THIS PATTERN IS FORECAST TO
PERSIST BEYOND 132-144 HRS. THE PERSISTENT RIDGE PATTERN WILL
CENTER ON A CLOSED HIGH THAT MEANDERS BETWEEN SANTA CRUZ IN
EASTERN BOLIVIA AND CHACO PARAGUAYO. A TROUGH TO THE NORTH
CENTERS ON A CLOSED HIGH BETWEEN WESTERN AMAPA-NORTHERN
PARA/RORAIMA. IN A CONVERGENT PATTERN ALOFT...THE TROUGH WILL
CONTINUE TO INHIBIT ORGANIZED DEEP CONVECTION ACROSS NORTHEAST
BRASIL. MODERATE CONVECTION WILL CONFINE TO TOCANTINS/GOIAS-BAHIA
AND MINAS GERAIS...AS COLD AIR ALOFT ADVECTS NORTH ALONG THE
EASTERN FRINGES OF THE SUBTROPICAL RIDGE. THIS WILL ENHANCE
CONVECTIVE INSTABILITY...WHICH IN COMBINATION WITH DIURNAL
HEATING WILL SUSTAIN SCATTERED CONVECTION. FOR THE MOST
PART...DAILY MAXIMA IN THIS AREA WILL PEAK AT 15-25MM/DAY. OTHER
CONVECTION WILL BUILD TO THE WEST...ACROSS NORTHERN BOLIVIA-
PERUVIAN JUNGLE/SIERRA AND AMAZONAS/RONDONIA IN WESTERN BRASIL.
THE MAXIMA IN THIS AREA WILL GENERALLY RANGE BETWEEN 15-
35MM/DAY...WITH MOST ACTIVE/INTENSE TO CONCENTRATE ON THE
PERUVIAN JUNGLE...WHERE WIDELY ISOLATED MAXIMA OF 25-50MM/DAY ARE
POSSIBLE ON DAYS 03-05.

FURTHERMORE...THE MODELS CONTINUE TO SHOW A MID LEVEL REFLECTION
OF THE SUBTROPICAL RIDGE PATTERN OVER NORTHERN SOUTH AMERICA. AT
500 HPA THE RIDGE EXTENDS WEST ACROSS THE BRAZILIAN STATES OF
MINAS GERAIS/NORTHERN SAO PAULO TO MATO GROSSO. THE MID LEVEL
RIDGE PATTERN IS FORECAST TO PERSIST DURING THE CYCLE. AS A
RESULT...POLAR PERTURBATIONS ENTERING THE CONTINENT WILL
REMAIN...FOR THE MOST PART...TO THE SOUTH OF 28S. AS THE RIDGE
PERSIST...A PERTURBATION IN THE NORTHERN STREAM IS TO QUICKLY
MOVE ACROSS CENTRAL CHILE TO CUYO IN ARGENTINA BY 24/30 HRS...AND
THROUGH 36 HRS IT IS TO RAPIDLY EJECT ACROSS THE BUENOS AIRES
PROVINCE/RIO DE LA PLATA BASIN. THIS FEATURE WILL SUSTAIN/INDUCE
24 HRS HEIGHT FALLS OF 75-100GPM. AT LOW LEVELS...A DEEP OCCLUDED
LOW LIES TO THE EAST OF THE MALVINAS...EXTENDING A FRONT TO THE
NORTHWEST ACROSS RIO DE LA PLATA-MENDOZA IN ARGENTINA. UNDER
INFLUENCE OF THE MID LEVEL TROUGH ON THE NORTHERN STREAM FLOW...
INTENSE CYCLOGENESIS IS EXPECTED ACROSS LOWER MESOPOTAMIA/
URUGUAY. THROUGH 30-36 HRS THE LOW WILL THEN MOVE OFF THE COAST
OF URUGUAY/SOUTHERN BRASIL...WITH FEATURE TO QUICKLY OCCLUDE OVER
THE WESTERN ATLANTIC. THE ASSOCIATED FRONT WILL THEN MOVE NORTH
INTO SOUTHERN BRASIL-SOUTHEAST PARAGUAY TO NORTHWEST PROVINCES IN
ARGENTINA. UNDER FAVORABLE JET DYNAMICS ALOFT...AND A FAIRLY
STRONG LOW LEVEL NORTHERLY JET...CONDITIONS WILL REMAIN FAVORABLE
FOR MCS FORMATION ALONG THE FRONT. ON DAY 01 WE NOW PROJECT
MAXIMA OF 40-90MM ACROSS SOUTHERN BRASIL-URUGUAY-MESOPOTAMIA
VALLEY...WITH A HIGH RISK OF SEVERE WEATHER. HEAVY RAINS WILL
PERSIST THROUGH 48-60 HRS...WHEN THE MAXIMA IS TO PEAK AT 40-
70MM/DAY. THROUGH 84 HRS IT WILL DECREASE TO 25-50MM/DAY. A SURGE
IN ACTIVITY...HOWEVER...IS POSSIBLE ON DAY 04 AS ANOTHER MID
LEVEL PERTURBATION STREAMS ACROSS CENTRAL/NORTHERN
ARGENTINA...WITH MAXIMA TO INCREASE TO 40-65MM/DAY.

ON THE SOUTHERN STREAM...A MID/UPPER LEVEL RIDGE EXTENDS SOUTH
ALONG 110W/120W TO 80S. THE RIDGE PATTERN IS TO PERSIST THROUGH
DAY 05...WITH SOME MODULATIONS AS PROGRESSIVE SOUTHERN STREAM
PERTURBATIONS SHEAR UNDER THIS AXIS. THE RIDGE PATTERN WILL
SUSTAIN THE NORTHWARD AMPLIFICATION OF A TROUGH ALONG 80W/90W TO
35S BY 24 HRS. A CUTOFF LOW IS TO FORM ALONG THIS AXIS NEAR 55S
80W BY 36-48 HRS...TO THEN MEANDER OFF THE SOUTHERN COAST OF
CHILE THROUGH 60-72 HRS. THIS LOW WILL ANCHOR A BROAD TROUGH OVER
SOUTHERN CHILE/PATAGONIA TO JUST NORTH OF THE ANTARCTIC
PENINSULA. THROUGH 96-108 HRS THE LOW AND TROUGH ARE TO DRIFT
ACROSS THE DRAKE PASSAGE TO THE SOUTH ATLANTIC. AT LOW LEVELS IT
WILL SUSTAIN A BROAD TROUGH PATTERN WITH A DEEP OCCLUDED LOW TO
THE SOUTH-SOUTHWEST OF TIERRA DEL FUEGO. THIS PATTERN WILL
PERSIST THROUGH 72-84 HRS...THEN SHIFT EAST LATER IN THE CYCLE.
POLAR FRONTS WILL REVOLVE AROUND THIS AXIS AS THEY MAKE LANDFALL
OVER SOUTHERN CHILE...WITH BOUNDARIES TO GENERALLY LIMIT TO SOUTH
OF 40S THROUGH 72-84 HRS. A NORTHWARD MODULATION IS POSSIBLE BY
96-120 HRS...AS THE LOW/TROUGH MIGRATES TO THE SOUTH ATLANTIC...
DRAWING THE FRONT NORTH ACROSS LA PAMPA TO CENTRAL CHILE.
RAINFALL MAXIMA OVER SOUTHERN CHILE WILL PEAK AT 15-35MM/DAY
THROUGH 48-72 HRS...WITH MODELS SHOWING STRONG UPWARD VERTICAL
MOTION ALONG THE COAST/INLAND INTO THE ANDES. IN THIS PATTERN
THERE IS A RISK/CHANCE OF DEEP CONVECTIVE CELLS TO DEVELOP...
WITH WIDELY ISOLATED THUNDERSTORMS TO SUSTAIN LOCALIZED HEAVY
RAINFALL AMOUNTS.

FERNANDES...CHM (BRASIL)
GARCIA...SENAMHI (PERU)
DAVISON...HPC (USA)