SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1151 AM EST FRI NOV 06 2009
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
 
MODEL COMPARISON (VALID FROM 00Z NOV 06). THE GFS AND UKMET
GLOBAL MODELS CONTINUE TO AGREE ON THE EVOLUTION ACROSS THE
DOMAIN THROUGH DAY 05-06. THE ECMWF REMAINS THE ODD MODEL...AS IT
IS THE ONLY ONE THAT DOES NOT FORECAST THE EASTERN PACIFIC LONG
WAVE RIDGE TO CRUMBLE LATER IN THE CYCLE. THE GLOBAL
ENSEMBLES...INCLUDING THE EUROPEAN MEMBERS...GENERALLY SUPPORT
THE DETERMINISTIC GFS/UKMET SOLUTIONS OVER THIS REGION.

THE EASTERN PACIFIC LONG WAVE RIDGE...AT 500 HPA...EXTENDS SOUTH
BETWEEN 90W-120W TO 65S/70S. THE STRONG RIDGE PATTERN WILL
PERSIST THROUGH 84-96 HRS...THEN BEGIN TO WEAKEN BETWEEN 120-132
HRS AS A NORTHER STREAM SHORT WAVE TROUGH PIERCES THIS AXIS.
BY 144 HRS IT IS TO NEARLY FLATTEN/COLLAPSE AS SUCCESSIVE
IMPULSES STREAK ACROSS THE EASTERN PACIFIC. THIS WILL SIGNAL THE
END OF THE LONG WAVE BLOCKAGE AND A CHANGE IN THE SYNOPTIC SCALE
PATTERN ACROSS THE DOMAIN.

EAST OF THIS AXIS...A MID LEVEL TROUGH OVER THE CONTINENT IS
FORECAST TO GRADUALLY PULL EAST DURING THE NEXT 36-48 HRS. IN
THIS PROCESS IT WILL SHEAR A SHORT WAVE TROUGH ON THE NORTHERN
STREAM FLOW...FORECAST TO MEANDER OFF THE CENTRAL COAST OF CHILE
EARLY IN THE FORECAST CYCLE. BUT BY 30-36 HRS...THE SHORT WAVE
TROUGH WILL SPILL ACROSS THE CENTRAL ANDES/CUYO IN ARGENTINA...
AND OFF THE COAST OF BUENOS AIRES/URUGUAY BY 48 HRS...TO THEN
RAPIDLY ACCELERATE INTO THE SOUTH ATLANTIC. AS THE SHORT WAVE
TROUGH SPILLS ACROSS THE ANDES...IT WILL INTERACT WITH AN
ELONGATED FRONT THAT STRETCHES ACROSS SOUTHERN BRASIL TO
NORTHWEST ARGENTINA. BY 30-36 HRS...AS A WAVE/LOW FORMS ALONG
THIS FRONT...THE MODELS THEN SHOW THE FRONTAL BOUNDARY SLIDING
NORTH ACROSS SANTA CATARINA IN SOUTHERN BRASIL TO PARAGUAY...
WHERE IT WILL MEANDER THROUGH 48-72 HRS. BY 96 HRS A WEAK FRONT
WILL MOVE NORTH ACROSS SAO PAULO...TO REACH RIO DE JANEIRO
LATER IN THE CYCLE. THE TAIL END OF THE FRONT OVER THE
CONTINENT WILL WEAKEN TO A NARROW LOW LEVEL TROUGH. AN 850 HPA
NORTHERLY JET ACROSS PARAGUAY WILL SUSTAIN A MOIST/WARM TRANSPORT
THAT IS TO CONVERGE ALONG THE QUASISTATIONARY FRONT...WHILE ALSO
SUSTAINING A HIGH SHEAR ENVIRONMENT. ALOFT...THE SUBTROPICAL/
POLAR JETS WILL ENTER THE CONTINENT AT A PERPENDICULAR ANGLE...
WHILE WILL CONTINUE FAVORING GENERATION OF LEE SIDE
PERTURBATIONS. AS THE FRONT BEGINS TO UNDULATE NORTH INTO
PARAGUAY-SOUTHERN BRASIL...THE MAXIMA WILL INCREASE TO 35-
70MM/DAY...WITH A SLIGHT DECREASE ON DAY 02 WHEN THE MAXIMA IS TO
PEAK AT 30-60MM/DAY. THE RISK OF SEVERE CONVECTION PERSISTS AS
MESO SCALE CONVECTIVE SYSTEMS DEVELOP ALONG THE MEANDERING
FRONT...WITH HIGHEST RISK OVER CENTRAL/SOUTHERN MESOPOTAMIA-
URUGUAY AND SOUTHERN BRASIL. AFTER 72-84 HRS...THE MAXIMA WILL
DECREASE TO 15-35MM/DAY WHILE CONCENTRATING ACROSS SOUTHERN/
SOUTHEAST BRASIL-PARAGUAY AND SOUTHEAST BOLIVIA. ALONG THE
COAST...ACROSS NORTHERN SAO PAULO/RIO DE JANEIRO...THE MAXIMA
WILL PEAK AT 20-45MM/DAY THROUGH 72-96 HRS AS A STRONG ONSHORE
FLOW ESTABLISHES BEHIND THE BAROCLINIC BOUNDARY.

UNDER INFLUENCE OF THE EASTERN PACIFIC RIDGE...A SURGE IN THE
SOUTHERN STREAM IS FORECAST EARLY THIS CYCLE...WHEN A TROUGH
ACROSS THE ANTARCTIC PENINSULA/BELLINGSHAUSEN SEA IS FORECAST TO
AMPLIFY NORTH. THE MID/UPPER LEVEL TROUGH WILL RAPIDLY AMPLIFY AS
IT MOVES NORTH INTO SOUTHERN CHILE/PATAGONIA THROUGH 48-72
HRS...WITH A BROAD TROUGH PATTERN TO ESTABLISH OVER THE SOUTHERN
CONE/WESTERN ATLANTIC BY 72-96 HRS. SECONDARY SHORT WAVE VORTICES
WILL ENTER THE BACKSIDE OF THIS TROUGH AND MOVE NORTH INTO
SOUTHERN CHILE/PATAGONIA. AT LOW LEVELS A PROGRESSIVE FRONT WILL
MOVE ACROSS TIERRA DEL FUEGO INTO PATAGONIA THROUGH 36 HRS...WITH
ADDITIONAL FRONTAL BOUNDARIES TO STREAM ACROSS THE SOUTHERN CONE
AS A DEEP TROUGH ESTABLISHES OVER THE SOUTHERN CONE/WESTERN
ATLANTIC. IN ADDITION TO SUSTAINING STRONG BOUNDARY LAYER
WINDS...THIS WILL FAVOR SCATTERED PRECIPITATION OVER SOUTHERN
CHILE WITH ORGANIZED SNOWFALL ON THE SOUTHERN ANDES...WITH
ACCUMULATION OF 05-10MM/DAY.

AT 200 HPA...THE SUBTROPICAL RIDGE EXTENDS FROM THE GALAPAGOS
ISLANDS-SOUTHEASTWARD ACROSS PERU/BOLIVIA-PARAGUAY TO THE WESTERN
ATLANTIC. ALTHOUGH IT IS TO INITIALLY CENTER ON TWO CLOSED
HIGHS...BY 24 HRS A SINGLE HIGH WILL REMAIN...TO CENTER OVER
BRASIL/BOLIVIA NEAR 15S 60W. THIS HIGH WILL THEN MEANDER BETWEEN
RONDONIA IN BRASIL AND SANTA CRUZ IN BOLIVIA THROUGH 96-108
HRS...THEN RELOCATES TO WESTERN MINAS GERAIS BY 120 HRS. ALSO...A
TROUGH DEVELOPED TO THE EAST OF THIS RIDGE...EXTENDING TO THE
NORTH OF 23S AND BETWEEN 50W-30W. A CLOSED LOW OVER EASTERN BAHIA
WILL ANCHOR THIS TROUGH DURING THE NEXT THREE DAYS...BUT BY 96-
120 HRS THE LOW WILL FILL AND THE TROUGH IS TO WEAKEN.
MEANWHILE...IT WILL SUSTAIN A CONVERGENT/SUBSIDENT PATTERN ALOFT
THAT IS TO INHIBIT ORGANIZED DEEP CONVECTION TO THE EAST OF 45W.
MOST ACTIVE CONVECTION DURING THE CYCLE IS EXPECTED TO THE
WEST...TO CONCENTRATE ON THE PERUVIAN/BOLIVIAN JUNGLE-AMAZONAS IN
WESTERN BRASIL WITH DAILY MAXIMA OF 20-45MM. THIS WILL DECREASE
TO 15-30MM/DAY BY MID CYCLE. OTHER CONVECTION IS EXPECTED ACROSS
WESTERN BAHIA-TOCANTINS/GOIAS AS A DIVERGENT PATTERN ALOFT
ESTABLISHES BETWEEN THE CONTINENTAL RIDGE AND THE TROUGH...WITH
DAILY MAXIMA OF 15-30MM THROUGH 36-48 HRS.

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