098
FXSA20 KWBC 261430
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
952 AM EDT FRI SEP 26 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 26). AFTER MAKING A SERIES
OF CORRECTIONS DURING THE LAST FOUR DAYS...THE GFS HAS FINALLY
REACHED A HAPPY MEDIUM...AND IT IS STARTING TO SHOW SOME
SEMBLANCE OF CONTINUITY. THE ECMWF AND UKMET ALSO APPEAR FAIRLY
STABLE...ONLY MAKING MINOR CORRECTIONS TO PREVIOUS FORECAST(S).
ALTHOUGH SOMEWHAT GUARDED...OUR CONFIDENCE IN THE OUTPUT IS
HIGHER TODAY.
AT 500 HPA...A SHORT WAVE TROUGH IS INITIALIZED OVER URUGUAY/
SOUTHERN BRASIL. THIS TROUGH MOVES TO THE NORTHEAST ACROSS
SOUTHERN/SOUTHEAST BRASIL THROUGH 36-48 HRS...THEN MEANDERS OFF
THE COAST OF BRASIL THROUGH 72-84 HRS. BY 96 HRS IT STARTS TO
MOVE ACROSS 30W...THEN DAMPENS/WEAKENS WHILE PULLING FARTHER EAST
BY 120 HRS. THIS TROUGH...IN INTERACTION WITH A SURFACE FRONT
OVER BRASIL...WILL SUPPORT AN AREA OF SCATTERED RAINFALL TO
AFFECT NORTHERN SAO PAULO-MINAS GERAIS AND ESPIRITO SANTO...WITH
MAXIMA OF 20-40MM THROUGH 30-36 HRS...AND 10-20MM/DAY AT 36-60
HRS. FURTHERMORE...AS THE TROUGH INTERACTS WITH THE FRONT...IT IS
TO INDUCE A WAVE/LOW OVER RIO DE JANEIRO...TO DRIFT OFF THE COAST
THROUGH 36-48 HRS. THIS WILL TIGHTEN THE GRADIENT AND SUSTAIN A
BRISK EASTERLY FLOW ACROSS SAO PAULO BRASIL. THE ENSUING FLOW
WILL CONVERGE ON THE COAST/SERRA DO MAR TO SUSTAIN SCATTERED
CONVECTION AND RAINFALL AMOUNTS OF 20-40MM/DAY EARLY IN THE
PERIOD...TO ALSO DECREASE TO 10-20MM/DAY THROUGH 48-72 HRS.
OVER THE PACIFIC...ALSO AT 500 HPA...A SOUTHERN STREAM TROUGH
WILL ENTER THE SOUTHERN CONE EARLY THIS PERIOD...AND THROUGH 48
HRS IT WILL MOVE ACROSS 50W/60W AND SOUTH OF 45S. ON THE NORTHERN
STREAM...A SHORT WAVE TROUGH IS TO LAG OFF THE CENTRAL/NORTHERN
COAST OF CHILE. MODELS NOW AGREE ON THE EVOLUTION/PROGRESSION OF
THIS PERTURBATION. THE SHORT WAVE TROUGH INITIALLY STALLS OVER
THE CENTRAL ANDES OF CHILE. BUT...BETWEEN 60-72 HRS IT WILL SPILL
ACROSS THE ANDES TO THE CENTRAL PROVINCES OF ARGENTINA AS ANOTHER
NORTHERN STREAM TROUGH ENTERS THE EASTERN PACIFIC. THE SHORT WAVE
TROUGH SLOWLY MOVES ACROSS CENTRAL ARGENTINA THROUGH 84-96
HRS...AND OFF THE COAST BY 96-108 HRS. NOTE THAT THE GLOBAL
MODELS HAVE BEEN TRENDING IN FAVOR OF A WEAKER PERTURBATION THAN
WHAT THEY ORIGINALLY PROJECTED. AT LOW LEVELS EXPECT A POLAR
FRONT TO EXTEND ACROSS PATAGONIA/LA PAMPA ARGENTINA-CONCEPCION
CHILE EARLY THIS PERIOD. THROUGH 24-36 HRS IT WILL START TO
UNDULATE NORTH INTO CENTRAL ARGENTINA/CHILE. BY 72-96 HRS IT
STARTS TO WEAKEN OVER THE EASTERN PACIFIC...WHILE IT PERSISTS
OVER THE CONTINENT AS A WAVE/LOW FORMS OVER CENTRAL CUYO. THIS
LOW DEEPENS THROUGH 96-108 HRS WHILE DRIFTING EAST-SOUTHEAST
ACROSS THE BUENOS AIRES PROVINCE...MEANWHILE PROPELLING THE FRONT
NORTH ACROSS NORTHERN PROVINCES OF ARGENTINA. AS THE FRONT
ACCELERATES...A PREFRONTAL SQUALL LINE WILL FORM AHEAD OF THIS
BOUNDARY...THAT IS LIKELY TO PERSIST ACROSS CORDOBA-CHACO
ARGENTINO ON DAY 04. ON DAY 05 ONLY THE FRONT REMAINS...WITH AXIS
ACROSS SOUTHERN BRASIL-PARAGUAY/NORTHERN PROVINCES OF ARGENTINA.
AS THE FRONT ENTERS SOUTHERN CHILE IT WILL FAVOR MOSTLY LIGHT
CONVECTION WITH MAXIMA TO PEAK AT 05-10MM/DAY. OVER THE CENTRAL
ANDES AND PORTIONS OF CUYO IN ARGENTINA MAXIMA WILL PEAK AT 10-
20MM/DAY ON DAY 01...WHILE ON DAY 02 THE MAXIMA WILL PEAK AT 20-
35MM AS THE BOUNDARY MEANDERS OVER CORDOBA-MENDOZA IN ARGENTINA.
WE NOW PROJECT AN INCREASE ACROSS CORDOBA-SAN LUIS-CHACO BY 60-96
HRS...WITH MAXIMA TO RANGE BETWEEN 25-50MM...WITH LESSER AMOUNTS
INTO PARAGUAY/MESOPOTAMIA VALLEY AND SOUTHERN BRASIL LATER IN THE
PERIOD. THERE IS A SLIGHT RISK OF SEVERE CONVECTION IN
ASSOCIATION WITH THE SQUALL LINE AS IT MOVES NORTH OF 28S/30S AND
FINDS MORE FAVORABLE ENVIRONMENT (READ MOISTURE).
THE NEXT MID LEVEL TROUGH WILL MOVE ACROSS 90W AND SOUTH OF 20S
BY 72 HRS. THROUGH 108 HRS IT WILL CONTINUE ALONG 80W...WITH
SHORT WAVE ENERGY FOCUSING ACROSS CHILE SOUTH OF CONCEPCION. BY
120 HRS IT WILL MOVE ACROSS 70W...FOCUSING ITS ENERGY OVER
PATAGONIA. THE ASSOCIATED FRONT ENTERS THE SOUTHERN CONE BY 108-
120 HRS...WITH SCATTERED CONVECTION EXPECTED BETWEEN TEMUCO-ISLA
DE CHILOE IN CENTRAL-SOUTHERN CHILE...WITH MAXIMA OF 10-20MM.
NORTH OVER THE CONTINENT...AT 200 HPA....THE SUBTROPICAL RIDGE
CONTINUES TO BECOME BETTER ORGANIZED/DEFINED...WITH CLOSED
CIRCULATION TO MEANDER NEAR 10S 70W DURING MOST OF THE CYCLE. THE
BROAD RIDGE IS TO ENVELOP CONTINENTAL AREA NORTH OF 30S. THE
INTENSIFICATION OF THE RIDGE IS TO ACCOMPANY A SURGE IN
CONVECTION ACROSS BRASIL TO PERU-ECUADOR/SOUTHERN COLOMBIA...WITH
MAXIMA OF 20-40MM/DAY TO CONCENTRATE ACROSS AMAZONAS-PARA-
PERUVIAN JUNGLE AND SOUTHERN COLOMBIA. ACROSS ECUADOR THE DAILY
MAXIMA IS TO GENERALLY PEAK AT 10-20MM/DAY...WITH LOCALLY HIGHER
AMOUNTS POSSIBLE DUE TO TOPOGRAPHICAL FORCING. AN INCREASE IN
ACTIVITY IS ALSO EXPECTED ACROSS CENTRAL/SOUTHERN JUNGLE OF PERU
INTO NORTHERN BOLIVIA/RONDONIA BRASIL BY 60-120 HRS AS PREFRONTAL
CONVECTION DEVELOPS IN THIS AREA AND THE LOW LEVEL THERMAL TROUGH
BECOMES BETTER ORGANIZED ACROSS THE JUNGLE.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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