SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1159 AM EDT FRI MAY 16 2008
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
 
MODEL COMPARISON (VALID FROM 00Z MAY 16). GLOBAL MODELS CONTINUE
TO DIVERGE ON THE EVOLUTION OF A MID LEVEL CYCLONE ENTERING THE
SOUTHERN CONE EARLY IN THE PERIOD. THE GFS REMAINS THE DEEPEST/
STRONGEST OF THE DETERMINISTIC SOLUTIONS...AND AS A DIRECT RESULT
IT FORESEES A SLOW EVOLUTION OF THE MID AND LOW LEVEL CYCLONES.
THE MEAN ENSEMBLE SOLUTION...HOWEVER...NOW SIDE QUITE WELL WITH
THE GFS SOLUTION(S)...GIVING SOME CONFIDENCE TO THIS FORECAST
DURING THE NEXT THREE DAYS. OVER THE EASTERN PACIFIC THE MODELS
AGREE QUITE WELL ON PERSISTENCE OF A SPLIT FLOW PATTERN DURING
THE NEXT THREE TO FOUR DAYS. THIS WILL ALLOW FOR NORTHERN STREAM
SYSTEM TO ESTABLISH/SUSTAIN A SUBTROPICAL CONNECTION AS THEY MOVE
ACROSS CENTRAL CHILE. AS A RESULT...CONDITIONS ARE QUITE
FAVORABLE FOR PERSISTENT HEAVY RAINS TO ACCOMPANY THE SURFACE
FRONTS AS THEY ENTER CHILE BETWEEN CONCEPCION-PUERTO MONTT.

UNDER INFLUENCE OF A MID LEVEL RIDGE ON THE SOUTHERN STREAM
FLOW...500 HPA SHORT WAVE TROUGH OVER THE DRAKE PASSAGE WILL MOVE
NORTH INTO SOUTHERN/CENTRAL PATAGONIA IN ARGENTINA BY 24 HRS...
WHERE THE SYSTEM IS EXPECTED TO RAPIDLY DEEPEN/OCCLUDE. THROUGH
48-54 HRS THE MID LEVEL CYCLONE WILL THEN TRACK EAST ACROSS THE
MALVINAS/FALKLAND ISLANDS. AS IT STARTS TO TRACK ACROSS THE SOUTH
ATLANTIC...THE LOW FILLS TO AN OPEN TROUGH...WITH AXIS TO EXTEND
ALONG 60S 40W TO JUST EAST OF THE BUENOS AIRES PROVINCE. THROUGH
96-120 HRS THE SYSTEM WILL SLOWLY MOVE ACROSS 15W/20W...MEANWHILE
AMPLIFYING NORTH TO 20S. AT LOW LEVELS EXPECT A DEEP OCCLUDED LOW
TO ENTER TIERRA DEL FUEGO EARLY THIS PERIOD...WITH A POLAR FRONT
TO PUSH NORTH ACROSS PATAGONIA. THIS BOUNDARY WILL THEN MERGE
WITH/INTERACT WITH AN ELONGATED FRONT OVER THE SOUTHERN CONE...
WITH ELONGATED BOUNDARY TO EXTEND ACROSS NORTHERN PATAGONIA-LA
PAMPA-TEMUCO CHILE INTO THE EASTERN PACIFIC. THIS FEATURE IS TO
CONTINUE NORTH ACROSS RIO DE LA PLATA BY 72 HRS...AND THROUGH 96
HRS IT WILL STALL OVER SOUTHERN BRASIL/URUGUAY-CORDOBA/MENDOZA IN
ARGENTINA. THE EVOLVING LOW LEVEL CYCLONE WILL SUSTAIN A
WARM/MOIST TRANSPORT ACROSS TIERRA DEL FUEGO INTO SOUTHERN/
CENTRAL PORTIONS OF PATAGONIA DURING THE NEXT 36-42 HRS. AS THE
MOISTURE LIFTS OVER A COLD POLAR CONTINENTAL AIR MASS... THIS
WILL FAVOR GENERATION OF SNOW/SNOW SHOWERS ACROSS SOUTHERN CHILE
AND CENTRAL/SOUTHERN PATAGONIA...WITH ACCUMULATION TO RANGE
BETWEEN 10-20CM. MOST INTENSE EXPECTED ON THE SOUTHERN ANDES.
MOST ACTIVE CONVECTION IS EXPECTED AS THE FRONT ORIGINALLY
MEANDERS BETWEEN CONCEPCION/PUERTO MONTT CHILE...WITH RAINFALL
AMOUNTS OF 20-40MM/DAY ON DAY 01...FORECAST TO RAPIDLY INCREASE
TO 35-70MM/DAY ON DAY 02. IN THIS AREA MODELS CONTINUE TO FORESEE
RISK OF DEEP CONVECTION...SO LOCALLY HIGHER AMOUNTS IN EXCESS OF
FORECASTED GUIDANCE ARE QUITE LIKELY. AS THE FRONT MOVES NORTH
ACROSS CENTRAL ARGENTINA...IT IS TO GENERALLY FAVOR LIGHT TO
MODERATE CONVECTION. THE MOST INTENSE IS EXPECTED OVER
MESOPOTAMIA VALLEY-URUGUAY-SOUTHERN BRASIL ON DAY 03...WITH
MAXIMA OF 15-30MM/DAY IN MIXED CONVECTION. THIS WILL DECREASE TO
10-20MM/DAY ON DAY 04. FURTHERMORE...STRONG BOUNDARY LAYER WINDS
OF 35-50KT ARE TO INITIALLY CONCENTRATE ACROSS EXTREME SOUTHERN
CHILE-CENTRAL/SOUTHERN PATAGONIA. THROUGH 42-48 HRS THE STRONG
WINDS WILL AFFECT AREAS FARTHER NORTH ACROSS SOUTHERN CENTRAL
CHILE AND LA PAMPA IN ARGENTINA.

AS PREVIOUSLY INDICATED...OVER THE FAR EASTERN PACIFIC...THE
MODELS INITIALIZE A BLOCKING RIDGE PATTERN ON THE SOUTHERN STREAM
FLOW. NORTH OF THIS RIDGE...IN A SPLIT FLOW PATTERN...NORTHERN
STREAM TROUGHS ARE TO SLOWLY STREAM TOWARD CENTRAL CHILE-
ARGENTINA. THE NORTHERN STREAM PERTURBATIONS WILL FOLLOW A STORM
TRACK THAT IS GOING TO TAKE THEM TOWARDS THE CONTINENT BETWEEN
40S-20S. A SUBTROPICAL/NORTHERN POLAR JET PAIR WILL ACCOMPANY
THESE PERTURBATIONS AS THEY ENTER THE CONTINENT. THIS WILL ALLOW
THESE PERTURBATIONS TO ESTABLISH/SUSTAIN A SUBTROPICAL
CONNECTION. MIXING RATIO VALUES ACROSS JUAN FERNANDEZ ISLAND WILL
PEAK AT 12-14G/KG...WHILE OVER CENTRAL CHILE THEY WILL RANGE
BETWEEN 09-11G/KG. THE FIRST MID LEVEL TROUGH...ALONG 90W AND
SOUTH OF 18S...IS TO ENTER CENTRAL CHILE EARLY THIS PERIOD...AND
SUPPORT THE AFOREMENTIONED FRONT STREAMING ACROSS ARGENTINA. THE
NEXT WILL MOVE ALONG 100W AND SOUTH OF 15S BY 72 HRS...AND
THROUGH 108-120 HRS IT WILL TRACK EAST TOWARDS THE CONTINENT
BETWEEN 40S-20S. THE LATTER WILL FOCUS ITS SHORT WAVE ENERGY OVER
CHILE BETWEEN 34S-41S. IN THIS PATTERN ORGANIZED CONVECTION COULD
REACH AS FAR NORTH AS CHILLAN/TALCA IN CENTRAL CHILE. RAINFALL
MAXIMA OVER THIS AREA WILL PEAK AT 25-50MM/DAY ON DAY 04...AND AS
HIGH AS 35-70MM/DAY ON DAY 05. ORGANIZED SNOWFALL IS POSSIBLE ON
ELEVATIONS ABOVE 3KM.

ALSO AT MID LEVELS..THE MODELS FORECAST ADDITIONAL SOUTHERN
STREAM PERTURBATIONS TO GRADUALLY AMPLIFY AS THEY ROUND THE APEX
OF THE BLOCKING RIDGE ON THE FAR EASTERN PACIFIC. ONE
PERTURBATION IN PARTICULAR WILL ENTER THE ANTARCTIC PENINSULA BY
48 HRS...WITH MODELS AGREEING QUITE WELL ON ITS GENERAL MOTION
DURING THE NEXT FOUR TO FIVE DAYS. THE WAVE WILL LIFT ACROSS THE
DRAKE PASSAGE BY 72 HRS...AND IT WILL DEEPEN JUST EAST OF THE
MALVINAS BY 108 HRS. IN WEAK UPPER LEVEL JET SUPPORT...THE SYSTEM
WILL BE SLOW TO MIGRATE...AND IT IS LIKELY TO THEN MEANDER OVER
THE WESTERN ATLANTIC THROUGH AT LEAST 120 HRS. THE ASSOCIATED
SURFACE FRONT WILL ENTER THE ANTARCTIC PENINSULA BY 42-48 HRS...
AND TIERRA DEL FUEGO/SOUTHERN PATAGONIA BY 66-72 HRS...WHERE IT
IS TO FAVOR MOSTLY LIGHT RAINFALL AMOUNTS.

ACROSS NORTHERN SOUTH AMERICA...AT 200 HPA...THE SUBTROPICAL
RIDGE WILL REMAIN ON AREA NORTH OF 20S AND WEST OF 65W. DURING
THE NEXT THREE TO FOUR DAYS. AT LOW LEVELS...AN ELONGATED FRONT
WILL EXTEND INTO THE TROPICS ACROSS BAHIA-MINAS GERAIS TO CENTRAL
BRASIL. BUT THIS BOUNDARY IS TO QUICKLY WEAKEN THROUGH 42-48
HRS. MEANWHILE...AN UPPER LEVEL SHORT WAVE TROUGH WILL MOVE
ACROSS CENTRAL BRASIL AND IT IS TO TRIGGER SCATTERED CONVECTION
ALONG THIS BOUNDARY...AND IT IS FORECAST TO FAVOR RAINFALL MAXIMA
OF 20-40MM OVER MATO GROSSO DO SUL/PARAGUAY. THE PREVAILING
EASTERLY FLOW WILL CONVERGE ON THE COAST OF BRASIL ON THE STATES
OF BAHIA-PERNAMBUCO...WHERE IT WILL FAVOR RAINFALL MAXIMA OF 20-
35MM/DAY THROUGH 36-48 HRS...AND 15-25MM/DAY THEREAFTER. THE
DECREASE IN ACTIVITY COULD BE ATTRIBUTED TO THE WANING SURFACE
FRONT. SEA BREEZE CONVERGENCE IS TO ALSO REMAIN ACTIVE FROM AMAPA
TO RIO GRANDE DO NORTE...WITH MAXIMA OF 15-30MM/DAY TO AFFECT
THIS REGION. MOST INTENSE IS EXPECTED ACROSS ILHA DE
MARAJO/AMAPA. ALSO EXPECT SCATTERED CONVECTION OVER NORTHERN
PARA-AMAZONAS IN WESTERN/CENTRAL BRASIL TO FAVOR RAINFALL MAXIMA
OF 20-40MM/DAY. OVER THE CAUCA VALLEY IN SOUTHWEST COLOMBIA INTO
ECUADOR/NORTHERN PERU...DAILY MAXIMA WILL PEAK AT 15-25MM.

CARVAJAL...DAC (ECUADOR)
COLMAN...DMH (PARAGUAY)
DAVISON...HPC (USA)