204
FXSA20 KWBC 281548
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1147 AM EDT THU JUN 28 2012
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
NOTE: UNDER FAVORABLE JET DYNAMICS...ENVIRONMENTAL CONDITIONS ARE
TO FAVOR A SURGE IN CONVECTIVE ACTIVITY ACROSS RIO DE LA PLATA
BASIN/URUGUAY-LOWER MESOPOTAMIA VALLEY BY 18-36 HRS. ALTHOUGH WE
PROJECT MAXIMA OF 30-60MM...STRONG VENTILATION ALOFT WILL LIKELY
FAVOR LOCALIZED MAXIMA OF 75-100MM. STRONG DOWNDRAFT/GUSTY WINDS
ARE TO ACCOMPANY THESE CELLS AS THEY SPREAD ACROSS URUGUAY.
MODEL COMPARISON (VALID FROM 00 UTC JUN 28): MODELS ARE
CONSISTENT ON THEIR EVOLUTION THROUGH 96-108 HRS...AS THEY SHOW
DEEP TROUGH PATTERN DOMINATING FLOW ACROSS THE SOUTHERN CONE TO
MID SECTIONS OF SOUTH AMERICA. THEY...HOWEVER...CONTINUE TO
DIVERGE INTO THE MEDIUM RANGE PERIOD...AS THEY FAIL TO REACH A
CONSENSUS ON INTENSITY/TRAJECTORY OF A PROGRESSIVE CLIPPER LOW
ENTERING THE CONTINENT.
AT MID/UPPER LEVELS...OVER THE SOUTH PACIFIC...THE MODELS SHOW A
BLOCKING RIDGE WITH AXIS SOUTH 130W/140W TO 60S. THE RIDGE WILL
REACH MAXIMUM AMPLITUDE BY 48-72 HRS...THEN HOLD THROUGH 120 HRS.
THIS RIDGE WILL INDUCE THE NORTHWARD AMPLIFICATION OF A TROUGH ON
THE SOUTHERN STREAM INTO THE DRAKE PASSAGE/SOUTHERN CONE OF SOUTH
AMERICA. AS THE TROUGH MEANDERS OVER THE SOUTHERN CONE...IT IS TO
THEN DRAW A SECONDARY VORTEX INLAND THROUGH 60-66 HRS...WITH AXIS
TO RAPIDLY AMPLIFY NORTH AS IT MOVES ACROSS PATAGONIA/LA PAMPA TO
CENTRAL PROVINCES IN ARGENTINA/CHILE BY 84-96 HRS. AT LOW
LEVELS...IT WILL SUSTAIN A BROAD TROUGH ACROSS THE DRAKE PASSAGE
TO THE SOUTHERN CONE. POLAR FRONTS WILL REVOLVE AROUND THIS
AXIS...TO INITIALLY CONFINE TO SOUTHERN CHILE/PATAGONIA IN
ARGENTINA. THIS PATTERN WILL HOLD THROUGH 60-72 HRS. BUT BY 84-96
HRS...AS A POLAR RIDGE BUILDS ACROSS THE SOUTHERN CONE...THE
FRONT WILL SURGE NORTH ACROSS BUENOS AIRES PROVINCE-CUYO IN
ARGENTINA/CENTRAL CHILE. THROUGH 108-120 HRS THE FRONT WILL THEN
STALL ACROSS RIO GRANDE DO SUL/NORTHERN URUGUAY TO NORTHERN
PROVINCES IN ARGENTINA/SOUTHERN BOLIVIA. THE DEEP TROUGH PATTERN
TO THE SOUTH IS TO CONTINUE FAVORING SCATTERED SNOW SHOWERS
ACROSS CHILE SOUTH OF ISLA DE CHILOE...WITH ACCUMULATION OF 05-
10MM/DAY (LIQUID WATER EQUIVALENT). HIGHER AMOUNTS ARE EXPECTED
BETWEEN CONCEPCION AND ISLA DE CHILOE...WITH ACCUMULATION OF 15-
25MM/DAY. THIS WILL PERSIST THROUGH 72-84 HRS. OTHER ACTIVITY IS
TO DEVELOP ACROSS THE CENTRAL ANDES OF CHILE...WITH SNOWFALL OVER
THE ANDES. AS THE FRONT SURGES NORTH ACROSS RIO DE LA PLATA
BASIN...IT WILL SUSTAIN SCATTERED CONVECTION WITH ACCUMULATION OF
15-20MM/DAY BY 96-108 HRS. A COLD SURGE IS TO ACCOMPANY THIS
BOUNDARY...WITH NEAR FREEZING TEMPERATURES EXPECTED INTO THE
BUENOS AIRES PROVINCE/CENTRAL ARGENTINA BY 132-144 HRS.
AS THE AFOREMENTIONED TROUGH AMPLIFIES ACROSS THE SOUTHERN
CONE...IT WILL DRAW A TROUGH ON THE NORTHERN STREAM INLAND ACROSS
CENTRAL CHILE TO CENTRAL ARGENTINA EARLY IN THE CYCLE. A
SUBTROPICAL/POLAR JET PAIR WILL ACCOMPANY THIS PERTURBATION...AND
THIS WILL SUSTAIN A DIVERGENT PATTERN ALOFT AS THEY PHASE OVER
RIO DE LA PLATA BASIN/MESOPOTAMIA VALLEY-URUGUAY. THE INFLOW OF
SHORT WAVE ENERGY/COLD AIR ALOFT WILL SUSTAIN HEIGHT FALLS OF 75-
100GPM ACROSS RIO DE LA PLATA BASIN. AT LOW LEVELS...A POLAR
FRONT WILL UNDULATE NORTH ACROSS URUGUAY-MENDOZA IN ARGENTINA-
CENTRAL CHILE EARLY THIS CYCLE. THE FRONT IS TO THEN REMAIN
NEARLY STATIONARY THROUGH 72-84 HRS. ALSO AT LOW LEVELS...A
NORTHERLY JET ACROSS BOLIVIA-PARAGUAY WILL SUSTAIN A WARM/MOIST
ADVECTIVE PATTERN...WITH BOUNDARY LAYER DEW POINT TEMPERATURES TO
INCREASE TO 18-19C LATER THIS AFTERNOON. MOISTURE WILL THEN POOL
ALONG THE POLAR FRONT...TO FAVOR ORGANIZED DEEP CONVECTION WITH
ACCUMULATION OF 30-60MM/DAY. THROUGH 72 HRS ACTIVITY WILL
WANE...DECREASING TO 05-10MM/DAY. ACROSS CENTRAL CHILE THIS WILL
FAVOR SCATTERED SNOW SHOWERS OVER THE ANDES WITH ACCUMULATION OF
05-10MM (LIQUID WATER EQUIVALENT).
DURING THE MEDIUM RANGE PERIOD...THE MODELS SHOW A PROGRESSIVE
PERTURBATION IN THE SOUTHERN STREAM QUICKLY STREAMING ACROSS THE
SOUTHEAST PACIFIC. AS PREVIOUSLY INDICATED...THEY TEND TO DIVERGE
ON INTENSITY/IMPACT THIS IS GOING TO HAVE ACROSS THE SOUTHERN
CONE. THE GFS AND UKMET GENERALLY SIDE IN FAVOR OF A MORE
PROGRESSIVE PERTURBATION...WHILE THE ECMWF FORECASTS THE LOW TO
STALL OVER TIERRA DEL FUEGO/PATAGONIA. THIS IS ADDING TO THE
UNCERTAINTY AND LOW CONFIDENCE IN THE LONG RANGE FORECAST. IF
THIS EVOLVES AS SLOW AS THE ECMWF SUGGESTS...WE COULD BE DEALING
WITH A HEAVY SNOWFALL EVENT.
AT 200 HPA...THE MODELS CONTINUE TO FORECAST THE SUBTROPICAL
RIDGE PATTERN TO REMAIN ILL ORGANIZED WHILE CONFINING TO
CONTINENTAL AREA TO THE NORTH OF 10S. LITTLE TO NO VARIABILITY IN
THIS PATTERN IS EXPECTED THROUGH DAY 02. BY 72-96 HRS...AS A
TROUGH AMPLIFIES ACROSS SOUTHEAST BRASIL-MATO GROSSO...THE RIDGE
TO THE NORTH IS TO NEARLY COLLAPSE/CRUMBLE. MEANWHILE...THE RIDGE
ALOFT IS TO CONTINUE VENTING DEEP CONVECTION ACROSS NORTHERN
PORTIONS OF BRASIL. ACROSS PARA-AMAZONAS TO SOUTHERN COLOMBIA
EXPECTING ACCUMULATION OTHER CONVECTION ALONG THE NORTHERN COAST
OF BRASIL IS TO SUSTAIN RAINFALL AMOUNTS OF 10-15MM/DAY...WITH A
GRADUAL SURGE IN ACTIVITY THROUGH DAY 02...WITH MAXIMA OF 15-
20MM/DAY. AS THE TROUGH ESTABLISHES ACROSS BRASIL...EXPECTING
COASTAL ACTIVITY ACROSS NORTHEAST BRASIL TO BECOME MORE
ACTIVE/BETTER ORGANIZED...WITH ACCUMULATION OF 15-20MM/DAY.
PARRENO...INAMHI (ECUADOR)
SANTAYANA...DNM (URUGUAY)
SOLORZANO...SMN(HONDURAS)
DAVISON...HPC (USA)
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