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South American Forecast Discussion
 
(Latest Discussion - Issued 1645Z May 23, 2017)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1245 PM EDT TUE MAY 23 2017

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

MODEL DISCUSSION (VALID FROM MAY 23 AT 0000 UTC): A MID/UPPER
LEVEL RIDGE OVER THE EASTERN SOUTH PACIFIC SUSTAINS THE NORTHWARD
AMPLIFICATION OF A LONG WAVE TROUGH ACROSS THE SOUTHERN CONE TO
THE CENTRAL COAST OF CHILE. LATER TODAY THE TROUGH IS TO SPLIT IN
TWO...WITH THE SOUTHERN HALF TO ACCELERATE ACROSS THE SOUTHWEST
ATLANTIC WHILE THE NORTHERN HALF STALLS OFF THE COAST OF CHILE.
THE LATTER WILL THEN EVOLVE INTO A CLOSED LOW NEAR ARCHIPELAGO DE
JUAN FERNANDEZ LATER ON WEDNESDAY/EARLY THURSDAY MORNING. THE MID
LEVEL LOW IS TO REMAIN NEARLY STATIONARY THROUGH FRIDAY MORNING.
IT WILL THEN SPILL ACROSS THE CENTRAL ANDES OF CHILE INTO THE
NORTHERN PROVINCES IN ARGENTINA LATER IN THE DAY/SATURDAY MORNING.
AS IT MEANDERS OFF THE CENTRAL COAST OF CHILE...THE MID LEVEL
TROUGH WILL FAVOR A DIVERGENT PATTERN ALOFT THAT IS TO VENT
SCATTERED CONVECTION ON THE CENTRAL ANDES. THIS IS TO RESULT IN
HEAVY SNOWFALL AMOUNTS ON ELEVATIONS ABOVE 3000 METERS...WITH
ACCUMULATION OF 05-10CM EXPECTED THROUGH WEDNESDAY MORNING AND
25-50CM ON WEDNESDAY TO THURSDAY. AT LOW LEVELS...BROAD RIDGE OVER
THE SOUTH ATLANTIC FAVORS A MOIST NORTHERLY FLOW ACROSS
BOLIVIA/MATO GROSSO DO SUL IN BRASIL TO PARAGUAY-CHACO
ARGENTINO/CORRIENTES. IN THIS AREA BOUNDARY LAYER DEW POINT
TEMPERATURES RANGE BETWEEN 18-21C. THE INFLOW OF WARM MOIST AIR
WILL FEED ORGANIZED DEEP CONVECTION DURING THE NEXT FEW DAYS AS
MOISTURE CONVERGES ALONG A SURFACE FRONT MEANDERING OVER SOUTHERN
BRASIL-NORTHERN ARGENTINA. UNDER FAVORABLE JET DYNAMICS ALOFT...
ENVIRONMENTAL CONDITIONS WILL BECOME HIGHLY FAVORABLE AND
CONDUCIVE TO GENERATION OF MESO SCALE CONVECTIVE SYSTEMS DURING
THE NEXT COUPLE OF DAYS. MOST ACTIVE CONVECTION IS TO CLUSTER
ACROSS SOUTHERN BRASIL-CORRIENTES/MISIONES IN ARGENTINA TO
SOUTHEAST PARAGUAY...WITH MAXIMA OF 30-60MM EXPECTED THROUGH
WEDNESDAY MORNING. ON WEDNESDAY TO THURSDAY THIS INCREASES TO
50-100MM...WITH FORECAST STABILITY INDICES SHOWING ENHANCED RISK
OF SEVERE WEATHER AND POTENTIAL FOR LARGE HAIL STONES. ON
THURSDAY-FRIDAY MAXIMA IN THIS AREA WILL THEN PEAK AT 35-70MM.
OTHER CONVECTION TO THE NORTH WILL THEN BUILD ACROSS CENTRAL
BOLIVIA ON THURSDAY...WITH ACCUMULATION OF 15-30MM.

FURTHERMORE...THE HIGH AMPLITUDE RIDGE TO THE WEST IS TO CONTINUE
STEERING PROGRESSIVE SHORT WAVE PERTURBATIONS ON THE SOUTHERN
STREAM FLOW ACROSS THE DRAKE PASSAGE TO EXTREME SOUTHERN PORTIONS
OF CHILE AND TIERRA DEL FUEGO. EMBEDDED IN THIS FLOW A FAIRLY
STRONG MID LEVEL VORTEX MAKES LANDFALL OVER THE SOUTHERN CONE
EARLY ON WEDNESDAY MORNING...TO RAPIDLY PULL ACROSS PATAGONIA TO
THE WESTERN ATLANTIC LATER IN THE DAY. AS IT MOVES TO THE WESTERN
ATLANTIC THE SHORT WAVE TROUGH WILL RAPIDLY EVOLVE INTO A LONG
WAVE AXIS...FAVORING A COLD ADVECTIVE PATTERN ACROSS
PATAGONIA-CENTRAL ARGENTINA THAT ANCHORS ON A 1027-1029 HPA HIGH
LATER IN THE CYCLE. THE INFLOW OF SOUTHERN STREAM ENERGY WILL
COMBINE WITH BELOW NORMAL TEMPERATURES TO LIKELY FAVOR SCATTERED
SNOW SHOWERS OVER CHILE SOUTH OF 50S. IN THIS AREA EXPECTING
ACCUMULATION OF 05-10CM TO CLUSTER ALONG THE COAST/SOUTHERN ANDES.

NORTH OVER THE CONTINENT...AT 200 HPA...CELL OF THE SUBTROPICAL
RIDGE ENVELOPS MOST OF BRASIL-PERU-ECUADOR. THIS PATTERN WILL HOLD
NEARLY UNCHANGED DURING THE NEXT 60-72 HRS...THEN WEAKEN LATER IN
THE CYCLE AS HIGHER AMPLITUDE TROUGH STREAMS ACROSS THE SOUTHERN
CONE. MEANWHILE...THE RIDGE ALOFT WILL VENT DIURNAL CONVECTION
ACROSS CENTRAL-WESTERN BRASIL. ACROSS CENTRAL BRASIL THE DAILY
MAXIMA IS TO RANGE BETWEEN 10-15MM...WHILE OVER NORTHWEST
BRASIL/NORTHERN PERU-SOUTHERN COLOMBIA TO EASTERN ECUADOR THE
MAXIMA IS TO PEAK AT 15-30MM. ON THE PERUVIAN JUNGLE TO THE SOUTH
MAXIMA WILL PEAK AT 05-10MM...WITH LOCALLY HIGHER AMOUNTS
POSSIBLE. OVER NORTHEAST BRASIL...MEANWHILE...AN OLD FRONT OVER
THE SOUTH ATLANTIC SUSTAINS A PREFRONTAL SHEAR LINE ACROSS
PARAIBA-NORTHERN BAHIA. THIS IS ENHANCING LOW LEVEL CONVERGENCE
WHILE TRIGGERING SCATTERED CONVECTION. IN THIS AREA EXPECTING
RAINFALL AMOUNTS OF 15-30MM/DAY WITH LOCALIZED MAXIMA OF 20-45MM.

ABDEMUR...FAA (ARGENTINA)
ESPINOLA...DNM (PARAGUAY)
DAVISON...WPC (USA)