Prognostic Meteorological Discussion
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FXSA20 KWBC 231627
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1226 PM EDT WED AUG 23 2017

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

MODEL DISCUSSION (VALID FROM AUG 23 AT 0000 UTC): UNDER INFLUENCE
OF A MID/UPPER LEVEL RIDGE ALONG 110W/120W...A HIGH AMPLITUDE LONG
WAVE TROUGH EXTENDS FROM THE BELLINGSHAUSEN SEA TO THE FAR EASTERN
PACIFIC/SOUTHERN CONE OF SOUTH AMERICA. AT LOW LEVELS...THE LONG
WAVE TROUGH SUSTAINS A SURFACE FRONT THAT IS TO EXTEND TO THE
NORTHWEST ACROSS THE BUENOS AIRES PROVINCE-MENDOZA IN ARGENTINA-LA
SERENA CHILE TO THE EASTERN PACIFIC. THE SURFACE FRONT IS TO THEN
REMAIN NEARLY STATIONARY OVER THE NEXT FEW DAYS. ALSO AT LOW
LEVELS...A NORTHERLY LOW LEVEL JET ACROSS PARAGUAY TO NORTHERN
ARGENTINA IS TO SUSTAIN A WARM/MOIST ADVECTIVE PATTERN...THAT IS
TO CONVERGE ALONG THE MEANDERING FRONT TO THE SOUTH. UNDER
FAVORABLE UPPER JET DYNAMICS ENVIRONMENTAL CONDITIONS WILL QUICKLY
BECOME FAVORABLE FOR GENERATION OF MESO SCALE CONVECTIVE SYSTEMS
ALONG THIS BOUNDARY ON THURSDAY-FRIDAY. EARLY IN THE CYCLE
EXPECTING SCATTERED CONVECTION ACROSS URUGUAY-ENTRE RIOS IN
ARGENTINA TO TRIGGER RAINFALL AMOUNTS OF 20-45MM. ON THURSDAY
CONVECTION BECOMES BETTER ORGANIZED...WITH MAXIMA TO INCREASE TO
35-70MM IN POTENTIALLY SEVERE CONVECTION. ALSO ON THURSDAY...THE
MODEL GUIDANCE SHOWS RISK OF LARGE HAIL STONES BETWEEN
CORDOBA-SOUTHERN ENTRE RIOS AND WESTERN BUENOS AIRES PROVINCE. ON
FRIDAY-SATURDAY THE RISK OF MCS AND POTENTIALLY SEVERE CONVECTION
FORMING ALONG THIS BOUNDARY WILL PERSIST...WITH MAXIMA TO PEAK AT
40-80MM. ON SATURDAY THE MAXIMA DECREASES TO 20-45MM WHILE
BUILDING NORTH ACROSS CORRIENTES IN ARGENTINA TO MISIONES AND
CENTRAL PARAGUAY.

AS THE LONG WAVE TROUGH MEANDERS OVER THE SOUTHERN CONE...
PROGRESSIVE MID LEVEL SHORT WAVE PERTURBATIONS ARE TO REVOLVE
AROUND THIS AXIS. IN THIS PATTERN A STRONG TROUGH REACHES SOUTHERN
CHILE LATER TODAY...TO QUICKLY PULL ACROSS PATAGONIA IN ARGENTINA
TO THE WESTERN ATLANTIC EARLY ON THURSDAY. AT LOW LEVELS THIS WILL
SUSTAIN A DEEP OCCLUDED LOW OVER SOUTHERN CHILE LATER
TODAY...WHERE IN A TIGHT GRADIENT THE LOW LEVEL WINDS WILL RANGE
BETWEEN 35-50KT. THE OCCLUDED LOW WILL TRACK TO THE SOUTHEAST
ACROSS THE DRAKE PASSAGE ON THURSDAY. THE ASSOCIATED FRONT MOVES
ACROSS PATAGONIA IN ARGENTINA TO CENTRAL CHILE ON THURSDAY...WHILE
ON FRIDAY IT REINFORCES THE OLD BOUNDARY TO THE NORTH. THE DEEP
POLAR TROUGH WILL SUSTAIN A COLD ADVECTIVE PATTERN THAT IS LIKELY
TO TRIGGER THUNDERSTORMS AS IT MAKES LANDFALL BETWEEN TEMUCO AND
ISLA DE CHILOE...TO RESULT IN RAINFALL AMOUNTS OF 20-40MM ON
WEDNESDAY-THURSDAY. SOUTH OF CHILOE THE MAXIMA WILL PEAK AT
10-15MM IN MIXED PRECIPITATION (RAIN AND SNOW). ON THURSDAY THE
AIR MASS WILL COOL ENOUGH TO SUSTAIN SCATTERED SNOW SHOWERS ACROSS
THE SOUTHERN ANDES/TIERRA DEL FUEGO...WHERE WE EXPECT SNOWFALL
AMOUNTS OF 05-10CM. OVER CENTRAL CHILE THIS IS TO THEN TRIGGER
SCATTERED CONVECTION WITH ACCUMULATION OF 15-25MM ON THURSDAY TO
FRIDAY.

THIS SETS THE STAGE FOR A DEEPER TROUGH IN THE SOUTHERN STREAM TO
FOLLOW...WITH MID LEVEL VORTEX NEARING SOUTHERN CHILE LATER ON
FRIDAY. BUILDING RIDGE TO THE WEST IS TO THEN DRIVE THIS FEATURE
NORTH ALONG THE COAST OF CHILE TO CENTRAL REGIONS ON
SUNDAY-MONDAY. AT LOW LEVELS IT SUSTAINS A DEEP OCCLUDED LOW OFF
THE SOUTH COAST OF CHILE LATER ON FRIDAY...THAT MEANDERS NORTH
ALONG THE COAST TO CENTRAL REGIONS LATER DURING THE WEEKEND. THE
ASSOCIATED FRONT IS TO THE MEANDER EAST ACROSS PATAGONIA IN
ARGENTINA TO CENTRAL REGIONS OF CHILE. AS IT ENTERS SOUTHERN CHILE
THIS IS TO SUSTAIN MODERATE TO HEAVY CONVECTION...WITH MAXIMA OF
20-35MM EXPECTED BETWEEN CONCEPCION-PUERTO MONTT...AND 15-20MM
BETWEEN PUERTO MONTT AND ISLA DE CHILOE.

CONSISTENT WITH PREVIOUS FORECAST...A SHORT WAVE TROUGH ON THE
NORTHERN STREAM MEANDERS OVER THE SOUTH ATLANTIC BETWEEN 30W-10W
AND TO THE SOUTH OF 20S. OVER THE NEXT FEW DAYS IT IS TO DRIFT TO
THE EAST-NORTHEAST. MEANWHILE...THIS SUSTAIN AN ELONGATED FRONT
THAT IS TO EXTEND WEST ACROSS THE ATLANTIC TO ESPIRITO SANTO/MINAS
GERAIS IN BRASIL. TRAILING END OF THIS BOUNDARY OVER THE CONTINENT
IS TO FRONTOLIZE LATER ON THURSDAY...WHILE THE REMNANTS OVER THE
ATLANTIC DISSIPATES LATER ON SATURDAY. MEANWHILE...THIS IS TO
ENHANCE COASTAL CONVERGENCE ACROSS ESPIRITO SANTO-EASTERN
BAHIA-PERNAMBUCO IN BRASIL. IN THIS AREA EXPECTING RAINFALL
AMOUNTS OF 05-10MM/DAY WITH ISOLATED MAXIMA OF 15-20MM. THROUGH
FRIDAY...AS THE FRONT WEAKENS... THIS DECREASES TO 00-05MM/DAY AND
ISOLATED MAXIMA OF 10MM.

NORTH OVER THE CONTINENT...A 200 HPA RIDGE IS TO EXTEND WEST
ACROSS THE TROPICAL ATLANTIC TO NORTHERN BRASIL. THE RIDGE IS TO
CONFINE TO AREA NORTH OF 10S DURING THE NEXT COUPLE OF DAYS...WITH
A SECONDARY HIGH/RIDGE TO DEVELOP OVER AMAZONAS IN BRASIL-NORTHERN
PERU/ECUADOR LATER ON FRIDAY-SATURDAY. THE RIDGE ALOFT IS TO VENT
CONVECTION ACROSS SOUTHERN COLOMBIA-NORTHERN AMAZONAS IN
BRASIL-NORTHERN JUNGLE OF PERU AND EASTERN ECUADOR. IN THIS AREA
EXPECTING RAINFALL AMOUNTS OF 05-10MM/DAY WITH MAXIMA OF 15-25MM
IN SCATTERED DEEP CONVECTION. OVER THE AMAZON RIVER DELTA REGION
AND AMAPA TO THE NORTH EXPECTING RAINFALL AMOUNTS OF 05-10MM/DAY
WITH LOCALIZED MAXIMA OF 15MM.

GONZALEZ...DMC (CHILE)
VANNUCCI...SMN (ARGENTINA)
DAVISON...WPC (USA)
$$




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