Prognostic Meteorological Discussion
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000
FXSA20 KWBC 181809
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
108 PM EST MON DEC 18 2017

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

MODEL DISCUSSION (VALID FROM DEC 18 AT 0000 UTC): GLOBAL MODELS
GENERALLY AGREE ON HOW PATTERN OVER THE EASTERN PACIFIC IS GOING
TO EVOLVE DURING THE NEXT SIX DAYS. OVER THE ATLANTIC THEY FOLLOW
A SIMILAR EVOLUTION THROUGH 72-96 HRS...THEN DISAGREE ON THE
AMPLITUDE OF A LONG WAVE TROUGH STREAMING ACROSS THE SOUTHERN CONE
TO THE WESTERN ATLANTIC. BOTH EUROPEAN MODELS FAVOR A
DEEPER/HIGHER AMPLITUDE TROUGH THAN WHAT THE GFS SUGGESTS.
ALTHOUGH THE MEAN OF THE GLOBAL ENSEMBLES FAVOR THE GFS
SOLUTION...THE VARIABILITY AMONG SOME OF THE ENSEMBLE MEMBERS IS
LARGE ENOUGH TO PUT THE FORECAST IN QUESTION.

OVER THE EASTERN PACIFIC...DEEPENING MID LEVEL TROUGH DOMINATES
THE FLOW...WITH AXIS TO AMPLIFY NORTH BETWEEN 110W-70W TO 35S. AS
IT NEARS THE SOUTHERN CONE LATER ON TUESDAY...THE TROUGH IS TO
FOCUS ITS SHORT WAVE ENERGY OVER CHILE/ARGENTINA TO THE SOUTH OF
40S...WHERE IT IS TO SUSTAIN HEIGHT FALLS IN EXCESS OF 200GPM. ON
WEDNESDAY THE TROUGH IS TO SLOWLY MOVE ACROSS THE SOUTHERN
CONE/DRAKE PASSAGE TO THE FAR WESTERN ATLANTIC...DAMPENING AS IT
CROSSES 40W ON THURSDAY. AT LOW LEVELS THIS WILL SUSTAIN A BROAD
TROUGH OVER THE SOUTHEAST PACIFIC THAT IS TO CENTER ON A DEEP
OCCLUDED LOW OVER THE BELLINGSHAUSEN SEA. POLAR FRONTS WILL EMBED
IN THIS FLOW...WITH ONE TO ENTER SOUTHERN CHILE EARLY ON TUESDAY.
AS IT NEARS THE SOUTHERN CONE THE FRONT WILL SUSTAIN A SUBTROPICAL
CONNECTION...ADVECTING WARM/MOIST AIR THAT IS TO CONVERGE ON THE
SOUTHERN REGIONS OF CHILE. THE FRONT THEN CONTINUES ACROSS
PATAGONIA ON TUESDAY...AND ON WEDNESDAY IT MOVES NORTH ACROSS LA
PAMPA TO THE CENTRAL PROVINCES IN ARGENTINA. OVER SOUTHERN CHILE
THIS IS TO FAVOR MODERATE RAINFALL AMOUNTS ON
TUESDAY-WEDNESDAY...WITH ACCUMULATION OF 20-30MM. DUE TO STRONG
TOPOGRAPHICAL FORCING LOCALLY HIGHER AMOUNTS ARE HIGHLY PROBABLE.
OVER CENTRAL ARGENTINA THE MAXIMA ON WEDNESDAY AFTERNOON WILL PEAK
AT 20-40MM...WHILE ON THURSDAY MORNING IT INCREASES TO 25-50MM AS
AN MCS FORMS ALONG THE SURFACE FRONT. ON BOTH DAYS THERE IS AN
ELEVATED RISK OF SEVERE THUNDERSTORMS BUILDING AHEAD OF THE
SURFACE FRONT...WITH FORECAST INDICES SHOWING A HIGHER POTENTIAL
FOR LARGE HAIL ON WEDNESDAY.

FARTHER NORTH...AN ELONGATED FRONT FLANKS THE EASTERN-NORTHERN
FRINGES OF A SURFACE RIDGE ON THE NORTHERN STREAM OFF THE COAST OF
ARGENTINA AND URUGUAY. THE FRONT STRETCHES ACROSS SOUTHERN STATES
OF BRASIL TO NORTHERN ARGENTINA. THIS BOUNDARY...AS IT
PERSISTS...WILL BE THE FOCUS OF LOW LEVEL MOISTURE CONVERGENCE. AT
500 HPA...A SHORT WAVE TROUGH ON THE NORTHERN STREAM IS TO ENTER
CENTRAL CHILE LATER TODAY...TO MOVE ACROSS RIO DE LA PLATA
BASIN/SOUTHERN BRASIL ON TUESDAY EVENING. ON WEDNESDAY-THURSDAY
THE SHORT WAVE PERTURBATION IS TO THEN MEANDER EAST ACROSS 40W. AS
IT ENTERS THE CONTINENT THE MID LEVEL VORTEX IS TO INTERACT WITH
THE SURFACE FRONT. ON THE SOUTHERN STATES OF BRASIL AND PARAGUAY
THIS IS TO INITIALLY FAVOR MAXIMA OF 15-20MM. ON TUESDAY AND
WEDNESDAY THIS IS TO BECOME BETTER ORGANIZED...TO SUSTAIN MODERATE
TO HEAVY CONVECTION WITH MAXIMA OF 25-50MM.

DEEPENING SHORT WAVE TROUGH ON THE SOUTHERN STREAM IS TO
FOLLOW...WITH MID LEVEL TROUGH TO MOVE ACROSS 90W AND SOUTH OF 35S
ON WEDNESDAY... EARING THE SOUTHERN REGIONS OF CHILE ON THURSDAY
AFTERNOON. ON FRIDAY THIS IS TO MOVE ACROSS 60W TO THE CENTRAL
PROVINCES IN ARGENTINA. THE ASSOCIATED FRONT RACES ACROSS THE
SOUTHEAST PACIFIC TO SOUTHERN PORTIONS OF CHILE EARLY ON THURSDAY
MORNING...MOVING ACROSS PATAGONIA TO THE WESTERN ATLANTIC LATER IN
THE DAY. THIS IS TO INTERACT WITH THE AFOREMENTIONED BOUNDARY TO
THE NORTH...DRIVING THIS FRONT FARTHER NORTH ACROSS RIO DE LA
PLATA BASIN-CENTRAL ARGENTINA ON THURSDAY-FRIDAY. ON THE SOUTHERN
REGIONS OF CHILE THE FRONT IS TO FAVOR SCATTERED CONVECTION WITH
MAXIMA OF 15-25MM ON THURSDAY-FRIDAY...WHILE OVER CENTRAL
ARGENTINA IT IS TO TRIGGER RAINFALL MAXIMA OF 25-50MM IN
POTENTIALLY SEVERE CONVECTION. ON FRIDAY THIS INCREASES TO 40-80MM
WHILE CLUSTERING ACROSS RIO DE LA PLATA BASIN/URUGUAY-NORTHERN
PROVINCES IN ARGENTINA.

NORTH OVER THE CONTINENT...AT 200 HPA...A CLOSED HIGH IS TO CENTER
OVER BOLIVIA DURING THE NEXT FEW DAYS...MEANWHILE ANCHORING A
RIDGE TO THE NORTH OF 30S AND WEST OF 40W/45W. AS THE RIDGE
MEANDERS OVER THE CONTINENT...AN UPPER LEVEL TROUGH IS TO AMPLIFY
ACROSS NORTHEAST BRASIL ON TUESDAY-WEDNESDAY. A LOW IS TO THEN
CLOSE NEAR 10S 40W ON WEDNESDAY-THURSDAY. AS THE TROUGH DEEPENS IT
IS TO THEN ENHANCE CONVECTIVE DEVELOPMENT ALONG THE NORTH COAST OF
BRASIL...WHERE THE MAXIMA IS TO PEAK AT 15-30MM/DAY ON
WEDNESDAY-FRIDAY. THE RIDGE ALOFT...MEANWHILE...WILL VENT
ORGANIZED CONVECTION ACROSS BOLIVIA-SOUTHERN JUNGLE OF
PERU-ACRE/RONDONIA-AMAZONAS IN BRASIL...WITH MAXIMA OF 35-70MM ON
MONDAY THROUGH WEDNESDAY. ON THURSDAY THIS DECREASES TO
20-40MM/DAY...WHILE ON FRIDAY THE MAXIMA WILL PEAK AT 20-30MM. THE
RIDGE ALOFT IS TO ALSO VENT DEEP CONVECTION ACROSS THE SOUTHERN
SIERRA OF PERU-ALTIPLANO OF BOLIVIA...WITH MAXIMA OF 10-15MM/DAY.
ON WEDNESDAY CONVECTION IS TO BECOME BETTER ORGANIZED...WITH
MAXIMA OF 15-30MM ALONG THE PERU/BOLIVIA BORDER...WHILE OVER THE
ALTIPLANO TO THE SOUTH THE MAXIMA WILL PEAK AT 20-40MM.

OVIEDO...SHN (ARGENTINA)
POMIER...SENAMHI (BOLIVIA)
RIVAS...SENAMHI (PERU)
DAVISON/GALVEZ...WPC (USA)
$$




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