THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


SOUTH AMERICAN MODEL DISCUSSION




SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1214 PM EDT THU MAR 23 2017

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

MODEL DISCUSSION (VALID FROM MARCH 23 AT 0000 UTC): MODELS ARE
SLOWLY REACHING A CONSENSUS ON HOW PATTERN OVER THE SOUTHERN CONE
IS GOING TO EVOLVE DURING THE MEDIUM RANGE PERIOD. MODELS ARE NOW
TRENDING IN SUPPORT OF A DEEPER/SLOWER TROUGH TO REMAIN OFF THE
SOUTH COAST OF CHILE. AS THE MODELS FALL IN AGREEMENT...OUR
CONFIDENCE IN THE FORECAST IS HIGHER THAN BEFORE.

DEEPENING MID/UPPER LEVEL TROUGH MOVES ACROSS THE EASTERN PACIFIC
EARLY ON FRIDAY MORNING. AS IT APPROACHES THE SOUTHERN CONE...THE
TROUGH WILL THEN CLASH WITH A RIDGE TO THE EAST. THE RIDGE IS TO
EXTEND SOUTH ALONG 60W TO THE ANTARCTIC PENINSULA. AS THEY
COLLIDE...THE TROUGH WILL SPLIT IN TWO...WITH A SHORT WAVE TROUGH
TO THE SOUTH TO QUICKLY SHEAR UNDER THE RIDGE AS IT STREAMS ACROSS
THE DRAKE PASSAGE ON SATURDAY EVENING. THE REMNANTS TO THE
NORTH... MEANWHILE...ARE TO MEANDER OFF THE SOUTH COAST OF CHILE
WHILE EVOLVING INTO A CLOSED LOW JUST WEST OF ISLA DE CHILOE ON
SATURDAY. AT LOW LEVELS...THE TROUGH ALOFT WILL SUSTAIN A
PROGRESSIVE FRONT THAT ENTERS SOUTHERN CHILE EARLY ON FRIDAY
MORNING. DEEPENING MID LEVEL LOW WILL THEN SUPPORT CYCLOGENESIS
ALONG THE MEANDERING FRONT AS IT LIES OFF THE SOUTH COAST OF
CHILE. THE LOW IS FORECAST TO GRADUALLY OCCLUDE ON SATURDAY. THE
SURFACE LOW AND ASSOCIATED FRONT WILL THEN MOVE EAST ACROSS
PATAGONIA/LA PAMPA IN ARGENTINA ON SUNDAY. AS IT ENTERS SOUTHERN
CHILE ON FRIDAY THIS IS TO ENHANCE MOISTURE CONVERGENCE TO TRIGGER
SCATTERED CONVECTION WITH ACCUMULATION OF 05-10MM. ON SATURDAY TO
SUNDAY THIS INCREASES TO 15-20MM...WITH SIMILAR AMOUNTS OVER
NORTHERN PATAGONIA-LA PAMPA-SOUTHERN CUYO IN ARGENTINA ON SUNDAY.

AS THE MID LEVEL LOW/TROUGH DEEPENS TO THE SOUTH...IT IS TO
INTERACT WITH A MEANDERING SHORT WAVE TROUGH OFF THE CENTRAL COAST
OF CHILE. THE SHORT WAVE TROUGH IS TO GRADUALLY WEAKEN THROUGH
SATURDAY EVENING WHILE SHEARING SHORT WAVE ENERGY ACROSS THE
CENTRAL/NORTHERN ANDES INTO NORTHERN ARGENTINA/MID SECTIONS OF
SOUTH AMERICA. THE INFLOW OF SHORT WAVE ENERGY IS TO INITIALLY
FAVOR SCATTERED CONVECTION ACROSS RIO DE LA PLATA BASIN/SOUTHERN
STATES OF BRASIL...WITH ACCUMULATION OF 10-15MM/DAY EXPECTED
THROUGH FRIDAY EVENING. A NORTHERLY LOW LEVEL JET ACROSS EASTERN
BOLIVIA/PARAGUAY IS TO THEN ENHANCE MOISTURE TRANSPORT/CONVERGENCE
ACROSS THE NORTHWEST PROVINCES IN ARGENTINA ON
SATURDAY-SUNDAY...WITH PRECIPITABLE WATER CONTENT TO INCREASE TO
25-50MM. THIS WILL COMBINE WITH INFLOW OF SHORT WAVE ENERGY ALOFT
TO TRIGGER SCATTERED CONVECTION ACROSS PARAGUAY-SOUTHERN BOLIVIA
AND NORTHERN ARGENTINA...WITH RAINFALL MAXIMA OF 20-35MM IN
POTENTIALLY SEVERE CONVECTION. ON SUNDAY THIS IS TO INCREASE TO
MAXIMA OF 20-45MM AS ENHANCED BY A SUBTROPICAL JET MAXIMA
STREAMING ACROSS MID SECTIONS OF SOUTH AMERICA.

MODELS THEN FORECAST THE NEXT MID/UPPER LEVEL TROUGH TO STREAM
ACROSS THE SOUTH PACIFIC...NEARING 120W LATER ON FRIDAY...AND
PULLING ACROSS 95W ON SATURDAY. BUILDING RIDGE TO THE WEST IS TO
THEN SUSTAIN THE RAPID AMPLIFICATION OF THIS TROUGH ALONG THE
SOUTH COAST OF CHILE ON SATURDAY. AS THE RIDGE TO THE WEST
UNDERCUTS THE LONG WAVE TROUGH...IT IS FORECAST TO EVOLVE INTO A
CLOSED LOW THAT MEANDERS OFF THE SOUTH COAST OF CHILE ON
SUNDAY-MONDAY. NOTE THAT THIS IS A MUCH SLOWER EVOLUTION THAN WHAT
THE MODELS PREVIOUSLY SUGGESTED. AT LOW LEVELS IT IS TO SUSTAIN A
PROGRESSIVE FRONT THAT REACHES SOUTHERN CHILE LATER ON SUNDAY. IN
A SIMILAR EVOLUTION TO THE FORMER...A LOW ALONG THIS AXIS IS TO
RAPIDLY OCCLUDE ON SUNDAY WHILE MEANDERING OFF THE SOUTH COAST OF
CHILE. THE DEEPENING TROUGH WILL THEN SUSTAIN A SUBTROPICAL
CONNECTION...ENHANCING MOISTURE CONVERGENCE ACROSS SOUTHERN
PORTIONS OF CHILE...WHERE WE EXPECT MAXIMA OF 15-20MM.

PATTERN OVER NORTHERN SOUTH AMERICA IS TO REMAIN UNDER INFLUENCE
OF THE SUBTROPICAL RIDGE THROUGHOUT THE MEDIUM RANGE PERIOD...WITH
AXIS TO ANCHOR ON A 200 HPA HIGH THAT IS TO MEANDER BETWEEN MATO
GROSSO IN BRASIL AND NORTHERN BOLIVIA. THE RIDGE ALOFT IS TO VENT
EQUATORIAL TROUGH/ITCZ RELATED CONVECTION OVER NORTHERN SOUTH
AMERICA. MOST ACTIVE EXPECTED TO THE NORTH OF 05S. ALONG THE NORTH
COAST OF BRASIL THIS IS TO GENERALLY FAVOR SCATTERED DEEP
CONVECTION WITH DAILY MAXIMA OF 20-35MM. OVER NORTHERN
PARA-AMAZONAS/RORAIMA IN BRASIL TO SOUTHERN COLOMBIA EXPECTING
MAXIMA OF 25-50MM DURING THE NEXT FEW DAYS. LESSER AMOUNTS TO THE
SOUTH WILL AFFECT SOUTHERN AMAZONAS/ACRE-RONDONIA AND THE CENTRAL
STATES OF BRASIL...WITH DAILY MAXIMA IN THIS AREA TO PEAK AT
20-30MM. ON THE SIERRA OF PERU AND HIGH JUNGLE THE DAILY MAXIMA
WILL PEAK AT 10-15MM/DAY. HIGHER AMOUNTS...HOWEVER...ARE EXPECTED
ALONG THE NORTH COAST AND WESTERN ECUADOR...WHERE UNDER INFLUENCE
OF THE SOUTHERN BRANCH OF THE EASTERN PACIFIC ITCZ EXPECTING
ORGANIZED DEEP CONVECTION OVER THE NEXT COUPLE OF DAYS. THROUGH
FRIDAY MORNING THIS IS TO FAVOR MAXIMA OF 50-100MM...WHILE ON
FRIDAY TO SATURDAY THE MAXIMA IS TO PEAK AT 35-70MM. WEAKENING OF
THE ONSHORE FLOW ON SATURDAY-SUNDAY WILL RESULT IN A FURTHER
REDUCTION...WITH MAXIMA OF 25-50MM.

ABDEMUR...FAA (ARGENTINA)
TEPES...DMC (CHILE)
DAVISON...WPC (USA)




Last Updated: 1214 PM EDT THU MAR 23 2017