Prognostic Meteorological Discussion
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FXSA20 KWBC 291639
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1239 PM EDT WED MAR 29 2017

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

MODEL DISCUSSION (VALID FROM MARCH 29 AT 0000 UTC): CONFIDENCE IN
THE FORECAST IS HIGH THROUGH 108-120 HRS...WITH ENSEMBLE MODEL
GUIDANCE PROVIDING STRONG SUPPORT TO THE DETERMINISTIC MODELS.

AT 500 HPA...A CLOSED LOW OFF THE SOUTH COAST OF CHILE ANCHORS A
TROUGH OVER THE EASTERN PACIFIC. THIS IS SHEARING SHORT WAVE
ENERGY ACROSS THE SOUTHERN ANDES INTO THE CENTRAL/SOUTHERN
PROVINCES IN ARGENTINA...WHERE IT IS TO FAVOR HEIGHT FALLS OF
75-125GPM DURING THE NEXT 24 HRS. BROAD TROUGH IS TO THEN REMAIN
NEARLY STATIONARY THROUGH MIDDAY ON SATURDAY...WHEN IT IS TO THEN
MOVE INLAND AS A PROGRESSIVE TROUGH APPROACHES FROM THE WEST. AT
250 HPA A NORTHERN POLAR JET MOVES ACROSS THE SOUTHERN CONE DURING
THE DAY TODAY... WHILE A SUBTROPICAL JET TO THE NORTH MOVES ACROSS
CENTRAL CHILE TO MENDOZA/CORDOBA IN ARGENTINA. AS THEY MEANDER
OVER THE CONTINENT... THESE JETS ARE TO COUPLE ON THEIR DIVERGENT
SIDE...TO SUSTAIN A BROAD AREA OF UPPER DIVERGENCE ACROSS
CENTRAL-SOUTHERN PROVINCES IN ARGENTINA. AT LOW LEVELS...THE
TROUGH ALOFT SUSTAINS A SURFACE FRONT THAT MEANDERS EAST ACROSS
PATAGONIA-CUYO/CENTRAL PROVINCES EARLY IN THE CYCLE. A LOW IS TO
THEN OCCLUDE OFF THE CENTRAL COAST OF PATAGONIA ON THURSDAY. AS IT
MEANDERS OVER THE CONTINENT...THE POLAR TROUGH WILL PRESS AGAINST
A BROAD CELL OF THE SUBTROPICAL RIDGE TO THE EAST. THE TIGHT
PRESSURE GRADIENT WILL THEN FAVOR A STRONG NORTHERLY JET ACROSS
ARGENTINA...ADVECTING WARM/MOIST AIR SOUTH. THE INFLOW OF COLD AIR
ALOFT WILL FAVOR A CONVECTIVELY UNSTABLE AIR MASS...WHILE THE
HORIZONTAL/VERTICAL WIND SHEAR WILL SUSTAIN HIGH HELICITY VALUES
THAT WOULD SUSTAIN LONG LASTING/POTENTIALLY SEVERE CONVECTION ON
WEDNESDAY-THURSDAY. OVER CENTRAL ARGENTINA-PATAGONIA EXPECTING
ACCUMULATION OF 75-125MM THROUGH THURSDAY MORNING. ON THURSDAY
ACTIVITY SPLITS IN THREE AREAS...ONE TO THE SOUTH IN ASSOCIATION
WITH THE OCCLUDING LOW WILL RESULT IN MAXIMA OF 30-60MM...WHILE
ANOTHER OVER CENTRAL ARGENTINA IS TO FAVOR MAXIMA OF 25-50MM IN
POTENTIALLY SEVERE CONVECTION. HIGHER AMOUNTS ARE EXPECTED ON THE
NORTHWEST PROVINCES WHERE THE MAXIMA ON THURSDAY IS TO PEAK AT
35-70MM. A SECOND ROUND OF POTENTIALLY SEVERE CONVECTION IS
EXPECTED ACROSS CENTRAL ARGENTINA ON THURSDAY TO FRIDAY...WHEN THE
TROUGH STARTS TO MOVE INLAND ACROSS CHILE...SUSTAINING HEIGHT
FALLS OF 50-75GPM AND TRIGGERING MODERATE TO HEAVY CONVECTION.
DURING THIS PERIOD THE MAXIMA IS TO PEAK AT 30-60MM.

A NORTH TO SOUTH RIDGE DOMINATES THE MID/UPPER LEVELS OVER
ARGENTINA/WESTERN ATLANTIC. THIS IS TO GRADUALLY MOVE FARTHER
OFFSHORE ON SATURDAY TO SUNDAY AS THE AFOREMENTIONED TROUGH MOVES
INLAND ACROSS CHILE TO ARGENTINA. MEANWHILE...THIS IS TO STEER
SHORT WAVE VORTICES NORTH ALONG THE SOUTH COAST OF BRASIL. THE
INFLOW OF SHORT WAVE ENERGY WILL SUSTAIN A TROUGH/CLOSED LOW TO
THE EAST OF SAO PAULO/RIO DE JANEIRO. THIS FEATURE IS TO PERSIST
DURING THE NEXT FEW DAYS. JET MAXIMA ARE TO REVOLVE AROUND THIS
TROUGH DURING THE NEXT FEW DAYS...FAVORING AN UPPER DIVERGENT
PATTERN THAT IS TO VENT DEEP CONVECTION ACROSS NORTHEAST SOUTH
AMERICA. AT LOW LEVELS...THE TROUGH ALOFT INTERACTS WITH AN
ELONGATED FRONT OVER THE SOUTH ATLANTIC...FAVORING CYCLOGENESIS
OFF THE SOUTHEAST COAST OF BRASIL. THE DEEPENING LOW DRIVES A
SURFACE FRONT NORTH ACROSS ESPIRITO SANTO-MINAS GERAIS-MATO GROSSO
IN BRASIL EARLY IN THE CYCLE...WHERE THE BOUNDARY IS TO REMAIN
DURING THE NEXT FEW DAYS. THIS IS TO TRIGGER MODERATE TO HEAVY
CONVECTION EARLY IN THE PERIOD WITH MAXIMA OF 35-70MM. ON THURSDAY
IT WEAKENS TO 20-35MM...WHILE ON FRIDAY-SATURDAY EXPECTING AN
INCREASE TO 40-80MM/DAY. OTHER CONVECTION TO THE WEST IS TO
CLUSTER OVER CENTRAL BRASIL ALONG AND NORTH OF THE SURFACE
FRONT...TO FAVOR SCATTERED CONVECTION WITH DAILY MAXIMA OF
20-30MM.

THE RELEASE OF LATENT HEAT ACROSS NORTHERN SOUTH AMERICA WILL HELP
SUSTAIN A BROAD CELL OF THE SUBTROPICAL RIDGE ACROSS
BRASIL-BOLIVIA-PERU. THE RIDGE IS TO ANCHOR ON A HIGH THAT IS TO
MEANDER BETWEEN CENTRAL BOLIVIA AND MATO GROSSO IN BRASIL. THE
RIDGE...AS IT PERSISTS...IS TO VENT CONVECTION ACROSS EQUATORIAL
SOUTH AMERICA...WITH MOST ACTIVE ACROSS RORAIMA-AMAZONAS IN BRASIL
TO NORTHERN PERU/SOUTHERN COLOMBIA. IN THIS AREA MAXIMA IS TO PEAK
AT 30-60MM THROUGH THURSDAY MORNING. THROUGH SATURDAY IT IS TO
THEN DECREASE TO 20-40MM. A SUBEQUATORIAL JET MAXIMA OVER NORTHERN
BRASIL IS TO THEN FAVOR A SURGE IN CONVECTION ACROSS WESTERN
ECUADOR/ NORTHERN PERU EARLY IN THE CYCLE. WE EXPECT RAINFALL
MAXIMA OF 75-150MM THROUGH THURSDAY MORNING...AND 75-125MM ON
THURSDAY TO FRIDAY. ON SATURDAY-SUNDAY IT WILL DECREASE TO 10-15MM.

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





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