Prognostic Meteorological Discussion
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495
FXSA20 KWBC 221619
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1219 PM EDT MON MAY 22 2017

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

MODEL DISCUSSION (VALID FROM MAY 22 AT 0000 UTC): A HIGH AMPLITUDE
LONG WAVE RIDGE OVER THE SOUTH PACIFIC SUSTAINS THE NORTHWARD
AMPLIFICATION OF A POLAR TROUGH ACROSS THE DRAKE PASSAGE TO THE
SOUTHERN CONE OF SOUTH AMERICA. THROUGH TUESDAY AFTERNOON THE
TROUGH IS TO AMPLIFY ALONG THE SOUTHERN TO CENTRAL COAST OF CHILE.
AXIS IS TO THEN SPLIT IN TWO...WITH SOUTHERN HALF TO ACCELERATE
ACROSS THE SOUTHWEST ATLANTIC WHILE THE NORTHERN HALF TRAILS OFF
THE COAST OF CHILE. THE LATTER WILL THEN EVOLVE INTO A CLOSED LOW
NEAR ARCHIPELAGO DE JUAN FERNANDEZ. THE LOW IS TO REMAIN NEARLY
STATIONARY THROUGH FRIDAY MORNING...SPILLING ACROSS THE ANDES INTO
THE NORTHERN PROVINCES IN ARGENTINA LATER IN THE DAY. AS IT
MEANDERS OFF THE CENTRAL COAST OF CHILE...THE MID LEVEL TROUGH
WILL TRIGGER SCATTERED CONVECTION ON THE CENTRAL ANDES. ON TUESDAY
TO WEDNESDAY THIS WILL AMOUNT TO 05-10CM...WHILE ON WEDNESDAY TO
THURSDAY IT INCREASES TO 25-50MM. AT LOW LEVELS...BROAD RIDGE OVER
THE SOUTH ATLANTIC WILL SUSTAIN A NORTHERLY LOW LEVEL JET ACROSS
PARAGUAY TO NORTHERN ARGENTINA EARLY IN THE CYCLE. THIS IS TO
SUSTAIN A WARM ADVECTIVE PATTERN ACROSS MID SECTIONS OF SOUTH
AMERICA. POLAR FRONT ADVANCING ACROSS ARGENTINA LATER ON
TUESDAY/EARLY WEDNESDAY MORNING WILL THEN CLASH WITH THE
WARM/MOIST AIR TO THE NORTH. UNDER FAVORABLE JET DYNAMICS
ALOFT...ENVIRONMENTAL CONDITIONS WILL BECOME HIGHLY FAVORABLE AND
CONDUCIVE TO GENERATION OF MESO SCALE CONVECTIVE SYSTEMS DURING
THE EARLY MORNING HOURS ON WEDNESDAY. THE FOCUS OF THE ORGANIZED
DEEP CONVECTION IS GOING TO BE ACROSS CORRIENTES-ENTRE RIOS IN
ARGENTINA-URUGUAY AND RIO GRANDE DO SUL IN BRASIL...WITH MAXIMA OF
35-70MM. DURING THAT PERIOD THERE IS AN ELEVATED RISK OF SEVERE
WEATHER...WITH POTENTIAL FOR LARGE HAIL. ON THURSDAY MORNING
ACTIVITY IS TO THEN CLUSTER OVER SOUTHERN BRASIL-MESOPOTAMIA
VALLEY IN ARGENTINA AND SOUTHEAST PARAGUAY...WITH MAXIMA OF
40-80MM IN SLIGHT RISK OF SEVERE CONVECTION.

FURTHERMORE...THE HIGH AMPLITUDE RIDGE TO THE WEST IS TO ALSO
STEER PROGRESSIVE SHORT WAVE PERTURBATIONS ON THE SOUTHERN STREAM
FLOW ACROSS THE DRAKE PASSAGE/TIERRA DEL FUEGO-SOUTHERN PATAGONIA.
IN THIS PATTERN A FAIRLY STRONG VORTEX MAKES LANDFALL OVER THE
SOUTHERN CONE EARLY ON WEDNESDAY MORNING...TO RAPIDLY PULL ACROSS
PATAGONIA TO THE WESTERN ATLANTIC LATER IN THE DAY. THIS WILL THEN
HELP SUSTAIN THE NORTHWARD AMPLIFICATION OF THE POLAR TROUGH
PATTERN ACROSS THE WESTERN ATLANTIC TO CENTRAL SOUTH AMERICA LATER
IN THE WEEK. MEANWHILE...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...OPEN CELL OF THE
SUBTROPICAL RIDGE ENVELOPS NORTHWEST SOUTH AMERICA. 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)
GALVEZ...WPC (USA)
$$





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