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Tropical Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1119Z Sep 13, 2020)
 
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
719 AM EDT SUN SEP 13 2020

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM SEP
13/11UTC: NHC IS ISSUING ADVISORIES FOR TROPICAL DEPRESSION
TWENTY. THE DEPRESSION IS FORECAST TO GRADUALLY INTENSIFY WHILE
SLOWLY RECURVING TO THE NORTHWEST BEFORE REACHING THE ISLAND CHAIN
LATER IN THE WEEK.

AT 500-250 HPA...THE MODELS INITIALIZE A BROAD UPPER LEVEL TROUGH
OVER THE WESTERN ATLANTIC...WITH AXIS BOTTOMING OUT OVER THE
NORTHEAST CARIBBEAN ISLANDS. THE UPPER LEVEL FEATURE IS TO HOLD
ITS GROUND THROUGH MONDAY AFTERNOON...THEN RAPIDLY WEAKEN ON
TUESDAY-WEDNESDAY WHILE LIFTING OVER A BROAD CELL OF THE
SUBTROPICAL RIDGE TO THE EAST. IT...HOWEVER...TENDS TO REGENERATE
LATER IN THE WEEK AS A RIDGE BUILDS OVER THE GULF OF
MEXICO/SOUTHEAST USA AND A SHORT WAVE PERTURBATION CLEARING THE
NORTHERN FRINGES OF THE RIDGE FEEDS INTO THIS FEATURE. THE ENSUING
WEAKNESS IN THE HEIGHT FIELD WOULD ALLOW TD-TWENTY TO LIFT TO THE
NORTHWEST BEFORE REACHING THE EASTERN CARIBBEAN ISLANDS.
MEANWHILE...THE COLD CORE UPPER LEVEL TROUGH WILL FAVOR A
DIVERGENT PATTERN OVER PUERTO RICO AND HISPANIOLA THAT IS TO VENT
CONVECTIVE DEVELOPMENT...WHILE SIMULTANEOUSLY  FAVORING A GRADUAL
INCREASE IN CONVECTIVE INSTABILITY.

AT LOW LEVELS...WINDS VEER TO THE SOUTH-SOUTHEAST DURING THE DAY
TODAY. THIS WILL ENHANCE MOISTURE TRANSPORT ACROSS THE EASTERN
CARIBBEAN...WITH PWAT CONTENT PEAKING BETWEEN 1.75-2.0 INCHES ON
MONDAY. THE SOUTHEASTERLY LOW LEVEL FLOW AND MID/UPPER LEVEL
SOUTHWESTERLY SHEAR WILL LIKELY FAVOR LONG LASTING DAYTIME
CONVECTION...WITH GFS GDI FORECAST STILL SHOWING POTENTIAL FOR
ORGANIZED DEEP CONVECTION ON MONDAY. DURING THE NEXT TWO DAYS THE
SOUTHEASTERLY STEERING FLOW WILL HELP FOCUS THE BULK OF THE
DIURNAL ACTIVITY ACROSS NORTHERN PUERTO RICO...WITH MOST INTENSE
EXPECTED BETWEEN AGUADA/AGUADILLA AND DORADO/TOA BAJA.
FURTHERMORE...IN THIS CYCLE THE GFS IS ALSO HINTING THE
POSSIBILITY OF A SECONDARY RAINFALL MAXIMA TO AFFECT SAN JUAN
METRO AREA AS ENHANCED BY LOW LEVEL MOISTURE CONVERGENCE. NOTE
THAT DUE TO THE DEEPER MOISTURE CONTENT...FAVORABLE UPPER DYNAMICS
AND DEEP LAYER INSTABILITY ENVELOPING THE FORECAST AREA...OUR QPF
FOR THE NEXT TWO DAYS OVER PUERTO RICO IS HIGHER THAN WHAT THE
DETERMINISTIC AND ENSEMBLE MODELS SUGGEST.

ON TUESDAY-WEDNESDAY MID/UPPER LEVEL FORCING WANES WHILE AT LOW
LEVELS EAST-SOUTHEASTERLY FLOW RETURNS TO THE FORECAST AREA.
DURING THIS PERIOD FOCUS OF THE DIURNAL ACTIVITY SHIFTS TO
INTERIOR PORTIONS OF PUERTO RICO...WHILE OVER THE VIRGIN
ISLES-VIEQUES AND CULEBRA FAIR WEATHER CONDITIONS ARE TO PREVAIL.
EARLY MORNING SHOWERS...HOWEVER...ARE EXPECTED ACROSS THE VIRGIN
ISLES ON WEDNESDAY AS A WEAK MID LEVEL PERTURBATION STREAMS FROM
THE EAST...TO ALSO ENHANCE DIURNAL ACTIVITY ACROSS WESTERN
INTERIOR PORTIONS OF PUERTO RICO DURING THE DAY.

DAVISON...WPC (USA)