236  
FXCA20 KWBC 231146  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
645 AM EST THU NOV 23 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO FROM NOV 23/06 UTC: HIGHLY  
AMPLIFIED SINUSOIDAL WAVE PATTERN PERSIST ACROSS THE FORECAST  
DOMAIN...WITH MID/UPPER LEVEL TROUGH EXTENDING OVER THE GULF OF  
MEXICO INDUCING THE NORTHWARD AMPLIFICATION OF THE SUBEQUATORIAL  
RIDGE ACROSS THE EASTERN CARIBBEAN TO THE WESTERN ATLANTIC. AT 500  
HPA THE RIDGE CENTERS ON A CLOSED HIGH OVER PUERTO RICO...WHERE IT  
FAVORS AN UPPER CONVERGENT/SUBSIDENT PATTERN. AT LOW LEVELS...A  
TUTT INDUCED PERTURBATION IN THE EASTERLY TRADES ENTERED THE  
LEEWARD ISLES. PLUME OF DEEP TROPICAL MOISTURE ACCOMPANIES THIS  
FEATURE...WITH PWAT CONTENT OF AROUND TWO INCHES. THE LEADING EDGE  
OF THE MOIST PLUME IS NEAR THE VIRGIN ISLES EARLY THIS MORNING.  
 
UPPER PATTERN IS TO GENERALLY HOLD NEARLY UNCHANGED THROUGH SUNDAY  
MORNING. AT 250 HPA A JET MAXIMA ROUNDING THE BASE OF THE TROUGH  
WILL THEN DRAW THIS AXIS EAST-NORTHEAST INTO THE WESTERN  
ATLANTIC-CUBA/NORTHWEST CARIBBEAN LATER DURING THE WEEKEND. AS THE  
TROUGH LIFTS OVER THE SUBEQUATORIAL RIDGE TO THE EAST...THE RIDGE  
IS FORECAST TO FLATTEN TO THE NORTH OF 20N LATER ON TUESDAY. THE  
MID/UPPER LEVEL RIDGE IS TO THEN CONFINE TO THE EASTERN  
CARIBBEAN/NORTHERN SOUTH AMERICA. MEANWHILE...CAP INVERSION ALOFT  
IS EXPECTED TO PERSIST...TO CONTINUE FAVORING A HOSTILE  
ENVIRONMENT FOR ORGANIZED DEEP CONVECTION TO DEVELOP. AT LOW  
LEVELS...IN A DEEP LAYER EASTERLY FLOW...THE TUTT INDUCED TROUGH  
AND ASSOCIATED MOIST PLUME ARE TO ADVECT ACROSS THE VIRGIN ISLES  
TO PUERTO RICO EARLY THIS AFTERNOON. HOWEVER...AS IT SLIDES UNDER  
THE MID LEVEL RIDGE...THE MODELS CONSISTENTLY SHOW EROSION OF  
AVAILABLE MOISTURE. THE BACK END OF THE SHALLOW MOIST PLUME IS TO  
THEN SLOWLY MOVE ACROSS PUERTO RICO ON SATURDAY MORNING. MODELS  
THEN AGREE ON A DRYER AIR MASS TO ENTRAIN FROM THE EAST...WITH  
STRONG CAP INVERSION DURING THE WEEKEND TO GENERALLY FAVOR FAIR  
WEATHER CONDITIONS ACROSS THE ENTIRE FORECAST AREA.  
 
LACKING UPPER LEVEL SUPPORT...AND IN THE ABSENCE OF VERTICAL WIND  
SHEAR...THE SHALLOW MOIST PLUME ADVECTING ACROSS THE FORECAST AREA  
IS TO ONLY FAVOR LIGHT-MODERATE RAINFALL AMOUNTS. SLOW MOVING  
SHALLOW CONVECTION IS TO GENERALLY CLUSTER ALONG THE  
CORDILLERA-WESTERN INTERIOR PORTIONS OF PUERTO RICO DURING THE  
AFTERNOON HOURS. ON FRIDAY THE GFS AND ECMWF GDI FORECAST SHOW  
SLIGHTLY HIGHER RISK OF MODERATE CONVECTION TO DEVELOP. THIS IS  
REFLECTED IN OUR QPF PROGS FOR THE DAY. ON SATURDAY-SUNDAY...AS A  
DRYER AIR MASS ENTRAINS FROM THE EAST...CONVECTION WILL GENERALLY  
LIMIT TO THE CORDILLERA AND WESTERN PUERTO RICO. IN THIS AREA ONLY  
EXPECTING TRACE/LIGHT RAINFALL AMOUNTS IN SHORT LIVED SHALLOW  
CONVECTION.  
 
DAVISON...WPC (USA)  
 

 
 
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