642  
FXCA62 TJSJ 190915  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
515 AM AST SUN MAY 19 2019  
   
SYNOPSIS
 
A SURGE IN LOW LEVEL MOISTURE WILL INCREASE SHOWERS  
ACROSS THE USVI AND EASTERN PR LATER TODAY AND THROUGH THE  
OVERNIGHT HOURS. SHOWERS AND THUNDERSTORMS ARE EXPECTED EACH AFTERNOON  
MAINLY ACROSS THE NORTHWEST QUADRANT OF PUERTO RICO FOR THE NEXT  
SEVERAL DAYS. ELSEWHERE, SCATTERED SHOWERS ARE POSSIBLE AND  
ISOLATED THUNDERSTORMS CAN NOT BE RULED OUT. AN UPPER LEVEL  
TROUGH WILL CONTINUE TO APPROACH THE FORECAST AREA FROM THE  
NORTHWEST AND MOVE JUST NORTH OF THE REGION BY THURSDAY. LIGHT TO  
MODERATE SOUTHEASTERLY TRADES WILL CONTINUE ACROSS THE REGION  
THROUGH NEXT WEEK.  
 

 
   
SHORT TERM...SUNDAY THROUGH TUESDAY
 
 
A MID TO UPPER-LEVEL TROUGH CURRENTLY NORTHWEST OF PUERTO RICO WILL  
CONTINUE TO MOVE TO THE EAST OVER THE NEXT SEVERAL DAYS. AS THE MID  
TO UPPER-LEVEL TROUGH MOVES CLOSER IT WILL WEAKEN THE MID TO UPPER-  
LEVEL RIDGE, AND CAUSE THE RIDGES TO GRADUALLY MOVE TO THE EAST. AT  
THE LOW-LEVELS A SURFACE HIGH OVER THE EASTERN ATLANTIC AND TROUGH  
350 MILES NORTH OF PUERTO RICO WILL INDUCE AN EAST TO SOUTHEAST  
WINDS OVER THE REGION. ALSO, AN INCREASE LOW-LEVEL MOISTURE OVER THE  
NEXT SEVERAL DAYS ACROSS PUERTO RICO AND THE LOCAL WATERS.  
 
AN INCREASE IN SHOWERS AND ISOLATED THUNDERSTORMS ARE FORECAST TO  
DEVELOP ACROSS NORTHWESTERN AREAS OF PUERTO RICO. AS A RESULT OF  
SIGNIFICANT SURFACE HEATING COMBINED WITH LOCAL AND DIURNAL EFFECTS.  
IN ADDITION, DUE TO PROXIMITY OF THE MID TO UPPER-LEVEL TROUGH AND  
SUB-TROPICAL JET A SLIGHT INCREASE IN UPPER-LEVEL DIVERGENCE IS  
EXPECTED MAINLY ACROSS NORTHWESTERN AREAS OF PUERTO RICO DURING THE  
AFTERNOON.  
 
THEREFORE, DURING THE REST OF OF THE SHORT-TERM PERIOD  
SHOWERS WILL MATURE TO THUNDERSTORMS ACROSS INTERIOR AND WESTERN  
AREAS OF PUERTO RICO. THESE SHOWERS COULD CAUSE URBAN AND SMALL  
STREAM FLOODING AND PONDING OF WATER ON ROADWAYS. ELSEWHERE,  
ISOLATED TO SCATTERED SHOWERS ARE POSSIBLE ACROSS THE U.S. VIRGIN  
ISLANDS, INTERIOR, AND EASTERN AREAS OF PUERTO RICO DURING THE EARLY  
MORNING AND AFTERNOON.  
   
LONG TERM...WEDNESDAY THROUGH SUNDAY
 
 
A MID TO UPPER LEVEL TROUGH IS FORECAST TO MOVE FROM THE WESTERN  
ATLANTIC AND OVER THE NORTHEASTERN CARIBBEAN BETWEEN WEDNESDAY AND  
THURSDAY. THIS WILL PROVIDE UPPER LEVEL SUPPORT FOR THE  
DEVELOPMENT OF SHOWERS AND THUNDERSTORMS ACROSS PR AND PORTIONS OF  
THE USVI. PRECIPITABLE WATER CONTENT IS FORECAST TO PEAK DURING  
THIS PERIOD BETWEEN 1.75-2.00 INCHES, THEREFORE THE POTENTIAL FOR  
FLOODING RAINS WILL BE HIGH.  
 
ANOTHER SHORT WAVE TROUGH IS FORECAST TO MOVE JUST NORTHEAST OF  
THE REGION DURING THE WEEKEND. THIS WILL CONTINUE TO PROVIDE  
DIVERGENCE ALOFT TO SUPPORT AFTERNOON CONVECTION. AT LOWER LEVELS,  
THE ABSENCE OF THE TRADE WIND CAP AND LIGHT SOUTHERLY STEERING  
WINDS WILL ALLOW FOR TROPICAL MOISTURE TO POOL OVER THE REGION.  
INDUCED PERTURBATIONS ACROSS THE EASTERN CARIBBEAN IN COMBINATION  
WITH TROUGH PATTERN ALOFT WILL INCREASE THE POTENTIAL FOR SHOWER  
ACTIVITY WITH POSSIBLE THUNDERSTORMS ACROSS THE CARIBBEAN WATERS  
AND ACROSS THE USVI/EASTERN SECTIONS OF PR LATE IN THE WEEKEND.  
 

 
   
AVIATION
 
VFR CONDS EXPECTED THROUGH THE FORECAST PERIOD WITH  
VCSH/VCTS AT TJMZ AND TJBQ BY 19/18Z. THEN SHRA/VCTS SHOULD END BY  
20/01Z WITH SCT TO BKN SKIES. WINDS WILL BE OUT OF THE EAST TO  
SOUTHEAST AT 10 TO 15 KTS WITH HIGHER GUSTS POSSIBLE.  
 

 
   
MARINE
 
SEAS ARE EXPECTED TO RANGE BETWEEN 3-5 FEET IN GENERAL  
WITH THE HIGHEST SEAS EXPECTED ACROSS THE ATLANTIC WATERS. EAST TO  
SOUTHEAST WINDS WILL PREVAIL BETWEEN 5-15 KTS WITH SEA BREEZE  
INFLUENCES AFTER 10 AM EACH DAY. A MODERATE RISK OF RIP CURRENTS  
WILL CONTINUE FOR THE NORTHERN BEACHES OF PUERTO RICO AND ST.  
THOMAS.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 90 78 89 77 / 30 40 30 30  
STT 86 77 86 77 / 40 30 30 50  
 

 
   
SJU WATCHES/WARNINGS/ADVISORIES
 
 
PR...NONE.  
VI...NONE.  
AM...NONE.  

 
 

 
 
SHORT TERM...TW  
LONG TERM....DS  
 
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