776  
FXCA62 TJSJ 200139  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
939 PM AST WED SEP 19 2018  
   
UPDATE
 
 
A SURGE OF MOISTURE IS PRODUCING SHOWER ACROSS THE SURROUNDING  
WATERS OF THE U.S. VIRGIN ISLANDS AND EASTERN PUERTO RICO. THIS  
ACTIVITY WILL CONTINUE OVERNIGHT AND SOME OF THEM COULD MOVED  
INLAND PRODUCING PERIODS OF MODERATE RAINFALL AT TIMES. HOWEVER,  
SAHARAN DUST PARTICULATE WAS DETECTED MAKING ITS WAY THROUGH THE  
REGION AND HAZY CONDITIONS WERE REPORTED LATER THIS AFTERNOON.  
AFTERNOON SHOWERS AND ONE OR TWO THUNDERSTORMS SHOULD NOT BE RULED  
OUT ACROSS THE NORTHWEST QUADRANT AND DOWNWIND FROM EL YUNQUE  
INTO THE SAN JUAN METRO BY THURSDAY. HEAT INDEX IN THE LOW TO MID  
100S ARE POSSIBLE ALONG THE COASTAL AREAS OF PUERTO RICO AND THE  
U.S. VIRGIN ISLANDS THE NEXT FEW DAYS.  
 

 
   
AVIATION
 
 
VFR CONDS WILL PREVAIL ACROSS THE ISLANDS THROUGH MUCH OF THE  
FORECAST PERIOD. HOWEVER A SAHARAN AIR LAYER WILL REDUCE  
VISIBILITIES BETWEEN 5 AND 10 MILES THROUGH MUCH OF THE FORECAST  
PERIOD. IN ADDITION, SHOWERS AND THUNDERSTORMS COULD FORM ACROSS  
THE NORTHWEST QUADRANT OF PUERTO RICO AND IN THE VCTY OF TJBQ/TJMZ  
BETWEEN 20/16-22Z. EASTERLY WINDS AT 7 TO 12 KNOTS EXPECTED  
OVERNIGHT BECOMING MORE ESE AT 10 TO 15 KT AND GUSTY AFT 20/13Z  
WITH SEA BREEZE VARIATIONS.  
 

 
   
MARINE
 
 
TRANQUIL MARINE CONDITIONS WITH E-ESE WINDS AT 10 TO 15 KNOTS AND  
SEAS AT 4 FEET OR LESS. WINDS AND SEAS ARE FORECAST TO INCREASE  
ACROSS THE ATLANTIC AND CARIBBEAN OFFSHORE WATERS ON THURSDAY.  
 

 
   
PREV DISCUSSION
 
/ISSUED 526 PM AST WED SEP 19 2018/  
 
SYNOPSIS...SAHARAN DUST HAS INVADED THE U.S. VIRGIN ISLANDS AND  
EASTERN PUERTO RICO AND WILL CONTINUE OVER THE AREA THROUGH  
FRIDAY. LITTLE SHOWER ACTIVITY WILL OCCUR AFTER TONIGHT EXCEPT IN  
NORTHWEST PUERTO RICO THURSDAY AND SOUTHWEST PUERTO RICO ON  
FRIDAY. WINDS BECOME MORE NORTHEASTERLY ON FRIDAY. AFTER FRIDAY  
MOISTURE INCREASES THROUGH MID WEEK NEXT WEEK WHEN ANOTHER  
TROPICAL WAVE IS EXPECTED.  
 
SHORT TERM...TONIGHT THROUGH SATURDAY...  
DRIER AIR HAS INVADED THE AREA AND IS DRASTICALLY REDUCING THE  
SHOWER ACTIVITY. NEVERTHELESS, THE BEST ACTIVITY OF THE TROPICAL  
WAVE IS YET TO FOLLOW, THOUGH IT WILL BE SAPPED BY THE VERY DRY  
AIR ACCOMPANYING THE DUST. AS EVIDENCE OF THIS ACTIVITY AND THE  
DRYNESS OF THE AIR AROUND IT MANY ELLIPTICAL OUTFLOW BOUNDARIES  
HAVE BEEN SEEN SOUTHEAST OF THE FORECAST AREA HEADED TOWARD THE  
AREA. MOST OF THESE WILL BE GONE BY TONIGHT. CURRENT EXPECTATIONS  
ARE THAT SOME SHOWERS OVERNIGHT WILL MAKE IT THROUGH, IMPACTING  
SOUTHEAST AND EASTERN PUERTO RICO THROUGH THE MORNING ON THURSDAY,  
THEN THE CONVECTION WILL DISAPPEAR OVER EASTERN PUERTO RICO AND  
WHAT IS LEFT OF THE MOISTURE IN NORTHWESTERN PUERTO RICO WILL  
SPAWN SOME THUNDERSTORMS THURSDAY AFTERNOON. THESE SHOULD BE  
BRIEF AND WILL LIKELY NOT PRODUCE SIGNIFICANT HEAVY RAINS. FRIDAY  
MORNING IS EXPECTED TO HAVE THE LOWEST PRECIPITABLE WATER OF THE  
NEXT 10 DAYS OVER THE AREA. THEN LOWER LEVEL FLOW BEGINS TO TURN  
TO THE NORTHEAST AND SOME MOISTURE IS BROUGHT INTO THE AREA FRIDAY  
NIGHT AFTER THE DUST HAS PASSED. THIS SHOULD YIELD SOME SHOWERS  
FRIDAY AFTERNOON IN THE SOUTHWEST AND THEN ON THE NORTHEAST COAST  
OF PUERTO RICO AND AROUND THE U.S. VIRGIN ISLANDS FRIDAY NIGHT,  
BUT AMOUNTS WILL BE LIGHT. ON SATURDAY AFTERNOON THERE IS A GOOD  
CHANCE FOR SHOWERS IN SOUTHWEST PUERTO RICO WITH ISOLATED SHOWERS  
ELSEWHERE.  
 
LONG TERM...SUNDAY THROUGH WEDNESDAY...  
LOW PRESSURE MIGRATING ACROSS THE WESTERN ATLANTIC WILL DOUBLE  
BACK AND GROW CLOSER TO THE AREA WHILE REMAINING NORTH OF 30  
DEGREES NORTH. NEVERTHELESS THE TROUGH THAT FORMS TO ITS SOUTHWEST  
WILL WEAKEN THE TRADE WIND FLOW AND IT WILL BEGIN TO SHIFT BACK TO  
THE EAST. LIMITED MOISTURE ABOVE 850 MB BUT ESPECIALLY AT MID  
LEVELS WILL ALLOW ONLY LIMITED SHOWER ACTIVITY WITH SOME  
THUNDERSTORMS SUNDAY AND THROUGH TUESDAY. A TROPICAL WAVE WILL  
MOVE THROUGH THE AREA ON WEDNESDAY DEPICTED BY THE GFS, BUT MOST  
OF ITS MOISTURE AND ENERGY WILL MOVE THROUGH THE CARIBBEAN TO THE  
SOUTH.  
 
AT UPPER LEVELS...ON SUNDAY MORNING A TROUGH EXTENDS OUT OF THE  
TUTT IN THE WESTERN TROPICAL ATLANTIC TO THE WEST NORTHWEST. IT  
SLOWLY MOVES SOUTH UNTIL IT REACHES THE AREA ON WEDNESDAY, THIS  
WILL HELP SOME OF THE ACTIVITY ON WEDNESDAY AS WILL THE PEAK OF  
THE COLUMNAR MOISTURE ON WEDNESDAY AND THURSDAY NEXT WEEK.  
 
AVIATION...MAINLY VFR CONDITIONS ARE EXPECTED TO PREVAIL ACROSS ALL  
TERMINAL THROUGH THE FORECAST PERIOD. SHRA/TSRA OVER WEST PR  
EXPECTED TO END AROUND 21Z. HZ DUE TO SUSPENDED SAHARAN DUST WILL  
CONTINUE ACROSS THE LOCAL TERMINALS. LOW LEVEL WINDS WILL  
CONTINUE MAINLY FROM THE ESE AT 15-25 KT. LIGHTER LAND BREEZE  
VARIATIONS EXPECTED ACROSS THE PR TERMINALS OVERNIGHT. MAXIMUM  
WINDS BLO FL540 NEXT 48 HRS ESE 20-25 KT ON 20/12Z BTWN  
FL023-080.  
 
MARINE...MARINE CONDITIONS ARE CURRENTLY RESPONDING TO MODERATE  
TO LOCALLY FRESH EAST SOUTHEAST TRADE WIND FLOW OF 10 TO 20 KT.  
THIS WILL BRING AREAS OF 6 FOOT SEAS IN THE CARIBBEAN AND ALONG  
THE NORTH COAST OF PUERTO RICO. STRONGEST WINDS WILL BE ON  
THURSDAY. A NORTH TO NORTHEAST SWELL WILL ENTER THE AREA ON  
SATURDAY NIGHT FROM THE REMNANTS OF FLORENCE. SEAS THEN ARE  
EXPECTED TO REMAIN BELOW 7 FEET.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 78 91 79 90 / 40 30 10 60  
STT 80 89 81 90 / 40 40 30 30  
 

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

 
 

 
 
UPDATE...CAM  
AVIATION...CAM  
 
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