251  
FXCA62 TJSJ 201358  
AFDSJU  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN JUAN PR  
958 AM AST FRI JUL 20 2018  
   
UPDATE
 
DRY AND STABLE AIR MASS WITH SAHARAN DUST PARTICULES  
HAS SPREAD OVER THE EASTERN CARIBBEAN. AS A RESULT, HAZY SKIES ARE  
DOMINATING THE WEATHER CONDITIONS ACROSS THE LOCAL ISLANDS THIS  
MORNING. VISIBILITIES HAVE BEEN REDUCED SOMEWHAT ACROSS PUERTO  
RICO, AND A FEW LOCATIONS HAVE REPORTED 6-8 MILES OF VISIBILITY  
THIS MORNING. STRONG INVERSION BETWEEN 4-8 KFT (900MB-750MB) WILL  
KEEP THE CHANCES OF SHOWERS LOW FOR THE REST OF TODAY. IF ANY  
SHOWER DEVELOPS OVER NORTHWEST PR THIS AFTERNOON, IT WILL BE  
BRIEF. DECREASED THE POPS OVER MOST OF THE REGION AS LITTLE OR NO  
SHOWER ACTIVITY IS FORECAST THROUGH THIS EVENING.  
 

 
   
AVIATION
 
VFR CONDS EXPECTED THRU THE FORECAST PERIOD. FEW SHRA  
POSSIBLE OVER NW PR IN THE AFTERNOON. SAHARAN DUST WILL CONTINUE  
TO REDUCE THE VIS ACROSS THE FLIGHT AREA, VIS RANGING BETWEEN 7-9  
SM. EAST WINDS OF 15-25 KT BELOW FL030.  
 

 
   
MARINE
 
CHOPPY CONDITIONS CONTINUE ACROSS MOST OF THE LOCAL  
WATERS. THEREFORE, SMALL CRAFT OPERATIONS SHOULD EXERCISE CAUTION  
DUE TO WIND OF 15-20 KTS AND SEAS OF 4-6 FEET. MARINE CONDITIONS  
WILL GRADUALLY IMPROVE EARLY NEXT WEEK.  
 

 
   
PREV DISCUSSION
 
/ISSUED 438 AM AST FRI JUL 20 2018/  
 
SYNOPSIS..STRONG SURFACE HIGH PRESSURE WILL CONTINUE TO PRODUCE  
MODERATE TO FRESH TRADE WINDS ACROSS THE REGION. PATCHES OF LOW  
LEVEL MOISTURE WILL AFFECT THE LOCAL REGION FROM TIME TO TIME.  
 
SHORT TERM...TODAY THROUGH SUNDAY...  
 
DRIER AIR AND SAHARAN DUST ARE MAKING ITS WAY BACK INTO THE LOCAL  
AREA. SHOWER ACTIVITY HAS BEEN GRADUALLY DIMINISHING OVERNIGHT AND  
THE SHOWER ACTIVITY IS EXPECTED TO REMAIN FAIRLY LIMITED TODAY AND  
SATURDAY AS THIS DRIER AIR MASS WITH A GOOD AMOUNT OF SAHARAN DUST  
REMAINS OVER THE LOCAL AREA. HAVING SAID THAT, THE LIMITED AVAILABLE  
MOISTURE SHOULD COMBINE WITH THE LOCAL EFFECTS AND DIURNAL HEATING  
THIS AFTERNOON AND CAUSE A FEW SHOWERS ACROSS NORTHWESTERN PR, BUT  
THE AMOUNTS ARE NOT EXPECTED TO BE ENOUGH TO CAUSE FLOODING, EXCEPT  
FOR PERHAPS A SMALL AREA DUE TO AN ISOLATED STRONG SHOWER THAT MAY  
CAUSE PONDING OF WATER. BUT GENERALLY HAZY SKIES AND FAIR WEATHER IS  
EXPECTED TODAY AND THROUGH SATURDAY MORNING. THE HIGH RESOLUTION  
MODELS ARE SUGGESTING THAT A SMALL AREA IN SOUTHWESTERN PR MAY  
OBSERVE HEAVY SHOWERS FOR A BRIEF AMOUNT OF TIME, OTHERWISE THE REST  
OF PR AND THE USVI SHOULD REMAIN WITH FAIR WEATHER AND HAZY SKIES.  
 
SUNDAY LOOKS LIKE IT HAS A BETTER CHANCE FOR SIGNIFICANT SHOWERS AND  
THUNDERSTORMS ACROSS WESTERN PR AND NEAR THE SAN JUAN METRO AREA,  
BUT THE REST OF PR AND THE USVI SHOULD STILL HAVE MAINLY FAIR  
WEATHER WITH BRIEF ISOLATED SHOWERS. THE AFTERNOON SHOWERS  
CONVECTION MAY ENHANCED BECAUSE OF AN UPPER LOW THAT MAY BE BETTER  
POSITIONED TO CAUSE DIVERGENCE IN THE UPPER LEVELS AND MUCH WEAKER  
CAP INVERSION, COMPARED TO SATURDAY, AS SUGGESTED BY THE FORECAST  
SOUNDINGS.  
 
LONG TERM...MONDAY THROUGH SATURDAY...  
 
STRONG SURFACE HIGH PRESSURE WILL MAINTAIN MODERATE TO FRESH TRADE  
WINDS ACROSS THE AREA THROUGH THE FORECAST PERIOD. THESE WINDS  
WILL TRANSPORT PATCHES OF LOW LEVEL MOISTURE THROUGH THE AREA FROM  
TIME TO TIME. NEXT TROPICAL WAVES WILL PASS MAINLY SOUTH OF THE  
AREA WITHOUT ANY SIGNIFICANT CHANGES IN THE CURRENT WEATHER  
PATTERN.  
 
AVIATION...VFR CONS EXPECTED TODAY WITH BRIEF -SHRA IN THE MORNING  
HOURS. HAZY CONDITIONS DUE TO SAHARAN DUST BUT VISIBILITY SHOULD  
REMAIN P6SM. WINDS WILL BE MAINLY FROM THE EAST AT AROUND 15KT AND  
GUSTY AFTER 20/13Z, SEA BREEZE VARIATIONS SHOULD DEVELOP THEREAFTER  
AS WELL. LIMITED SHRA ACTIVITY IS POSSIBLE NEAR TJBQ AND TJMZ AFTER  
20/16Z. WINDS TO DECREASE TO 5-10KT AFTER 20/23Z.  
 
MARINE...SEAS UP TO 6 FEET AND WINDS UP TO 20 KNOTS ARE EXPECTED  
TO PREVAIL ACROSS MOST LOCAL WATERS THROUGH AT LEAST SUNDAY. AS A  
RESULT, SMALL CRAFT OPERATORS SHOULD EXERCISE CAUTION.  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
SJU 89 78 89 79 / 20 10 10 20  
STT 90 79 89 78 / 20 10 10 10  
 

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

 
 

 
 
SHORT TERM...ER/GL  
 
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