554  
FXPQ50 PGUM 180132  
AFDGUM  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE TIYAN GU  
1132 AM CHST SUN APR 18 2021  
   
UPDATE
 
 
SENT OUT A QUICK UPDATE TO INCREASE POPS TO SCATTERED SHOWERS FOR  
THE WESTERN SIDE OF GUAM. VISIBLE IMAGERY SHOWS ISLAND CONVECTION  
ALREADY DEVELOPING, WITH AN ONSHORE BREEZE AND SHOWERS WITH LARGE  
DROPLET SIZES REPORTED BY AN OFF DUTY EMPLOYEE IN AGAT. GIVEN THE  
LIGHT WINDS AT THE AIRPORT, ONLY 8 KNOTS OF WIND AT 925 MB, AND  
THE LATEST VISIBLE IMAGERY LOOP, THIS WILL LIKELY CONTINUE INTO  
THE AFTERNOON HOURS. HOW MUCH OF THE WESTERN SIDE OF THE ISLAND  
(IMMEDIATE COAST OF SLIGHTLY INLAND) REMAINS TO BE SEEN.  
 

 
   
PREV DISCUSSION
 
/ISSUED 820 AM CHST SUN APR 18 2021/  
 
MARIANAS SYNOPSIS...  
EAST TO NORTHEAST WINDS PREVAIL OVER THE MARIANAS THIS MORNING.  
SATELLITE IMAGERY SHOWS ONLY A FEW CLOUDS AND SHOWERS OVER THE AREA.  
 
DISCUSSION...  
NO CHANGES MADE TO THE CURRENT FORECAST. MODELS KEEP 700 MB RELATIVE  
HUMIDITY BELOW 50 PERCENT THROUGH MONDAY NIGHT. THIS SHALLOW LEVEL OF  
MOISTURE WILL KEEP SHOWERS TO A MINIMUM THROUGH THAT TIME. SATELLITE  
IMAGERY THIS MORNING DEPICTED AN BAND OF SHOWERS AND THUNDERSTORMS TO  
THE EAST OF THE LOCAL AREA. MODELS ACTUALLY SHOW THIS BAND AND BRING  
IT TOWARDS THE MARIANAS REACHING THE AREA LATER TONIGHT. THE MODELS  
TEND TO WEAKEN THE LINE AND WHILE THERE MIGHT BE A SLIGHT INCREASE IN  
CLOUDS AND THE CHANCE OF SHOWERS IT SHOULD NOT CHANGE THE CURRENT  
FORECAST OF PARTLY CLOUDY SKIES AND ISOLATED SHOWERS.  
 
LIGHT WINDS AND A SLIGHT INCREASE IN MOISTURE COMBINED WITH DAYTIME  
HEATING WILL EQUAL A CHANCE OF AFTERNOON SHOWERS OVER THE ISLANDS  
TUESDAY THROUGH THURSDAY. DESPITE THE SLIGHT INCREASE IN MOISTURE THE  
PRECIPITABLE WATER INDEX WILL REMAIN RELATIVE LOW SO THAT AFTERNOON  
THUNDERSTORMS ARE LESS LIKELY.  
 
MODELS SHOW THE EASTERLY WINDS DECREASING FROM 10 TO 15 MPH TO 10 MPH  
BY MONDAY NIGHT. THE WINDS WILL BECOME VARIABLE BY THURSDAY AND  
THEN SWITCH TO SOUTHWEST BY FRIDAY.  
 
MARINE/SURF...  
RITIDIAN AND TANAPAG BUOYS SHOWED SEAS AROUND 8 FEET...ALTHOUGH THERE  
WERE A FEW 9 TO 10 FOOT SEAS OBSERVED BY THE BUOYS. MODELS DEPICT  
SEAS BETWEEN 5 TO 7 FEET THE NEXT FEW DAYS AND JUST A COMPARISON  
BETWEEN THE TWO WOULD INDICATE THE MODELS ARE UNDER FORECASTING THE  
CONDITIONS. THE SWELLS ARE ALSO NOT REPRESENTATIVE EITHER SO HAD TO  
MANUALLY EDIT ALL THE PERIODS. KEPT THE SWELL HEIGHT AT 7 FEET AND  
HAD THE DIRECTION FROM WEST-SOUTHWEST THE NEXT FEW DAYS WEDNESDAY.  
ADDED A NORTHEAST TRADE-WIND SWELL AT 3 FEET AS THE BUOYS DO SHOW  
SOMETHING SIMILAR CONDITIONS.  
 
AS INDICATED PREVIOUSLY, KEPT SEA HEIGHTS AT 8 FEET AS THIS WAS THE  
PREDOMINATE LEVEL THROUGH THE MORNING. DESPITE THE MODELS BEING TOO  
LOW THEY KEPT THE HEIGHTS CONSTANT ALL WEEK SO FOLLOWED THAT IDEA. IF  
THE TREND OF 9 TO 10 FEET CONTINUES THEN A SMALL CRAFT ADVISORY MAY  
BE NEEDED. ALSO THE SWELL HEIGHT HAS INCREASED FROM 7 FEET TO AT  
TIMES 8 FEET, ALMOST 9 FEET. IF THE SWELL WAS 7 FEET AT 11 SECONDS  
THE SURF WOULD BY 9 TO 11 FEET. IF THE SWELL REMAINED AT 9 FEET AT  
13 SECONDS THE SWELL WOULD BE UP TO 12 FEET. WILL MONITOR AND IF THE  
HIGHER SEA AND SWELL HEIGHT PERSIST FORECAST MAY BE UPDATED.  
 
FIRE WEATHER...  
THE KDBI HAS INCREASED TO 712 TODAY. A WETTING RAIN IS NOT EXPECTED  
THE NEXT FEW DAYS. WINDS ARE EXPECTED TO REMAIN BELOW FIRE ADVISORY  
CRITERIA ALTHOUGH DRY CONDITIONS WILL BE FAVORABLE FOR SPOT FIRES.  
AFTERNOON SHOWERS MAY BECOME SCATTERED TUESDAY THROUGH THURSDAY  
WHICH MAY PRODUCE A WETTING RAIN.  
 
EASTERN MICRONESIA...  
THE MAIN WEATHER FEATURES AFFECTING EASTERN MICRONESIA ARE TWO TRADE  
WIND TROUGHS, ONE OF WHICH EXTENDS FROM A WEAK CIRCULATION SOUTH-  
SOUTHWEST OF POHNPEI. THE MOST ACTIVE CONVECTION IS MAINLY SOUTH OF  
THE MAIN ISLANDS. THIS IS A GOOD THING, AS KOSRAE NEEDS SOME TIME TO  
DRY OUT AFTER THE EXCESSIVE RAIN OF YESTERDAY. NEVERTHELESS, THERE  
WILL BE SCATTERED SHOWERS AT POHNPEI AT MAJURO FOR MUCH OF THE NEXT  
FEW DAYS. POHNPEI IS GETTING SHOWERS FROM THE TROUGH RUNNING BETWEEN  
THE CIRCULATION AND WAKE ISLAND. MAJURO IS GETTING SHOWERS FROM THE  
TROUGH JUST EAST OF THEM. IN THE LAST FEW HOURS, A LOT OF SHOWERS  
HAVE BLOWN UP EAST OF KOSRAE. SINCE WE HAVE ISOLATED SHOWERS FOR  
KOSRAE, THIS WILL HAVE TO BE WATCHED. THESE TROUGHS ARE EXPECTED TO  
MOVE WESTWARD WITH TIME, SO MAJURO WILL CLEAR OUT AROUND THE END OF  
THE PERIOD, HOWEVER THAT CLEARER WEATHER WILL NOT REACH KOSRAE AND  
POHNPEI WITHIN THE FORECAST PERIOD.  
 
MOST WINDS WILL BE AROUND 10 KNOTS, ALTHOUGH THE GFS SOLUTION DOES  
SUGGEST POHNPEI COULD REACH 20 KNOTS AT TIMES. THIS IS STILL BELOW  
SMALL CRAFT ADVISORY CRITERIA, SO OTHER THAN THUNDERSTORMS HAVING  
LIGHTNING NEAR THEM, NO WEATHER RELATED HAZARDS ARE ANTICIPATED IN  
EASTERN MICRONESIA.  
 
WESTERN MICRONESIA...  
THE MAIN FEATURES AFFECTING WESTERN MICRONESIA ARE A MONSOON TROUGH  
EXTENDING EASTWARD FROM SUPER TYPHOON SURIGAE TO JUST NORTHEAST OF  
YAP AND A TROUGH RUNNING FROM THERE TO THE WEAK CIRCULATION SOUTH-  
SOUTHEAST OF CHUUK. THE LAST RAIN BAND OF SURIGAE IS NEAR KOROR.  
FOR NOW, HANDLED THIS BY ADDING THUNDERSTORMS. IF THIS MOVES OVER  
KOROR, WE COULD HAVE SCATTERED SHOWERS, SO THIS WILL BE THE MAJOR  
RISK TO THE KOROR FORECAST.  
 
NO WEATHER RELATED HAZARDS ARE ANTICIPATED AT CHUUK THROUGH MONDAY,  
AND THEN IT'S JUST LIGHTNING NEAR THUNDERSTORMS. NOW THAT SEAS HAVE  
DIMINISHED AT YAP, NO WEATHER RELATED HAZARDS ARE EXPECTED THERE  
THROUGH THE PERIOD EXCEPT SURF, ALTHOUGH IT SHOULD BE NOTED THAT THE  
SEAS ARE JUST BELOW SMALL CRAFT ADVISORY CRITERIA, AND THEY ARE  
PRODUCING HAZARDOUS SURF. KOROR STILL HAS AN ACTIVE SMALL CRAFT  
ADVISORY THROUGH TODAY AT LEAST. TOUCHING ON THE HAZARDOUS SURF, IT'S  
EXPECTED TO LAST AT LEAST THROUGH MIDWEEK AS SUPER TYPHOON SURIGAE  
CONTINUES TO PUMP OUT SWELL, AND COULD LAST ALL WEEK.  
 

 
   
GUM WATCHES/WARNINGS/ADVISORIES
 
 
GU...HIGH SURF ADVISORY UNTIL 7 AM CHST SATURDAY FOR GUZ001>004.  
 
MARIANAS WATERS...NONE.  

 
 

 
 
MARIANAS: DOLL  
 
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