012  
FXPQ50 PGUM 132057  
AFDGUM  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE TIYAN GU  
657 AM CHST FRI AUG 14 2020  
   
MARIANAS SYNOPSIS
 
 
RADAR AND SATELLITE SHOW ISOLATED SHOWERS FORMING ACROSS THE  
MARIANAS AS DIVERGENCE ALOFT IS ASSOCIATED WITH WEAK SURFACE  
CONVERGENCE NEAR ROTA. BUOYS ACROSS THE REGION SHOW SEAS IN THE 4  
TO 5 FOOT RANGE.  
 

 
   
DISCUSSION
 
 
THE SOUNDING FOR GUAM SHOWS A SURFACE INVERSION, THIS CAP NEAR  
THE SURFACE HELPED REDUCE CONVECTION OVERNIGHT. THE SOUNDINGS  
ALSO SHOWS WINDS LESS THAN 10 KNOTS UP TO THE TROPOPAUSE;  
ADDITIONALLY THERE IS ALSO DEEP MOISTURE PRESENT IN THE SOUNDING.  
LIGHT WINDS AND DEEP MOISTURE ARE THE TWO MAIN INGREDIENTS  
NECESSARY FOR ISLAND-EFFECT THUNDERSTORMS. SINCE THIS MORNING  
STARTED OFF MOSTLY CLEAR FOR GUAM DAY HEATING SHOULD QUICKLY ERODE  
OUR INVERSION AND THIS AFTERNOON SHOWERS AND THUNDERSTORMS ARE  
LIKELY TO FORM. THROUGH THE WEEKEND ISLAND-EFFECT SHOWERS AND  
THUNDERSTORMS WILL BE THE MOST LIKELY SOURCE OF RAIN FOR GUAM.  
 
THE GENERAL PATTERN FOR THE CNMI WILL REMAIN UNSETTLED AS WELL  
WITH SURFACE ANALYSIS SHOWING A COMPLICATED MESS. THERE IS A ONE  
SURFACE TROUGH MOVING TOWARDS TINIAN AND SAIPAN FROM THE SOUTH  
NEAR YAP AND ANOTHER MOVING IN FROM THE EAST. EVEN THOUGH THESE  
ARE WEAK SYSTEMS, SHOWERS THAT DO FORM WILL HAVE PLENTY OF DEEP  
MOISTURE AVAILABLE AND HEAVY SHOWERS ARE POSSIBLE. LOOKING AT THE  
MODELS THE GFS IS SHOWING THE DEEP MOISTURE BEING REPLACED BY A  
DRY LAYER ON SUNDAY, WHILE THIS WON'T BE ENOUGH TO STOP ISOLATED  
SHOWERS IT COULD PUT AN END TO THE SCATTERED SHOWERS THAT ARE  
FORECASTED FOR MONDAY. HOWEVER, THE EURO AND GFS ENSEMBLE STILL  
SHOW AN ACTIVE PERIOD OF CONVECTION MONDAY. BECAUSE OF  
DISAGREEMENT IN THE MODELS WE LOOKED AT MODEL INITIALIZATION AND  
THE CURRENT WEATHER PATTERN. THE GFS ENSEMBLE APPEARS TO HAVE THE  
BEST GRASP ON THE SITUATION WHILE THE GFS IS UNDERESTIMATING THE  
CONVECTION CURRENTLY IN THE REGION. WITH THIS GENERALLY ACTIVE  
PATTERN CURRENTLY ACROSS THE REGION THE DECISION WAS MADE TO LEAVE  
THE FORECAST UNCHANGED. IF THE OTHER MODELS START TO PICK UP ON  
THE DRY LAYER THEN THE SHOWERS ON MONDAY MAY BE DOWNGRADED FROM  
SCATTERED (35%) TO ISOLATED (20%).  
 
COMBINED SEAS WILL REMAIN LOW ACROSS THE REGION STAYING BETWEEN 3  
TO 6 FEET THROUGH THE WEEKEND. THEN SEAS WILL BEGIN TO FALL AGAIN  
SETTLING BETWEEN 2 TO 4 FEET BY MID NEXT WEEK.  
 

 
   
EASTERN MICRONESIA
 
 
AN AREA OF HIGH PRESSURE COVERS THE EASTERN HALF OF MICRONESIA  
AND WILL KEEP SHOWERS TO A MINIMUM AT POHNPEI, KOSRAE, AND MAJURO  
TODAY. A CIRCULATION WAS CROSSING THE DATE LINE THIS MORNING JUST  
EAST OF MAJURO. THE ARE OF HIGH PRESSURE MENTIONED EARLIER WILL  
KEEP SHOWERS ISOLATED AT POHNPEI AND KOSRAE THROUGH SATURDAY  
NIGHT. MODELS SHOW THE CIRCULATION PASSING NORTH OF MAJURO BY  
SATURDAY NIGHT. LOW-LEVEL CONVERGENCE SOUTH OF THE CIRCULATION  
WILL PRODUCE ISOLATED THUNDERSTORMS AT MAJURO SATURDAY AND  
SATURDAY NIGHT AND SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS  
SUNDAY AND MONDAY. THE CIRCULATION WILL CONTINUE TO MOVE SLOWLY  
TOWARDS THE WEST-NORTHWEST AND BE LOCATED NEAR ENEWETAK TUESDAY.  
MOST OF THE ACTIVITY ASSOCIATED WITH THE CIRCULATION WILL REMAIN  
NORTH OF POHNPEI AND KOSRAE. ENOUGH LOW-LEVEL CONVERGENCE WILL BE  
PRESENT, HOWEVER, TO PRODUCE MOSTLY CLOUDY SKIES, ISOLATED SHOWERS  
AND THUNDERSTORMS AT THESE TWO LOCATIONS SUNDAY THROUGH TUESDAY.  
 

 
   
WESTERN MICRONESIA
 
 
SATELLITE IMAGERY THIS MORNING SHOWED MOSTLY CLOUDY SKIES, SCATTERED  
SHOWERS AND ISOLATED THUNDERSTORMS AT PALAU THIS MORNING. SATELLITE  
TRENDS AND MODELS AGREE THAT THESE SHOWERS WILL TAPER OFF BY THE  
AFTERNOON. WEAK LOW-LEVEL CONVERGENCE AND ATMOSPHERIC INSTABILITY  
WILL PRODUCE ISOLATED SHOWERS AND THUNDERSTORMS AT YAP TODAY AND  
TONIGHT AND THROUGH SUNDAY AT PALAU. MODELS HINT AT A MONSOON TROUGH  
WITH A FEW CIRCULATIONS MOVING ALONG IT FORMING NORTH OF 10N BY THE  
BEGINNING OF NEXT WEEK. LOW-LEVEL CONVERGENCE SOUTH OF THESE WEATHER  
FEATURES WILL GENERATE SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS  
AT PALAU AND YAP MONDAY AND TUESDAY.  
 
A WEAK RIDGE OF HIGH PRESSURE WILL KEEP SKIES PARTLY CLOUDY AND  
SHOWERS ISOLATED AT CHUUK FOR PART OF THE DAY. SATELLITE IMAGERY THIS  
MORNING SHOWED AN AREA OF SCATTERED SHOWERS AND ISOLATED  
THUNDERSTORMS TO THE EAST OF CHUUK THIS MORNING. THIS AREA IS  
ASSOCIATED WITH A CIRCULATION. MODELS DO SHOW THESE SHOWERS AND HINT  
THAT THEY WILL DECREASE DURING THE DAY, ALTHOUGH ENOUGH WILL REMAIN  
TO BRING MOSTLY CLOUDY SKIES TO CHUUK TONIGHT. THE MONSOON TROUGH  
MENTIONED ABOVE FORMING NORTH OF PALAU AND YAP WILL ALSO FORM NORTH  
OF CHUUK BY THE WEEKEND. MOST OF THE ACTIVITY WILL REMAIN NORTH OF  
CHUUK, BUT ENOUGH LOW-LEVEL CONVERGENCE WILL BE PRESENT TO PRODUCE  
MOSTLY CLOUDY SKIES, ISOLATED SHOWERS AND THUNDERSTORMS AT THAT  
LOCATION SATURDAY NIGHT THROUGH MONDAY.  
 

 
   
GUM WATCHES/WARNINGS/ADVISORIES
 
 
GU...NONE.  
MARIANAS WATERS...NONE.  

 
 

 
 
SLAUGHTER/ZIOBRO  
 
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