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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