747
FXPQ50 PGUM 092011
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
611 AM CHST WED SEP 10 2025
MARIANAS UPDATE
THE SHOWERS PUSHED THROUGH THE MARIANAS A LITTLE QUICKER THAN
EXPECTED, SO JUST UPDATED TO REMOVE THE SCATTERED SHOWERS THIS
MORNING. OTHERWISE EVERYTHING WAS IN GOOD SHAPE. WINDS STILL LOOK A
LITTLE STRONG FOR ISLAND CONVECTION TODAY, BUT ARE EXPECTED TO WEAKEN
BY FRIDAY, SO THAT COULD BE A CONCERN BY THEN.
EASTERN MICRONESIA UPDATE
A RATHER ROBUST ITCZ (INTER TROPICAL CONVERGENCE ZONE) CONTINUE TO
AFFECT MAJURO. ADDED 10 PERCENT TO THE POP'S TO MAKE THEM HIGH END
SCATTERED VERSUS MID RANGE, MOST EVERYTHING WAS WELL IN ORDER THOUGH.
INCREASED THE CLOUDS AT KOSRAE TO PARTLY SUNNY WITH ALL KINDS OF
DEBRIS CLOUDINESS IN THE AREA. POHNPEI WAS LOOKING GOOD AS IT WAS FOR
TODAY, SO NO UPDATES WERE NEEDED THERE.
WESTERN MICRONESIA UPDATE
FORECAST WAS EXPECTING SCATTERED SHOWERS AT CHUUK, AND SATELLITE
SHOWED THEM COMING IN, SO ROLLED AHEAD WITH THAT, NO CHANGES FOR
CHUUK. WITH A BIG SLUG OF WEATHER HEADED FOR YAP THOUGH, HAD TO
INCREASE THEM TO SCATTERED AT 30 PERCENT. KOROR PALAU WAS GOOD TO GO
AS IT WAS, NO CHANGES NEEDED. IT'S STARTING TO LOOK LIKE THE NASCENT
DISTURBANCE WILL MOST LIKELY BE CLEAR OF MICRONESIA BEFORE DEVELOPING
SIGNIFICANTLY.
PREV DISCUSSION
/ISSUED 605 PM CHST TUE SEP 9 2025/
MARIANAS SYNOPSIS...
A FEW SHOWERS AND THUNDERSTORMS ARE PRESENT ACROSS THE MARIANAS,
THOUGH A BROADER DISTURBANCE IS WELL SOUTH, MOVING INTO EASTERN YAP
STATE. A SURFACE TROUGH EXTENDING NORTHWARD IS NOW CROSSING 150E EAST
OF THE MARIANAS, BRINGING WITH IT STRONGER WINDS AND INCREASED
SHOWERS AND THUNDERSTORMS. REGIONAL BUOYS SHOW SEAS OF 4 TO 5 FEET
AROUND THE MARIANAS. THE RITIDIAN BUOY INDICATES A DOMINANT NORTHERLY
SWELL OF ABOUT 4 TO 5 FT.
DISCUSSION...
THE TROUGH TO THE EAST IS SET TO MOVE THROUGH THE MARIANAS LATER
TONIGHT AND INTO EARLY WEDNESDAY. HAVE INCREASED WINDS AND SHOWERS
AFTER MIDNIGHT AND FOR THE FIRST PART OF WEDNESDAY FOR ITS PASSAGE.
DRIER WEATHER WILL RETURN FOR THE LATTER HALF OF THE WEEK SO HAVE
MAINTAINED 20 POPS IN THE LONGER RUN.
MARINE...
HAVE REFRESHED THE MARINE GRIDS TO BETTER REFLECT THE PRESENCE OF THE
LONGER-PERIOD NORTHERLY SWELL. THIS NORTH SWELL WILL SLOWLY SHIFT TO
NORTH-NORTHEAST, THEN NORTHEAST BY LATE WEEK AS IT DIMINISHES. NO
SURF HAZARDS ARE ANTICIPATED THROUGH THE WEEKEND, THOUGH NORTH SWELL
WILL MAINTAIN A MODERATE RISK OF RIP CURRENTS ALONG NORTH REEFS THE
NEXT FEW DAYS.
EASTERN MICRONESIA...
THE INTER-TROPICAL CONVERGENCE ZONE (ITCZ) REALLY CAME TO LIFE
OVERNIGHT, AS YESTERDAY IT WAS CONVECTIVELY-VOID, WHERE AS TODAY IT
HAS WIDESPREAD CONVECTION BETWEEN 2N AND 11N FROM EASTERN YAP SATE,
ACROSS ALL OF EASTERN MICRONESIA, TO THE DATE LINE AND BEYOND.
CONVERGENCE HAS STRENGTHENED ALONG A MORE DEFINED SURFACE TROUGH TO
AID IN THIS PROCESS, WITH THE TROUGH STRETCHING FROM KOSRAE TO
BETWEEN KWAJALEIN AND MAJURO, ENDING NEAR 11N175E (NORTHEAST OF
MAJURO). SOME OF THIS RAIN HAS BEEN QUITE HEAVY WITH KWAJALEIN IN THE
RMI HAVING HAD VISIBILITIES DROP TO 1 3/4 OF A MILE, AND 1 MILE AT
KOSRAE. WITHIN THE ITCZ, CLOUD TOP TEMPERATURES HAVE ROUTINELY
SURPASSED -80 DEG. C, INDICATIVE OF THE HEAVY RAINFALL IN ADDITION TO
THE LOWERED VISIBILITIES.
SO, WILL THE ITCZ CONTINUE TO BE A RAIN PRODUCER FOR THE REGION? THE
SHORT ANSWER IS YES, AT LEAST THROUGH THE NEXT 48 TO 72 HOURS, BUT
THAT'S ABOUT IT. THIS IS DUE TO CONTINUED MODEL TRENDS OF WEAKENING
IT'S (THE ITCZ) CONVECTIVE SIGNATURE DUE TO A SLIGHT WEAKENING IN THE
AREA OF CONVERGENCE AND THE EMBEDDED TROUGH LIFTING NORTH AND WEST
WITH TIME, POTENTIALLY ASSOCIATED WITH A WEAK CIRCULATION THAT
DEVELOPS NORTH OF CHUUK/EAST OF THE MARIANAS. IF THIS CIRCULATION
DEVELOPS, INFLOW INTO IT WILL INDEED HELP LIFT THE TROUGH NORTHWARD.
AS FOR SEAS, THE SURFACE TROUGH MENTIONED ABOVE MAY ALLOW SEAS TO
BRIEFLY INCREASE LOCALLY TO NEAR 6 FEET AS IT PASSES BY KOSRAE AND
POHNPEI THIS WEEK, BUT, IN GENERAL SEAS WILL REMAIN IN THE 3 TO 5
FOOT RANGE BEFORE DECREASING A FOOT FOR THE WEEKEND. LIGHT AND GENTLE
WINDS WILL PREVAIL.
WESTERN MICRONESIA...
A WEAK TROPICAL DISTURBANCE IS MOVING THROUGH THE REGION, WITH
SATELLITE IMAGERY AND SCATTEROMETER DATA SUGGESTING A WEAK
CIRCULATION IS LOCATED SOUTHWEST OF CHUUK NEAR 7N148E, EMBEDDED
WITHIN A SURFACE TROUGH THAT EXTENDS FROM NORTH OF POHNPEI AND
THROUGH MOST OF YAP STATE. THESE FEATURES ARE PRODUCING SCATTERED
SHOWERS AND ISOLATED THUNDERSTORMS ACROSS YAP AND CHUUK, WITH MOST
OF THE HEAVIER CONVECTION CURRENTLY ACROSS THE OUTER ISLANDS OF
EASTERN YAP STATE AND WESTERN CHUUK STATE. AT PALAU, A POCKET OF
DRIER AIR IS CURRENTLY OVER THE REGION, WITH THE TOTAL PRECIPITAL
WATER (TPW) SATELLITE LOOP SHOWING VALUES AROUND 2.0 TO 2.1 INCHES,
COMPARED TO THE TROPICAL AIRMASS THAT EXTENDS ACROSS YAP AND CHUUK
WITH VALUES AROUND 2.3 TO 2.6 INCHES. THIS DRIER AIR HELPED TO LIMIT
ISLAND-EFFECT TO JUST LIGHT TO MODERATE SHOWERS, AND SUPPRESSED
THUNDERSTORMS FROM DEVELOPING AT PALAU, DESPITE THE LIGHT NORTHWEST
WINDS TODAY.
THE WEAK TROPICAL DISTURBANCE WILL CONTINUE TO DRIFT WESTWARD,
MOVING TOWARD YAP AND PALAU OVER THE NEXT FEW DAYS. A SURFACE TROUGH
ASSOCIATED WITH THE ITCZ IN EASTERN MICRONESIA WILL CONTINUE TO
SUPPORT SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS THROUGH
WEDNESDAY NIGHT AT CHUUK, BEFORE SHOWERS DIMINISH IN COVERAGE AND
LIFT WEST-NORTHWEST LATER THIS WEEK AND INTO THE WEEKEND. FOR PALAU
AND YAP, LATEST COUPLE OF RUNS OF THE GFS AND ECMWF SUGGEST THAT THE
CONVECTION CURRENTLY ALONG 140E, NEAR ULITHI AND EAST-SOUTHEAST OF
YAP PROPER, WILL DIMINISH OVERNIGHT, LEADING TO A BRIEF PERIOD OF
DRY WEATHER FOR YAP AND PALAU WEDNESDAY. HOWEVER, THERE IS THE
POSSIBILITY THAT THIS CONVECTION WILL HOLD TOGETHER, SO DECIDED TO
INCREASED CLOUD COVER FOR WEDNESDAY BUT KEPT THE POTENTIAL FOR
SHOWERS AROUND 20 PERCENT TO MATCH MODEL TRENDS. MOVING INTO
THURSDAY AND FRIDAY, THERE IS A HIGH DEGREE OF VARIABILITY IN HOW
MODELS MODELS HANDLE THE DEVELOPMENT OF APPROACHING TROPICAL
DISTURBANCE. ALL MODELS AGREE THAT THE POTENTIAL FOR SHOWERS AND
THUNDERSTORMS WILL INCREASE THURSDAY AND FRIDAY, BUT THE GFS IS VERY
AGGRESSIVE AND SHOWS THE CIRCULATION BECOMING MORE DEFINED AND WINDS
INCREASING AROUND THE CIRCULATION AS THE SYSTEM DEVELOPS. THE ECMWF
IS ON THE VERY OTHER END OF THE SPECTRUM, SHOWING VERY LITTLE DEVELOP
AND KEEPING THE CIRCULATION BROAD AND MAINLY EMBEDDED WITHIN THE
SURFACE TROUGH AS THE FEATURE MOVES ACROSS THE REGION AND INTO THE
PHILIPPINE SEA, WEST OF PALAU. THE CANADIAN IS INBETWEEN BOTH MODELS,
SUPPORTING ECMWF THAT THE CIRCULATION WILL REMAIN WEAK AND BROAD
UNTIL PASSING WEST OF PALAU, THEN SHOWING A STRENGTHENING TREND
SIMILAR TO THE GFS, BUT ONCE THE TROPICAL DISTURBANCE IS MOVING
NORTHWEST AND AWAY FROM PALAU AND YAP. LOOKING AT THE ENSEMBLE
GUIDANCE FOR ALL THREE MODELS, THE GFS ENSEMBLE IS STILL THE MOST
AGGRESSIVE, BUT CURRENTLY ONLY SHOWS A 30 PERCENT CHANCE THAT WINDS
GREATER THAN 25 MPH (22 KT) WILL OCCUR NEAR PALAU AND YAP OVER A 48
HR WINDOW. FOR NOW KEPT WINDS FAIRLY LIGHT TO GENTLE IN FORECAST BUT
DID MENTION THE POTENTIAL FOR GUSTS AROUND 20 KT NEAR SHOWERS FOR YAP
AND PALAU. WE WILL CONTINUE TO MONITOR THE TROPICAL DISTURBANCE AND
WILL DISCUSS ANY CHANGES IN FUTURE DISCUSSIONS.
ALTIMETRY DATA SHOWS BENIGN SEAS OF 2 TO 3 FEET NEAR YAP AND PALAU
AND 3 TO 4 FEET NEAR CHUUK. A NORTH TO NORTHEAST SWELL AROUND 2 TO 3
FT WILL MOVE THROUGH THE REGION WEDNESDAY AND THURSDAY, PUSHING SEAS
UP TO AROUND 3 TO 4 FEET AT PALAU AND YAP AND 5 FEET AT CHUUK.
BESIDES FOR THE POTENTIAL FOR GUSTY WINDS NEAR HEAVIER SHOWERS, MODEL
GUIDANCE SUPPORTS WINDS REMAINING MOSTLY LIGHT TO GENTLE, WITH SOME
PERIODS OF MODERATE WINDS (~15 KT).
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS/EAST AND WEST MICRONESIA UPDATE: STANKO
MARIANAS: AYDLETT
EAST MICRONESIA: DOLL
WEST MICRONESIA: SCHANK
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