784
FXPQ50 PGUM 202338 AAA
AFDGUM
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE TIYAN GU
938 AM CHST MON JUL 21 2025
UPDATE
CONVECTION HAS MOVED OUT OF GUAM AND ROTA THIS MORNING, WITH
SCATTERED SHOWERS AND THE POSSIBILITY OF THUNDERSTORMS CONTINUING
ACROSS TINIAN AND SAIPAN. DUE TO LIGHT SOUTHERLY WINDS, GUAM SHOULD
SEE ISLAND CONVECTION DURING THE AFTERNOON ACROSS MOST OF THE ISLAND.
PREV DISCUSSION
/ISSUED 559 AM CHST MON JUL 21 2025/
MARIANAS UPDATE...
LITTLE CHANGE HAS BEEN MADE TO THE FORECAST IN THE MARIANAS.
LIBERATION DAY FOR GUAM STILL LOOKS TO BE A LITTLE WET, WITH PATCHY
SHOWERS AND SOME THUNDERSTORMS BROUGHT BY LIGHT TO GENTLE SOUTHERLIES
EXPECTED FOR MUCH OF THE DAY. AFTER, SHOWERS WILL TAPER DOWN SLIGHTLY
OVERNIGHT AS THE WINDS SHIFT TO THE SOUTHEAST. REGARDING SURF; THE
EAST SWELL IS EXPECTED TO WEAKEN ENOUGH TO ALLOW RIP CURRENT RISK TO
DROP TO LOW ALONG ALL REEFS TODAY THROUGH TUESDAY, BEFORE BECOMING
MODERATE ALONG EAST REEFS ONCE TRADE SWELL STRENGTHENS SLIGHTLY BY
MIDWEEK.
TROPICAL SYSTEMS UPDATE...
A TROPICAL DISTURBANCE, KNOWN AS JTWC'S INVEST 97W, IS STILL CENTERED
NEAR 12.5N AND 135.0E, ABOUT 280 MILES NORTHWEST OF YAP PROPER. 97W
REMAINS LARGELY DISORGANIZED, WITH A VERY ROUGH CENTER OF ROTATION
THAT WOBBLES ALONG A MINOR AXIS EMBEDDED WITHIN THE GENERAL FLOW OF
THE MONSOON TROUGH, PRODUCING POCKETS OF SPORADIC SHOWERS AND SOME
THUNDERSTORMS WITHIN 175 MILES FROM THE APPROXIMATE CENTER. ALTHOUGH
THE POTENTIAL TO DEVELOP INTO A SIGNIFICANT TROPICAL CYCLONE OVER
THE NEXT 24 HOURS IS UNLIKELY, MODEL GUIDANCE SHOWS 97W MOVING NORTH-
NORTHWEST AS IT GRADUALLY INTENSIFIES OVER THE NEXT DAY OR SO.
EASTERN MICRONESIA UPDATE...
SATELLITE SHOWS AN ACTIVE INTERTROPICAL CONVERGENCE ZONE (ITCZ)
ACROSS THE REGION THIS MORNING. NO MAJOR CHANGES WERE MADE IN THE
FORECAST. CLOUDY SKIES AND NUMEROUS LOCALLY HEAVY SHOWERS ARE STILL
EXPECTED TODAY, DECREASING TO SCATTERED COVERAGE OVER KOSRAE AND
MAJURO TONIGHT AS THE ITCZ SHIFTS NORTH AND BECOMES MORE FRAGMENTED.
WESTERN MICRONESIA UPDATE...
MODERATE TO DEEP CONVECTION CONTINUES NORTH OF PALAU AND YAP, ALONG
THE MONSOON TROUGH AND NEAR INVEST 97W NEAR 13N135E. SHOWERS ON THE
SOUTHERN FRINGE OF THIS PATTERN IS STARTING TO DEVELOP OVER
NORTHWESTERN WATERS OF PALAU, BUT THE BULK OF SHOWERS REMAIN WEST OF
YAP THIS MORNING. CONDITIONS ARE BEGINNING TO DETERIORATE TODAY AT
CHUUK AS A DISTURBANCE AT THE LEADING EDGE OF THE ITCZ BEGINS TO
CLOSE IN. THE ONLY CHANGES IN THE FORECAST WERE TO INCREASE THE
CHANCES OF SHOWERS AT PALAU/CHUUK, WHERE LOCALLY HEAVY SHOWERS ARE
POSSIBLE THIS MORNING AT PALAU AND FOR CHUUK THROUGH TONIGHT. LATEST
ALTIMETRY DATA SHOW SEAS HAVE DECREASED TO 3 TO 5 FEET ACROSS THE
REGION.
PREV DISCUSSION... /ISSUED 612 PM CHST SUN JUL 20 2025/
MARIANAS SYNOPSIS...
SATELLITE AND RADAR OBSERVATIONS SHOW SCATTERED TO NUMEROUS SHOWERS
ACROSS THE MARIANAS. RITIDIAN AND IPAN BUOYS SHOW COMBINED SEAS
BETWEEN 3 AND 5 FEET.
DISCUSSION...
SHORT TERM (TONIGHT THROUGH WEDNESDAY):
THE SHORT TERM FORECAST WILL SEE A GENERAL DOWNTREND IN SHOWERS
THROUGH TUESDAY, WITH SCATTERED SHOWERS PERSISTING THROUGH MONDAY AND
THEN DECREASING TO ISOLATED SHOWERS MONDAY NIGHT AND TUESDAY.
MONSOONAL CONVERGENCE WILL BE THE MAIN SOURCE OF LIFT FOR SHOWERS AND
THUNDERSTORMS. BY MONDAY NIGHT WE BRIEFLY START TO LOSE THE MONSOONAL
CONVERGENCE AND SHOWER COVERAGE DECREASES. TUESDAY NIGHT INTO
WEDNESDAY, BOTH THE GFS AND ECMWF SHOW THE MONSOON TROUGH RE-
ESTABLISHING JUST SOUTH OF GUAM AND SCATTERED SHOWERS WILL RETURN TO
THE MARIANAS.
LONG TERM (WEDNESDAY NIGHT THROUGH SUNDAY NIGHT):
THE LONG TERM FORECAST REMAINS COMPLICATED WITH A WIDE RANGE OF
SOLUTIONS. WHAT WE ARE EXPECTING IS AN ENHANCEMENT OF THE MONSOON
TROUGH INTERACTING WITH A TROPICAL DISTURBANCE. GUIDANCE VARIES
WHETHER THE TROPICAL DISTURBANCE WILL BE CLOSE TO THE MARIANAS AND
THE MAIN PRODUCER OF HEAVY RAINFALL, OR IF THE MONSOON TROUGH WILL BE
DRAPED OVER THE MARIANAS AND THE MAIN PRODUCER OF HEAVY RAINFALL.
WHAT THEY ARE ALL SHOWING IS HEAVY RAINFALL FOR THE MARIANAS FROM
THURSDAY THROUGH SUNDAY NIGHT. SEE HYDROLOGY SECTION FOR MORE
INFORMATION.
NEARLY ALL GLOBAL MODELS SHOW A CIRCULATION DEVELOPING NEAR THE
MARIANAS LATER THIS WEEK. THE MODELS AND ENSEMBLES VARY DRASTICALLY
ON LOCATION AND STRENGTH OF THIS CIRCULATION, BUT THERE IS GOOD
AGREEMENT WITH A LOCATION NEAR THE MARIANAS. THIS SYSTEM LOOKS TO
BRING HEAVY RAIN TO THE MARIANAS LATE THIS WEEK INTO THE NEXT
WEEKEND. SEE TROPICAL SYSTEMS SECTION FOR MORE DETAILS.
MARINE...
COMBINED SEAS OF 3 TO 5 FEET PREVAIL ACROSS THE REGION. THE MAIN
CHANGE FOR THIS FORECAST WAS TO DECREASE THE WESTERLY SWELL IN THE
SHORT TERM TO BELOW 2 FEET AND REMOVE MENTION OF THE SWELL FROM THE
SURF FORECAST. RITIDIAN BUOY SHOWS NO ARRIVAL OF A WESTERLY SWELL AT
THIS TIME AND GUIDANCE DOESN'T SUGGEST ONE MOVES INTO THE REGION FOR
ANOTHER FEW DAYS. TOWARDS THE END OF THE WEEK, WE ARE LOOKING AT A
POTENTIAL TROPICAL DISTURBANCE NEAR THE MARIANAS LEADING AN INCREASE
IN SEAS AND A WESTERLY SWELL DEPENDING ON THE LOCATION AND STRENGTH
OF THE TROPICAL DISTURBANCE.
HYDROLOGY...
A TROPICAL DISTURBANCE COULD BRING HEAVY RAIN TO THE MARIANAS FROM
THURSDAY EVENING THROUGH SUNDAY. MODELS DIFFER ON LOCATION AND
STRENGTH OF THIS CIRCULATION, HOWEVER, THEY DO AGREE THAT HEAVY
RAINFALL IS LIKELY AS EARLY AS THURSDAY, CONTINUING THROUGH THE
WEEKEND.
THERE IS A POTENTIAL FOR GUAM AND THE CNMI TO SEE RAINFALL AMOUNTS
OF 5 TO 10 INCHES BY SUNDAY, THOUGH LOCALLY HEAVIER AMOUNTS ARE
POSSIBLE. THIS RAINFALL IS EXPECTED TO PRODUCE MINOR FLOODING ACROSS
THE ISLANDS, WITH FLASH FLOODING POSSIBLE IN THE AREAS OF HEAVIER
RAINFALL, ESPECIALLY OVER SOUTHERN GUAM.
RESIDENTS IN THE MARIANAS NEED TO CLOSELY MONITOR THIS DEVELOPING
SITUATION AS WATCHES, ADVISORIES AND WARNINGS COULD BE ISSUED LATER.
TROPICAL SYSTEMS...
A SURFACE CIRCULATION IS SEEN ON VISIBLE SATELLITE ROUGHLY 280 MILES
NORTHWEST OF YAP, DESIGNATED AS INVEST 97W BY THE JOINT TYPHOON
WARNING CENTER (JTWC). INVEST 97W IS CURRENTLY LOCATED NEAR 12.5N
135.0E, WITH CONVECTION BUILDING ALONG ITS WESTERN FLANK.
SCATTEROMETRY FROM THIS MORNING INDICATES WINDS OF 15 TO 20 KNOTS
ABOUT THE CIRCULATION CENTER. THIS SYSTEM IS EXPECTED TO MOVE NORTH-
NORTHWEST OVER THE PHILIPPINE SEA OVER THE NEXT FEW DAYS AS IT MORPHS
INTO A BROADER CYCLONIC CIRCULATION, EXITING THE REGION BY THE END
OF THE WEEK.
ELSEWHERE ACROSS THE REGION THERE ARE CURRENTLY NO DISTURBANCES OR
TROPICAL CYCLONES. HOWEVER, LATEST MODEL GUIDANCE IS IN VERY GOOD
AGREEMENT THAT A DISTURBANCE WILL FORM NEAR THE MARIANAS LATE THIS
WEEK INTO THE WEEKEND. NEARLY ALL OF THE MODELS, BOTH DETERMINISTIC
AND ENSEMBLES, AGREE THAT A TC WILL DEVELOP IN THE VICINITY OF THE
MARIANAS BETWEEN THURSDAY AND SATURDAY. THEY DO VARY A LITTLE ON
EXACT LOCATION, WITH CIRCULATIONS BEING PREDICTED OVER DIFFERENT
PORTIONS OF THE MARIANAS. THE MODELS DO ALSO VARY ON INTENSITY.
THEREFORE, THERE IS A HIGH DEGREE OF UNCERTAINTY FOR ACTUAL LOCATION
AND STRENGTH, BUT A MUCH LOWER DEGREE OF UNCERTAINTY THAT SOMETHING
WILL OCCUR IN THE VICINITY OF THE MARIANAS. LATEST MODEL GUIDANCE
SHOWS HEAVY RAINFALL WITH A GENERAL RANGE OF 5 TO 10 INCHES FROM
THURSDAY THROUGH SUNDAY NIGHT. SOME DETERMINISTIC GUIDANCES SUGGESTS
THE POSSIBILITY OF 12+ INCHES OVER A LOCALIZED AREA DURING THE SAME
TIME FRAME. THE POTENTIAL FOR DEVELOPMENT WILL BE MONITORED CLOSELY
OVER THE NEXT SEVERAL DAYS.
EASTERN MICRONESIA...
AN ACTIVE INTER-TROPICAL CONVERGENCE ZONE (ITCZ) REMAINS THE PRIMARY
PLAYER ACROSS EASTERN MICRONESIA THIS AFTERNOON, STRETCHING EAST FROM
POHNPEI TO BEYOND THE DATE LINE. SEVERAL WEAK TRADE-WIND TROUGHS ARE
ALSO MOVING THROUGH THE REGION. THESE TROUGHS ARE INTERACTING WITH
THE ITCZ TO PRODUCE AREAS OF NUMEROUS SHOWERS AND ISOLATED
THUNDERSTORMS. LOCALLY HEAVY SHOWERS ARE POSSIBLE ACROSS THE MARSHALL
ISLANDS TONIGHT THROUGH MONDAY NIGHT AND ACROSS POHNPEI MONDAY AND
MONDAY NIGHT. THE ITCZ IS EXPECTED TO DRIFT A BIT FARTHER NORTH
MONDAY NIGHT, SO THE POTENTIAL FOR HEAVY SHOWERS AT KOSRAE WILL
DECREASE MONDAY AFTERNOON. THE ITCZ LOOKS TO BEGIN TO BREAK DOWN
EARLY IN THE WEEK, THOUGH AN OVERALL WET PATTERN WILL CONTINUE
ACROSS POHNPEI AND MAJURO THROUGH MIDWEEK, WHILE KOSRAE SEES A DRIER
PATTERN TUESDAY.
GENTLE TO MODERATE WINDS WILL CONTINUE FOR KOSRAE AND MAJURO THROUGH
MONDAY WHILE WINDS WILL BECOME LIGHT TO GENTLE FOR POHNPEI MONDAY
MORNING. HOWEVER, HEAVIER SHOWERS AND THUNDERSTORMS WILL PRODUCE
OCCASIONAL STRONG GUSTS AT KOSRAE THROUGH MONDAY AND AT POHNPEI AND
MAJURO THROUGH MONDAY NIGHT. OTHERWISE, LIGHT TO GENTLE WINDS ARE
EXPECTED ACROSS THE REGION FOR THE REMAINDER OF THE WEEK. COMBINED
SEAS ARE NOT EXPECTED TO CHANGE MUCH OVER THE NEXT FEW DAYS FOR
KOSRAE AND POHNPEI, REMAINING BETWEEN 3 AND 5 FEET THROUGH THE WEEK.
FOR MAJURO, COMBINED SEAS OF 6 TO 8 FEET TONIGHT WILL SUBSIDE TO
BETWEEN 3 AND 5 FEET BY MIDWEEK.
WESTERN MICRONESIA...
AN UNSETTLED MONSOON WEATHER PATTERN CONTINUES ACROSS FAR WESTERN
MICRONESIA, BUT CONDITIONS WERE RELATIVELY QUIET THIS AFTERNOON WITH
JUST ISOLATED SHOWERS OVER THE REPUBLIC OF PALAU AND YAP PROPER. THE
PORTION OF THE MONSOON TROUGH AXIS THAT PASSES JUST NORTH OF YAP HAS
SHIFTED FURTHER NORTH, BUT VISIBLE SATELLITE REVEALS INCREASING
SHOWERS WITHIN THE AREA OF TROUGHING THAT DIPS SOUTHEASTWARD OVER
WESTERN YAP STATE. CONVECTION IS FOCUSED WITHIN THE STRONGER
CONVERGENCE JUST SOUTH OF YAP PROPER, WITH SHOWERS BUILDING ACROSS
THE AREA BETWEEN NGULU AND SOROL, AND WEST OF EAURIPIK. TO THE EAST,
SATELLITE SHOWS ISOLATED SHOWERS AND PARTLY CLOUDY SKIES OVER CHUUK.
NORTHWEST OF YAP, VISIBLE SATELLITE PLAINLY REVEALS A TIGHT SURFACE
CIRCULATION EMBEDDED WITHIN THE MONSOON TROUGH, LOCATED NEAR 12.5N
135.0E. CONVECTION IS RAPIDLY BUILDING ALONG THE WESTERN FLANK OF THE
CIRCULATION, AND SCATTEROMETRY FROM THIS MORNING REVEALS WIND SPEEDS
OF 15 TO 20 KTS. JTWC HAS DESIGNATED THIS CIRCULATION AS INVEST 97W,
AND IT WILL BE CLOSELY MONITORED AS IT MOVES NORTH AND AWAY FROM YAP
AND PALAU.
THE AREA OF WEAK RIDGING OVER PALAU WILL MAINTAIN MOSTLY DRY
CONDITIONS THERE THIS EVENING, BEFORE INCREASING CONVERGENCE INTO THE
MONSOON TROUGH AND INVEST 97W BRINGS HIGHER SHOWER COVERAGE LATE
OVERNIGHT. THE MONSOON TROUGH AND EMBEDDED DISTURBANCE WILL SHIFT
NORTH THROUGH MUCH OF THIS NEXT WEEK AS THEY MORPH INTO A BROADER
CYCLONIC CIRCULATION OVER THE PHILIPPINE SEA. MODELS GENERALLY FOCUS
MORE OF THE CONVERGENCE AND SHOWERS NEAR YAP OVER THE NEXT FEW DAYS,
NEAR THE TAIL END OF CONVERGENT FLOW INTO THE DEVELOPING SYSTEM.
CHUUK REMAINS QUIET THIS EVENING, BUT CAN EXPECT INCREASINGLY
UNSETTLED CONDITIONS THROUGH THE FIRST HALF OF THIS WEEK. AN AREA OF
TROUGHING AND CONVERGENCE ALONG THE WESTERN EDGE OF THE ITCZ,
CURRENTLY LOCATED NEAR POHNPEI, WILL SHIFT WEST INTO THE AREA THROUGH
TOMORROW MORNING, BRINGING AT LEAST SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS OVER THE NEXT FEW DAYS. THIS DISTURBANCE WILL PASS
NORTHWEST OF CHUUK BY WEDNESDAY, AFTER WHICH MODELS SUGGEST A QUIETER
RIDGING PATTERN WILL SET UP NEAR WENO. HOWEVER, MODEL GUIDANCE
STRONGLY SUGGESTS A TROPICAL CYCLONE OF SOME INTENSITY WILL FORM NEAR
THE MARIANAS BETWEEN THURSDAY AND SATURDAY, AND MOIST CONVERGENT
FLOW INTO THIS SYSTEM COULD BRING PERIODS OF SHOWERS OVER CHUUK LATE
IN THE WEEK.
COMBINED SEAS OF 4 TO 6 FEET WILL CONTINUE FOR PALAU AND YAP OVER THE
NEXT FEW DAYS, COMPRISED OF A WEST TO SOUTHWEST MONSOON SWELL AND
WEAKENING EASTERLY TRADE SWELL. A 2 TO 3 FOOT NORTHWEST SWELL LOOKS
TO ARRIVE IN YAP AND PALAU WATERS BY MIDWEEK, EMANATING FROM A
DEVELOPING DISTURBANCE OVER THE PHILIPPINE SEA. FOR NOW, BENIGN
MARINE CONDITIONS ARE EXPECTED TO CONTINUE THROUGH THE WEEK.
COMBINED SEAS OF 2 TO 4 FEET ARE EXPECTED TO CONTINUE FOR CHUUK
WATERS FOR MUCH OF THE FORECAST PERIOD, DRIVEN BY THE PRIMARY TRADE
SWELL. LIGHT TO MODERATE SOUTHWEST TO WEST MONSOON FLOW IS EXPECTED
FOR PALAU THROUGH THE WEEK, WITH LIGHT TO GENTLE WINDS FOR YAP.
MOSTLY LIGHT WINDS ARE EXPECTED FOR CHUUK INTO EARLY NEXT WEEK.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
UPDATE: KLEESCHULTE
MARIANAS UPDATE: MONTVILA
EAST/WEST MICRONESIA UPDATE: CRUZ
MARIANAS: WILLIAMS
EAST MICRONESIA: KLEESCHULTE
WEST MICRONESIA: DECOU
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