192
FXPQ50 PGUM 200734
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
534 PM CHST SUN JUL 20 2025
MARIANAS SYNOPSIS
SATELLITE AND RADAR OBSERVATIONS SHOW SCATTERED TO NUMEROUS SHOWERS
ACROSS THE MARIANAS. RITIDIAN AND IPAN BOUYS 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
THERE ARE CURRENTLY NO DISTURBANCES OR TROPICAL CYCLONES WITHIN THE
REGION. 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 POSSIBLITY 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 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 REVEALS A PLAINLY VISIBLE SURFACE
CIRCULATION, LOCATED NEAR 12.6N 135.2E. CONVECTION IS RAPIDLY
BUILDING ALONG THE WESTERN FLANK OF THE CIRCULATION, AND
SCATTEROMETRY FROM THIS MORNING REVEALS A TIGHT CIRCULATION WITH
WINDS OF 15 TO 20 KTS. AS OF 06Z, THERE HAS BEEN NO DESIGNATION BY
THE JOINT TYPHOON WARNING CENTER (JTWC), BUT IT IS POSSIBLE THAT AN
INVEST WILL BE ISSUED OVER THE NEXT 12 HOURS OR SO.
THE AREA OF WEAK RIDGING OVER PALAU WILL MAINTAIN MOSTLY DRY
CONDITIONS THERE THIS EVENING, BEFORE INCREASING CONVERGENCE INTO THE
MONSOON TROUGH AND DEVELOPING DISTURBANCE BRINGS HIGHER SHOWER
COVERAGE LATE OVERNIGHT. THE MONSOON TROUGH AND EMBEDDED DISTURBANCE
WILL SHIFT NORTH THROUGH MUCH OF THIS NEXT WEEK AS IT LOOKS TO
MORPH INTO A BROAD 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 GYRE.
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.
MARIANAS: WILLIAMS
EAST MICRONESIA: KLEESCHULTE
WEST MICRONESIA: DECOU
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