809
FXPQ50 PGUM 252122
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
ISSUED BY NATIONAL WEATHER SERVICE HONOLULU HI
722 AM CHST THU JUN 26 2025
MARIANAS UPDATE
AFTER A QUIET AND STABLE NIGHT, CONVECTION HAS REDEVELOPED IN THE
REGION. THE PAST COUPLE OF HOURS HAVE FEATURED A LINE OF NORTHWARD-
MOVING SHOWERS TRAINING RIGHT OVER GUAM WITH NEARLY NO MOTION EAST
OR WEST. THIS LINE HAS DEVELOPED ALONG THE CONVERGENCE BOUNDARY
BETWEEN EASTERLY WINDS EAST OF THE MARIANAS AND SOUTHERLY WINDS
FLOWING COUNTER-CLOCKWISE AROUND THE NEARBY DISTURBANCE, 97W,
CURRENTLY WEST OF GUAM. LATEST MODEL DATA STILL POINTS TO A WET 3-5
DAYS AHEAD, THOUGH AS THE PATTERN EVOLVES, THE HEAVIER SHOWERS WILL
BE DEPENDENT ON THE DISTURBANCE AND THE SURROUNDING FLOW. SEE THE
HYDROLOGY SECTION, BELOW, FOR MORE ON RAINFALL. STRONGER WINDS
PREVIOUSLY OVER THE CNMI HAVE SHIFTED WEST, AS SEEN IN OVERNIGHT
SCATTEROMETRY, SO THE SMALL CRAFT ADVY HAS BEEN ALLOWED TO EXPIRE FOR
TINIAN AND SAIPAN WATERS.
AVIATION
SOUTH-SOUTHEAST WINDS WILL PERSIST ACROSS GUAM FOR MUCH OF TODAY
RESULTING IN CROSS WINDS AT THE GUAM AIRPORT. A NORTHWARD-MOVING
DISTURBANCE WEST OF GUAM WILL KEEP THE LOCAL WIND PATTERN IN PLACE
THROUGH EARLY AFTERNOON BEFORE WINDS WEAKEN LATE AFTERNOON AND BECOME
VARIABLE OVERNIGHT.
HYDROLOGY UPDATE
THIS MORNING'S RAIN OVER GUAM IS, PERHAPS, A GOOD PRECURSOR FOR THE
NEXT SEVERAL DAYS. SIMILAR TO HOW THIS MORNING'S HEAVY RAIN IS VERY
LOCALIZED TO GUAM ALONG THE CONVERGENCE BOUNDARY, THE ANTICIPATED
SHOWERS OF THE NEXT SEVERAL DAYS WILL BE SIMILARLY LOCALIZED AND
HIGHLY DEPENDENT ON HOW 97W EVOLVES AND WHERE IT TREKS. LATEST
ENSEMBLE GUIDANCE HAS DECREASED THU-SUN RAINFALL POTENTIAL, BUT ONLY
SLIGHTLY, AND THIS IS LIKELY DUE TO MODEL TRENDS POINTING MORE TO 97W
STAYING JUST WEST OF THE ISLANDS.
TROPICAL SYSTEMS UPDATE
LATEST MODEL GUIDANCE STILL POINTS TO THE DISTURBANCE, DUBBED 97W BY
JTWC, TO TAKE SHAPE WEST OF THE MARIANAS. OVERNIGHT SATELLITE IMAGERY
SHOWED A WEAK CIRCULATION NEAR 12N141E, IN LINE WITH A SURFACE TROUGH
REVEALED IN 12Z SCATTEROMETRY. SINCE THEN, THE CIRCULATION HAS LIFTED
NORTHWARD TO NEAR 14N141E. MOST GUIDANCE KEEPS 97W WEST OF THE
ISLANDS AND HEADING NW, BUT THE DETERMINISTIC GFS STILL AGGRESSIVELY
PUSHES A BUDDING TROPICAL CYCLONE EASTWARD THROUGH THE MARIANAS TO
EVENTUALLY MERGE WITH ANOTHER CIRCULATION EAST OF THE ISLANDS. THIS
IS IN LINE WITH THE MORE AGGRESSIVE OUTLIERS OF THE GEFS MEMBERS.
CONCERNING THE POTENTIAL FOR TROPICAL CYCLONE WINDS IN THE MARIANAS,
THE DESI GRAND ENSEMBLE (CONSISTING OF 100 MODEL ENSEMBLE MEMBERS
FROM 3 DIFFERENT MODELS) SHOWS A VERY LOW PROBABILITY OF 5 PERCENT
CHANCE LATE WEEKEND OR EARLY NEXT WEEK. THIS POTENTIAL IS LARGELY
DRIVEN BY THE MORE AGGRESSIVE GEFS. THE SHARED HIGH-END
INTENSIFICATION THAT HAS BEEN EXHIBITED IN SOME GFS RUNS AND ALSO IN
RECENT RUNS OF THE HI-RES ARW-WRF SUGGEST SOME CONVECTIVE FEEDBACK
RESULTING IN AN ERRONEOUSLY SMALL TC AND UNREALISTICALLY-STRONG WINDS
FOR SUCH A SMALL SYSTEM. CURRENT FORECAST PHILOSOPHY STANDS ALONGSIDE
THE BULK OF ENSEMBLE DATA MAINTAINING A SLOW-DEVELOPING DISTURBANCE
NEAR OR JUST WEST OF THE MARIANAS AND A VERY WET PATTERN AHEAD.
EASTERN MICRONESIA UPDATE
LATEST SATELLITE IMAGERY SHOWS AN ACTIVE ITCZ NESTLED JUST SOUTH OF
POHNPEI AND KOSRAE SLOPING ENE TO CROSS THE DATE LINE BETWEEN 4N AND
8N. KOSRAE AND POHNPEI WILL BOTH SEE SHOWERS AND THUNDERSTORMS
INCREASING THROUGH THE MORNING WHILE MAJURO WILL STAY FAIRLY DRY WITH
MODERATE TO FRESH TRADES. SHOWERS WILL SPREAD NORTHWARD TO MAJURO
TONIGHT. REGIONAL SEAS WILL REMAIN A MIX OF TRADE AND SE SWELL. SEAS
WILL RANGE BETWEEN 4 AND 6 FEET FOR POHNPEI AND KOSRAE WHILE 6 TO 8
FT SEAS ARE EXPECTED IN THE SHORT TERM FOR MAJURO, ESPECIALLY WATERS
TO THE EAST FROM STRONG TRADES NEARER AND EAST OF THE DATE LINE.
MAJURO SEAS WILL DIMINISH 2 TO 3 FEET OVER THE NEXT COUPLE OF DAYS.
WESTERN MICRONESIA UPDATE
A WEAK SW MONSOON FLOW IS SLOWLY TAKING ROOT OVER YAP AND PALAU AS
THE DISTURBANCE, 97W, LIFTS NORTHWARD FROM ITS CURRENT POSITION NEAR
12N141E. SHOWERS AND THUNDERSTORMS WILL FLARE UP FROM TIME TO TIME
UNTIL THE MONSOON FLOW STRENGTHENS INTO THE 10-20 KT RANGE LATE WEEK
OR OVER THE WEEKEND. CHUUK REMAINS UNDER A SURFACE TROUGH SITTING
JUST EAST SERVING AS THE BOUNDARY BETWEEN EASTERLY TRADES TO THE EAST
AND WEST TO NORTHWEST FLOW TO THE WEST. CHUUK CONDITIONS WILL BE
QUITE CHANGEABLE THE NEXT FEW DAYS AS THE BROADER SYNOPTIC PATTERN
EVOLVES IN RESPONSE TO 97W AS IT LIFTS NORTHWARD THE NEXT FEW DAYS.
SEAS WILL REMAIN FAIRLY QUIET ACROSS THE REGION, A MIX OF EASTERLY
SWELL, AND AN EVENTUALLY-DEVELOPING SOUTHWEST SWELL FOR YAP AND
PALAU.
PREV DISCUSSION
/ISSUED ISSUED BY NATIONAL WEATHER SERVICE HONOLULU HI/
1001 PM UPDATE...
CONDITIONS ACROSS THE MARIANAS HAVE MARKEDLY QUIETED FOLLOWING THE
ACTIVE WEATHER OF EARLIER TODAY. THIS CONVECTIVE PAUSE IS ULTIMATELY
EXPECTED TO BE SHORT-LIVED SO HAVE ONLY INDICATED A DRIER TREND FOR
THE OVERNIGHT HOURS WITH SHOWERS AND THUNDERSTORMS ANTICIPATED TO
TREND UPWARD BY DAYBREAK THURSDAY.
MARIANAS SYNOPSIS... SATELLITE AND LOCAL SURFACE OBSERVATIONS ARE
SHOWING MOSTLY CLOUDY SKIES AND SCATTERED SHOWERS, WITH FRESH TO
STRONG GUSTS MEASURED WITH THE PASSING SHOWERS. NEARBY BUOYS ARE
INDICATING SEAS OF 4 TO 6 FEET.
DISCUSSION...
MOIST, CONVERGENT SOUTHEAST FLOW ALONG THE EASTERN EDGE OF THE
MONSOON TROUGH WILL PERSIST OVER THE MARIANAS. SCATTEROMETRY
CONTINUES TO SHOW STRONGER EAST TO SOUTHEAST FLOW PUSHING INTO THE
MARIANAS AND GUSTS TO 30+ MPH HAVE BEEN MEASURED THIS AFTERNOON
ACROSS THE MARIANAS WITH THE PASSING SHOWERS, ESPECIALLY AT TINIAN
AND SAIPAN. THE LATEST GFS AND MULTIPLE ENSEMBLE MEMBERS CONTINUE TO
DEVELOP A TC NEAR TO WEST OF THE MARIANAS AND MEANDER IT THROUGH THE
ISLANDS OVER WEEKEND, WHILE THE REST OF MODEL GUIDANCE IS MUCH LESS
ENTHUSIASTIC. WHILE THIS CASTS DOUBT ON THE POTENTIAL FOR WIND
HAZARDS, THE CONTINUING EXPECTATION IS FOR A VERY WET PERIOD
BEGINNING THURSDAY AND CONTINUING THROUGH THE WEEKEND, WITH NUMEROUS,
LOCALLY HEAVY SHOWERS AND ISOLATED THUNDERSTORMS. A FLOOD WATCH HAS
BEEN ISSUED FOR THIS TIME FRAME.
MARINE/SURF...
GENTLE TO MODERATE EAST TO SOUTHEAST WINDS FOR GUAM AND ROTA, AND
MODERATE TO FRESH WINDS FOR SAIPAN AND TINIAN WINDS WILL CONTINUE
THROUGH THURSDAY BEFORE DIMINISHING AND SHIFTING TO SOUTH OR
SOUTHWEST BY FRIDAY. PASSING SHOWERS WILL BRING WIND GUSTS OF 25 TO
30 KT TO SAIPAN AND TINIAN TONIGHT AND A SMALL CRAFT ADVISORY HAS
BEEN ISSUED. A WET PATTERN WILL PERSIST THE NEXT SEVERAL DAYS WITH
HEAVIER SHOWERS REDEVELOPING ACROSS THE REGION AROUND THURSDAY.
COMBINED SEAS OF 4 TO 6 FEET WILL BUILD A FOOT FOR GUAM AND ROTA
WATERS BY FRIDAY. ACROSS SAIPAN AND TINIAN, SEAS WILL BE HIGHER,
BUILDING TO BETWEEN 6 AND 8 FT THURSDAY, THEN FALLING ABOUT A FOOT BY
SUNDAY.
A MODERATE RISK OF RIP CURRENTS IS EXPECTED ALONG EAST FACING REEFS
OF THE MARIANAS FOR THE NEXT FEW DAYS. A MODERATE RISK IS ALSO
EXPECTED ALONG SOUTH FACING REEFS OF TINIAN AND SAIPAN.
HYDROLOGY...
A TROPICAL DISTURBANCE ANTICIPATED TO DEVELOP LATER THIS WEEK
IS EXPECTED TO BRING WIDESPREAD SHOWERS BEGINNING THURSDAY AND
CONTINUING THROUGH THE WEEKEND. RAINFALL AMOUNTS OF 5 TO 10 INCHES
ARE POSSIBLE, WITH LOCALLY HEAVY AMOUNTS POSSIBLE, ESPECIALLY ACROSS
GUAM AND ROTA. THIS RAINFALL IS EXPECTED TO PRODUCE MINOR FLOODING
ACROSS THE ISLANDS, WITH FLASH FLOODING POSSIBLE IN THE AREAS OF
HEAVIER RAINFALL.
RESIDENTS IN THE MARIANAS, ESPECIALLY THOSE IN FLOOD-PRONE AREAS,
SHOULD MONITOR THIS DEVELOPING WEATHER PATTERN CLOSELY AS WATCHES,
ADVISORIES AND WARNINGS COULD BE ISSUED LATE THIS WEEK.
TROPICAL SYSTEMS...
THE MOST RECENT RUNS OF THE GFS AND MULTIPLE MEMBERS OF THE GEFS
CONTINUE TO SHOW A TROPICAL CYCLONE DEVELOPING NEAR TO WEST OF THE
MARIANAS BY THE END OF THE WEEK AND MEANDER OVER OR NEAR THE ISLANDS
THROUGH THE WEEKEND. HOWEVER, THE REST OF MODEL GUIDANCE IS MUCH LESS
ENTHUSIASTIC, WITH GENERALLY A SLOWER DEVELOPING TROPICAL CYCLONE
WITH A MORE PROGRESSIVE TRACK TO THE WEST OF THE MARIANAS. WHILE THIS
CASTS DOUBT ON THE POTENTIAL FOR WIND HAZARDS, THE CONTINUING
EXPECTATION IS FOR A VERY WET PERIOD IN THE COMING DAYS, WITH GUSTY
WINDS NEAR SHOWERS AND THUNDERSTORMS. THE JOINT TYPHOON WARNING
CENTER (JTWC) AS UPGRADED THE POTENTIAL FORMATION AREA TO 60 PERCENT
FOR THE AREA OVER TO WEST OF GUAM AND THE MARIANAS.
EASTERN MICRONESIA...
THE INTER-TROPICAL CONVERGENCE ZONE (ITCZ) IS CURRENTLY SOUTH OF THE
FORECAST POINTS AND IS EXPECTED TO DRIFT NORTH AND WEST AND GRADUALLY
WEAKEN. EXPECT SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS THROUGH
THE END OF THE WORK WEEK, WITH PROBABILITY OF PRECIPITATION (POP) OF
30 TO 50 PERCENT. GUSTY WINDS WILL BE POSSIBLE AT TIMES NEAR HEAVIER
SHOWERS. THE ITCZ IS EXPECTED TO DISSIPATE, AND MAINLY ISOLATED TO
LOW-END SCATTERED SHOWERS ARE EXPECTED OVER THE WEEKEND. SHOWERS AND
THUNDERSTORM CHANCES INCREASE EARLY NEXT WEEK AS THE ITCZ LOOKS TO
RE-ESTABLISH ITSELF OVER EASTERN MICRONESIA.
COMBINED SEAS OF 4 TO 5 FEET LOOK TO CONTINUE FOR POHNPEI AND KOSRAE
THROUGH THE FORECAST PERIOD, AS EASTERLY SWELL SLOWLY DIMINISHES AND
A SOUTHEASTERLY SWELL STRENGTHENS. SLIGHTLY HIGHER TRADE SWELL IS
LIKELY FOR WELL-EXPOSED WATERS AROUND MAJURO, WITH SEAS UP TO 7 FEET.
WINDS WILL GENERALLY BE GENTLE TO MODERATE FOR THE PERIOD. WINDS LOOK
TO BECOME FRESH AT TIMES THROUGH THURSDAY FOR MAJURO. RESIDENTS
SHOULD BE AWARE OF THE LARGE TIDES THE NEXT SEVERAL DAYS DUE TO THE
NEW MOON PHASE DESPITE THE FACT NO NOTABLE SWELLS ARE CURRENTLY
FORECAST.
WESTERN MICRONESIA...
CHUUK SITS BETWEEN THE PRIMARY SYNOPTIC PLAYERS WITH THE MONSOON TO
THE WEST AND THE TRADES AND ITCZ TO THE EAST. SUBTLE FEATURES
EMBEDDED IN THE LOW-LEVEL FLOW ARE LIKELY TO TRIGGER BURSTS OF
SHOWERS AND THUNDERSTORMS AT TIMES THROUGH EARLY WEEKEND, THEN MAINLY
ISOLATED TRADE SHOWERS FOR LATE WEEKEND INTO EARLY NEXT WEEK. WINDS
WILL BE LIGHT TO GENTLE. SEAS FOR 3 TO 5 FEET LOOK TO DECREASE
SLIGHTLY BY THE WEEKEND AS THE TRADE SWELL SUBSIDES.
FURTHER WEST, MODEST MONSOON FLOW WILL RESULT IN LOW-END SCATTERED
SHOWERS AND ISOLATED THUNDERSTORMS FOR YAP AND PALAU THROUGH FRIDAY.
OVER THE WEEKEND, THE WEST TO SOUTHWEST MONSOON FLOW IS EXPECTED TO
INCREASE AS THE A POTENTIAL TROPICAL DISTURBANCE LOOKS TO DEVELOP
NEAR TO WEST OF THE MARIANAS, BUT LIKELY STAYING WELL NORTH OF YAP
AND PALAU, WITH SHOWERS AND THUNDERSTORM POTENTIAL TRENDING HIGHER.
SEAS WILL BE BENIGN FOR PALAU AND YAP DUE TO THE LACK OF ELEVATED
WINDS LOCALLY OR IN A DISTANT SENSE. SEAS OF 2 TO 3 FEET AND LIGHT TO
GENTLE WINDS LOOK TO CONTINUE THROUGH THE END OF THE WORK WEEK. WINDS
AND SEAS WILL TREND UP THROUGH THE WEEKEND INTO EARLY NEXT WEEK AS
THE MONSOON FLOW INCREASES. MODERATE TO FRESH WINDS AND SEAS OF 4 TO
6 FEET ARE EXPECTED BY SUNDAY. RESIDENTS SHOULD BE AWARE OF THE
LARGE TIDES THE NEXT SEVERAL DAYS DUE TO THE NEW MOON PHASE DESPITE
THE FACT NO NOTABLE SWELLS ARE CURRENTLY FORECAST.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS: AYDLETT
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