634
FXPQ50 PGUM 242047
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
ISSUED BY NATIONAL WEATHER SERVICE HONOLULU HI
647 AM CHST WED JUN 25 2025
MARIANAS UPDATE
SHOWERS AND A FEW THUNDERSTORMS HAVE PERSISTED FOR MUCH OF THE NIGHT
AROUND GUAM BUT ARE SHOWING SIGNS OF DIMINISHING WITH LIGHT RAINFALL
TAPERING OFF, AS NOTED IN MORNING RADAR IMAGERY AND WARMING CLOUD
TOPS IN IR IMAGERY. THIS DOWNTREND WILL ONLY BE SHORT-LIVED AS
MODERATE TO FRESH SOUTHEAST WINDS BRINGS MORE SHOWERS INTO THE
MARIANAS TODAY. OVERNIGHT SCATTEROMETRY SHOWED STRONGER SE FLOW LINED
UP TO THE EAST OF TINIAN AND SAIPAN WITH THE GFS SHOWING THESE WINDS
PASSING OVER THE TWO ISLANDS THROUGH TOMORROW. THESE STRONGER WINDS
WILL COME CLOSE TO THE 22 KT CRITERION FOR A SMALL CRAFT ADVISORY.
THURSDAY, WINDS ACROSS THE MARIANAS WILL WEAKEN AS A DISTURBANCE
LIFTS NORTHWARD INTO THE REGION. THE 12Z GFS CONTINUES TO DOUBLE DOWN
ON A DEVELOPING TC 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 IN THE
COMING DAYS. THE HYDROLOGIC OUTLOOK WAS UPDATED THIS MORNING AND A
FLOOD WATCH MAY BE WORTH CONSIDERING LATER, PENDING AN ASSESSMENT OF
THE UPCOMING MODEL CYCLE.
HAVE REFRESHED MARINE GRIDS TO BETTER PARALLEL THE STRONGER WINDS
SEEN ACROSS TINIAN AND SAIPAN. THIS HAS, IN TURN, RESULTED IN AN
EARLIER INCREASE IN SURF HEIGHTS AND THE ELEVATION OF THE RIP CURRENT
RISK TO MODERATE.
EASTERN MICRONESIA UPDATE
THE OVERNIGHT HOURS HAVE SEEN A SIGNIFICANT REDUCTION IN CONVECTION
ACROSS THE REGION, ESPECIALLY NEAR POHNPEI AND KOSRAE. WIDESPREAD
CLOUDINESS PERSISTS WITH DWINDLING SHOWERS. KOSRAE RECEIVED THE
LION'S SHARE OF RAINFALL, RECORDING 5IN IN 24HR IN THE 12Z SYNOPTIC
OBSERVATION. TO THE SOUTHEAST, ITCZ CONVECTION PERSISTS BETWEEN 3N
AND 5N TO THE EAST OF KOSRAE. WHILE THE ITCZ SHOWS LITTLE WESTWARD
MOTION THE NEXT COUPLE OF DAYS, IT'LL BE THE FOCUS OF A WESTWARD PUSH
OF CONVECTION STARTING FRIDAY. MARINE-WISE, SEAS WILL BE FAIRLY
BENIGN WITH THE EXCEPTION OF AN ARRIVING SOUTHEAST SWELL FOR KOSRAE
AND POHNPEI COURTESY A LONG FETCH OF 20-30 KT SOUTHEAST WINDS ALONG
THE SOLOMON ISLANDS. ADDITIONALLY, SEAS AND EAST SWELL WILL CONTINUE
BETWEEN 5 AND 7 FEET EAST OF THE MARSHALLS.
WESTERN MICRONESIA UPDATE
PALAU, YAP AND CHUUK HAVE ALL SEEN LOCAL WEATHER DRYING OUT FOR THE
SHORT TERM. REGIONAL WEATHER IS ENTERING A STATE OF FLUX DENOTED BY
WEAK FLOW SURROUNDING A DISORGANIZED DISTURBANCE TRYING TO SLOWLY
MATERIALIZE BETWEEN YAP AND CHUUK, SOUTH OF GUAM. OTHER THAN A FEW
SHORT-TERM ADJUSTMENTS TO ACCOUNT FOR RECENT SATELLITE TRENDS AND
TIMING, THE REMAINDER OF THE FORECAST STILL LOOKS GOOD WITH A WEAK SW
MONSOON FLOW DEVELOPING OVER YAP AND PALAU AROUND THURSDAY AND
STRENGTHENING LATE IN THE WEEKEND. CHUUK WILL SEE LIGHT SOUTH TO
SOUTHEAST WINDS AND DRIER CONDITIONS FOR LATE WEEK.
FOR YAP AND PALAU, THE WATERS WILL BE BENIGN WITH LIGHT SWELL. LATE
WEEK, SHORT-PERIOD WIND WAVES WILL BEGIN TO INCREASE IN RESPONSE TO
THE SW WINDS. A LONGER-PERIOD (8 SEC AND GREATER) SWELL SHOULD BEGIN
TO DEVELOP EARLY NEXT WEEK AS THE SW FLOW EXPANDS IN THE REGION.
PREV DISCUSSION
/ISSUED ISSUED BY NATIONAL WEATHER SERVICE HONOLULU HI/
MARIANAS SYNOPSIS...
THE MARIANAS SIT SANDWICHED BETWEEN A MERIDIONAL ORIENTED (SSW-TO-
NNE) MONSOON TROUGH TO THE WEST AND A STEADY TRADE-WIND REGIME TO THE
EAST. RADAR SHOWS CLUSTERS OF SHOWERS SWEEPING THROUGH GUAM, WITH
LESSER AMOUNTS ACROSS THE MAIN ISLANDS OF THE CNMI. RITIDIAN AND IPAN
PEG SEAS UNDER 4 FEET WITH A COMBO OF TRADE SWELL AND WIND WAVES.
TROPICAL STORM SEPAT (02W) REMAINS FAR TO THE NORTH NEAR 30N.
A WETTER PATTERN IS STILL IN STORE LATER THIS WEEK AS THE MONSOON
TROUGH RESETS AND PROTRUDES INTO THE MARIANAS. SEAS WILL REMAIN
FAIRLY BENIGN THIS WEEK, DOMINATED BY TRADE SWELL. EAST SWELL AND
SURF IS SLATED TO SPIKE BRIEFLY THE SECOND HALF OF WEDNESDAY INTO
THURSDAY MORNING AS A SLIGHT E-SE TRADE SURGE MOVES THROUGH.
DISCUSSION...
TRANSITION TO WET SEASON CONTINUES WITH MOISTURE-LADEN TRADES (2.22"
PWAT) INCITING A FEW ROUNDS OF THUNDERSTORMS JUST WEST OF GUAM THIS
AFTERNOON. A SIMILAR THEME MAY BE IN STORE THE NEXT DAY OR SO AS A
COUPLE OF WEAK TROUGHS SLIDE THROUGH, RESULTING IN AN UPTICK IN BOTH
WINDS AND SHOWERS TONIGHT, AND PERHAPS AGAIN ~WEDNESDAY NIGHT.
WINDS THEN BEGIN TO SHIFT SOUTHERLY OR EVEN SOUTHWESTERLY ON
FRIDAY/SATURDAY AS THE MERIDIONALLY-ORIENTED MONSOON TROUGH
REORGANIZES AND EXTENDS EASTWARD INTO THE MARIANAS. MODEL GUIDANCE
IS STILL MIXED ON EXACTLY WHAT THIS REORGANIZATION ULTIMATELY LOOKS
LIKE. WHILE DEVELOPMENT OF A TROPICAL CYCLONE IN THE VICINITY OF THE
MARIANAS IS POSSIBLE, THE ONGOING DIFFERENCES IN GUIDANCE BETWEEN
VARIOUS DETERMINISTIC GLOBAL MODELS, AND SUBSTANTIAL MODEL SPREAD IN
ENSEMBLE MEMBER DATA LEADS TO CONTINUED LOW CONFIDENCE AT THIS TIME.
HEAVY RAIN IS THE BIGGEST CLEAR CONCERN AT THIS TIME FOR GUAM UP TO
SAIPAN.
MARINE...
E TO SE TRADES BUILD A NOTCH TOMORROW AND WEDNESDAY NIGHT, ALLOWING
SEAS (AND SHORT-PERIOD SURF) TO RISE ENOUGH TO TRIGGER A MODERATE RIP
RISK FOR EAST FACING REEFS. WINDS AND SEAS THEN RELAX A NOTCH ON
THURSDAY AS THE PATTERN EASES AND THE TROUGH MOVES IN. NORMALLY CALM
AND SMOOTH WEST FACING COASTS MAY SEE A BUMP IN CHOP THIS WEEKEND AS
THE MONSOON TRIES TO EDGE IN FROM THE WEST.
HYDROLOGY...
THERE IS A POTENTIAL FOR GUAM AND THE CNMI TO SEE RAINFALL AMOUNTS
VARYING FROM 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.
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...
WITH TROPICAL STORM SEPAT FAR TO THE NORTH, OUR ATTENTION TURNS TO
THE REORGANIZATION OF THE MONSOON TROUGH IN THE VICINITY OF THE
MARIANAS.
THE GFS, IN ADDITION TO BEING THE MOST AGGRESSIVE IN RATE
OF DEVELOPMENT, HAS STRONGLY FAVORED THE EASTERN SIDE OF THE ISLANDS
FOR GENESIS. HOWEVER, RECENT RUNS OF THE GEFS HAVE SHOWN SOME
WESTWARD SPREAD IN THE POTENTIAL DEVELOPMENT AREA. WHILE ITS STILL
TOO SOON TO STATE ANY CERTAINTY, ONE THING TO WATCH FOR IN THE NEXT
24-36 HOURS WILL BE FOR A PUSH OF WESTERLY WINDS FROM THE YAP AREA TO
THE SOUTHEAST OF GUAM BY TONIGHT OR EARLY WED. THAT WOULD LEND MORE
SUPPORT TO GFS GUIDANCE. HOWEVER THINGS SHAPE UP LATE THIS WEEK, THE
COMMON OUTCOME AMONG MODELS IS FOR A MUCH WETTER PATTERN IN THE
MARIANAS.
EASTERN MICRONESIA...
AN ACTIVE PORTION OF THE INTERTROPICAL CONVERGENCE ZONE (ITCZ)
IS PRODUCING SHOWERS AND THUNDERSTORMS OVER KOSRAE, POHNPEI AND THE
RALIK CHAIN OF THE MARSHALLS. WAVES OF SHOWERY WEATHER ARE LIKELY TO
CONTINUE FOR POHNPEI AND KOSRAE THE NEXT SEVERAL DAYS, WITH GUSTY
CONDITIONS AT TIMES. MAJURO STILL LOOKS TO SIT ON THE EASTERN EDGE OF
THIS REGIME WITH ISOLATED TO LOW-END SCATTERED SHOWERS EXPECTED
THROUGH AT LEAST MIDWEEK.
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. 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 THE NEXT COUPLE OF DAYS. MODELS
HINT AT A MORE STABLE, WEAK TRADE FLOW LATE IN THE WEEK. SEAS LOWER
THROUGH THE REMAINDER OF THE WEEK AS LOCAL WINDS AND UPSTREAM SOURCES
TAPER OFF.
LITTLE CHANGES WERE MADE TO THE FORECASTS FARTHER WEST WITH PALAU AND
YAP LIKELY TO SEE A MONSOONAL PATTERN SLOWLY RE-ESTABLISHING. WSO
YAP REPORTED OVER 2 INCHES IN A SHORT TIME FRAME THIS AFTERNOON,
SUPPORTING PREVIOUS FORECASTS OF LOCALLY HEAVY RAINS. A QUIETER
PATTERN LOOKS TO SET UP FOR PALAU AND YAP WEDNESDAY THROUGH THE
LATTER HALF OF THE WEEK AS THE MONSOON TROUGH SHIFTS NORTH. HOWEVER,
THE WEATHER PATTERN AND POSITIONING OF THE MONSOON TROUGH WILL
PARTIALLY DEPEND ON THE DEVELOPMENT AND MOVEMENT OF A YET-TO-FORM
TROPICAL DISTURBANCE, WHICH STILL LOOKS TO ASSEMBLE NEAR THE MARIANAS
IN THE LATTER HALF OF THE WEEK. SEAS WILL BE BENIGN FOR PALAU AND
YAP DUE TO THE LACK OF ELEVATED WINDS LOCALLY OR IN A DISTANT SENSE.
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 UPDATE: W. AYDLETT
EAST/WEST MICRONESIA UPDATE: W. AYDLETT
MARIANAS: BUKUNT/W. AYDLETT
TROPICAL: BUKUNT/W. AYDLETT
EAST MICRONESIA: BUKUNT
WEST MICRONESIA: BUKUNT
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