553
FXPQ50 PGUM 151921
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
521 AM CHST TUE SEP 16 2025
MARIANAS UPDATE
THE MARIANAS FORECASTS LOOKED GOOD WITH SCATTERED SHOWERS COMING UP
FOR TODAY. IT MIGHT TAKE THEM A BIT TO GET GOING, BUT THEY PROBABLY
WILL. PERHAPS A BRIEF DELAY COULD GET EVERYONE'S MORNING COMMUTE OUT
OF THE WAY THOUGH. COMBINED SEAS OF 3 TO 4 FEET COULD DIMINISH EVEN
FURTHER AROUND WEDNESDAY NIGHT. THE BUOYS REVEAL THAT THE CURRENT
COMBINED SEA HEIGHTS ARE INDEED 3 TO 4 FEET.
EASTERN MICRONESIA UPDATE
EASTERN MICRONESIA IS CURRENTLY BEING AFFECTED MAINLY BY A DEVELOPING
ITCZ AND SURFACE TROUGHS MOVING THROUGH IT. THIS IS CAUSING AREAS OF
SCATTERED SHOWERS MOVING THROUGH EASTERN MICRONESIA, AT A FAST ENOUGH
RATE TO ALL BLEND TOGETHER FOR THE NEXT DAY OR TWO, SO ALL ISLANDS
HAVE A SIMILAR FORECAST.
FOR ALL OF EASTERN MICRONESIA, SEA CONDITIONS ARE BENIGN. SWELLS CAP
OUT AT 5 FEET, AND WINDS MAX OUT AT 10 KNOTS. NO SIGNIFICANT CHANGE
IS EXPECTED THROUGH THE WEEKEND.
WESTERN MICRONESIA UPDATE
WESTERN MICRONESIA IS CURRENTLY BEING AFFECTED BY TRADE-WIND TROUGHS
THAT HAVE BEEN MOVING IN FROM EASTERN MICRONESIA. TIMING THESE
TROUGHS IS THE MAIN FORECAST PROBLEM. YAP AND KOROR LOOK FAIRLY DRY
THROUGH TONIGHT. THEY THEN LOOK WETTER WEDNESDAY AND THURSDAY, THEN
DRIER AGAIN FRIDAY AND SATURDAY. MEANWHILE CHUUK LOOKS WET MOST OF
THE WEEK, BUT IS DRIER WEDNESDAY.
FOR ALL OF WESTERN MICRONESIA, SEA CONDITIONS ARE BENIGN. WINDS CAP
OUT AT 15 KNOTS FOR CHUUK DURING THE FORECAST PERIOD (THROUGH THIS
WEEKEND). SEAS WILL MAX OUT AT 4 FEET. FOR YAP AND KOROR PALAU, WINDS
WILL MAINLY TOP OUT AT 5 KNOTS, THE POSSIBLE EXCEPTION BEING SATURDAY
WHICH MAY HAVE 10 KNOTS AT KOROR AS THE NEXT MONSOON PUSH BEGINS TO
SURGE OUT OF ASIA. SEAS WILL MAX OUT AT 4 FEET FOR YAP AND 3 FEET FOR
KOROR. WE MAY HAVE TO WATCH THAT MONSOON SURGE NEXT WEEK THOUGH.
PREV DISCUSSION
/ISSUED 615 PM CHST MON SEP 15 2025/
MARIANAS SYNOPSIS...
SATELLITE IMAGERY SHOWS CLOUDY SKIES, WHILE RADAR IMAGERY SHOWER
HIGH-END SCATTERED SHOWERS WITHIN THE MARIANAS AND ADJACENT COASTAL
WATERS. ALTIMETRY SHOWS COMBINED SEAS OF 3 TO 4 FEET AND RITIDIAN AND
IPAN BUOYS SHOW AROUND 3 TO 4 FEET.
DISCUSSION...
THERE IS A LOT OF UNCERTAINTY IN THE IMMEDIATE FORECAST. THERE IS A
QUASI-STATIONARY TROUGH (QST) TO THE EAST OF THE MARIANA'S COASTAL
WATERS AND A BRANCH OF THE TROPICAL UPPER-TROPOSPHERIC TROUGH (TUTT)
STILL REMAINS TO THE WEST. THE QST IS VERY ACTIVE AND PRODUCING
MULTIPLE OUTFLOW BOUNDERS THAT ARE PASSING OVER THE GUAM AND CNMI.
THESE BOUNDARY ARE PRODUCING FAST-MOVING AND SHORT-LIVED SHOWERS
THAT HAVE A POTENTIAL TO DROP A LOT OF RAIN, CREATING THE CONCERN
FOR URBAN AND SMALL STREAM FLOODING. THIS CAN VERY WELL BE ONE OF
THOSE CASES WHERE IT LOOKS LIKE WE SHOULD BE IN NUMEROUS SHOWERS
CATEGORY OR IT COULD FALL APART LEAVING THE MARIANAS WITH JUST
ISOLATED SHOWERS. GIVEN THE VAST RANGE A HIGH-END, LOCALLY HEAVY
SCATTERED SHOWERS WAS SEEN AS A GOOD MIDDLE GROUND APPROACH. ONCE
THIS TROUGH MOVES THROUGH THE REGION AN OVERALL DRIER PATTERN IS
EXPECTED FOR LATTER HALF OF THE WEEK.
MARINE...
BENIGN MARINE CONDITIONS CONTINUE THROUGH THE MIDDLE OF THE WEEK
WITH COMBINED SEAS AROUND 3 TO 4 FEET FROM AN EASTERLY TRADE SWELL.
SEAS MAY FALL BY A FOOT NEAR THE END OF THE WORK WEEK. WINDS ARE
EXPECTED TO LIGHT TO GENTLE THROUGH TUESDAY, THEN COULD BRIEFLY PICK
UP TO LIGHT TO MODERATE FOR TUESDAY NIGHT AND WEDNESDAY BEFORE
FALLING AGAIN FOR THURSDAY AND FRIDAY. THERE WILL BE A MODERATE RISK
OF RIP CURRENTS ALONG EAST FACING REEFS OF THE MARIANAS THROUGH
TUESDAY, THEN IT COULD BECOME LOW ALONG ALL REEFS. THERE WILL ALSO BE
A MODERATE RISK OF LIGHTNING ALL THIS WEEK.
EASTERN MICRONESIA...
SCATTERED TO LOCALLY NUMEROUS SHOWERS CONTINUE TO DEVELOP ACROSS
POHNPEI, ESPECIALLY WESTERN SECTIONS, WITH SCATTERED SHOWERS ALSO
EXTENDING FROM POHNPEI EAST-SOUTHEAST TO NEAR KOSRAE. THIS AREA IS IN
A REGION OF MODEST CONVERGENCE, BUT IS LOCATED JUST AHEAD OF AN
UPPER-LEVEL TROUGH AXIS. THIS HAS ALLOWED MOISTURE TO "POOL" JUST
AHEAD OF THIS UPPER FEATURE WITH PWATS (PRECIPITABLE WATER VALUES)
IN EXCESS OF 2.5 INCHES ACROSS MOST LOCATIONS. MOISTURE IS SOMEWHAT
LOWER TO THE SOUTH AND EAST TOWARDS KOSRAE AND MAJURO, BUT THERE'S
NOT A BIG DIFFERENCE. MOISTURE ALSO IS ATTEMPTING TO INCREASE ALONG A
LEAD SURFACE TROUGH THAT STRETCHES FROM SOUTH OF MAJURO TO IT'S
EAST-NORTHEAST. THIS LEAD TROUGH WILL HEAD WEST THIS WEEK, WITH YET
ANOTHER TROUGH CROSSING THE DATE LINE BY THURSDAY.
THEN, THE ITCZ (INTER-TROPICAL CONVERGENCE ZONE) LOOKS TO BECOME
BETTER ESTABLISHED, STRENGTHENING BY THE WEEKEND WITH INCREASING
CONVECTIVE COVERAGE AND VIGOR, LIFTING NORTH TO NEAR 10N TOWARDS THE
START OF NEXT WEEK. GIVEN THE ACTIVE PERIOD, SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS LOOK TO PERSIST THROUGH MUCH OF THE WEEK,
BECOMING LOCALLY NUMEROUS AND HEAVY FROM TIME TO TIME. SEAS LOOK TO
HOLD IN THE 3 TO 5 FOOT RANGE THIS WEEK, AS LIGHT TO GENTLE WINDS
PREVAIL.
WESTERN MICRONESIA...
THE PRIMARY MONSOON TROUGH AXIS REMAINS ACROSS THE PHILIPPINES AND
NORTHWEST OF PALAU AND YAP, KEEPING BOTH LOCATIONS EAST OF THE WET
MONSOON PATTERN EXTENDING FROM SOUTHEAST ASIA. SATELLITE IMAGERY
SHOWS FAIRLY DRY WEATHER ACROSS PALAU AND YAP, WHILE SOME ISOLATED
SHOWERS ARE SEEN TO THE NORTH OF YAP PROPER AND ACROSS THE OUTER
ISLANDS OF THE STATE. OVER BY CHUUK, AN APPROACHING TRADE-WIND
TROUGH IS GENERATING SCATTERED SHOWERS ACROSS CHUUK LAGOON, AND IS
ASSOCIATED WITH A POCKET OF INCREASED MOISTURE, ACCORDING TO THE
BLENDED TOTAL PRECIPITABLE WATER (TPW) SATELLITE LOOP. THIS POCKET
OF MOISTURE WILL INCREASE THE POTENTIAL FOR SHOWERS TO BECOME
NUMEROUS OVERNIGHT, WITH SOME GUSTS AROUND 20 KT.
MODELS ARE NOW SUPPORTING AN OVERALL DRIER PATTERN AT YAP AND PALAU
FOR TUESDAY AS THE DISTURBANCE NEAR THE MARIANAS IS EXPECTED TO PASS
FURTHER NORTH OF PALAU AND YAP THEN PREVIOUSLY EXPECTED, POTENTIALLY
CONNECTING WITH THE MONSOON TROUGH IN THE PHILIPPINE SEA. NOW IT IS
POSSIBLE THAT AS THE DISTURBANCE CONNECTS WITH THE MONSOON TROUGH,
THIS WILL CREATE CONDITIONS FAVORABLE FOR A CIRCULATION TO DEVELOP
AND POTENTIALLY DEVELOP TO A TROPICAL CYCLONE LATER THIS WEEK.
DEPENDING ON FACTORS SUCH AS LOCATION AND HOW QUICKLY THIS
CIRCULATION DEVELOPS AND MOVES WEST-NORTHWEST, PALAU AND YAP COULD
ENTER A WET MONSOONAL PATTERN AS SOUTH TO SOUTHWEST WINDS SLOWLY
BUILD OVER FAR WESTERN MICRONESIA, OR BOTH LOCATIONS REMAIN ON THE
EASTERN EDGE OF THE MONSOON PATTERN, KEEPING WINDS LIGHT AND SHOWERS
MORE EPISODIC IN NATURE, LEADING TO A MIXTURE OF SUNSHINE AND
SCATTERED SHOWERS. RIGHT NOW MODEL GUIDANCE IS SUPPORTING THE LATTER
PATTERN, BUT THIS IS A TREND TO MONITOR OVER THE NEXT FEW DAYS. FOR
CHUUK, THE OVERALL PATTERN IS NOT EXPECTED TO CHANGE MUCH. SHOWERS
AND THUNDERSTORMS HAVE MOSTLY BEEN ASSOCIATED WITH PASSING TRADE-WIND
TROUGHS, ALLOWING FOR LULLS IN SHOWERS TO OCCUR BEFORE INCREASING
AGAIN AS THE NEXT TROUGH MOVES IN. EVEN AS THE ITCZ SLOWLY DEVELOPS
ACROSS THE REGION, THIS PATTERN IS EXPECTED TO CONTINUE THROUGH THE
REST OF THE WEEK.
SEAS REMAIN AROUND 2 FEET AT PALAU, ACCORDING TO ALTIMETRY DATA,
WHILE SEAS ARE 3 TO 4 FEET ACROSS MOST OF YAP AND CHUUK STATES. THE
PRIMARY SWELL ACROSS REGION REMAINS EASTERLY AND WAVE MODELS
CONTINUE TO SUPPORT THIS PATTERN THROUGH THE REST OF THE WEEK.
HOWEVER, THERE IS THE POTENTIAL THAT A SOUTHWEST SWELL COULD DEVELOP
IN FAR WESTERN MICRONESIA LATER THIS WEEK IF THE MONSOON PATTERN NEAR
THE PHILIPPINES STRENGTHENS, BUT THIS WILL LARGELY DEPEND ON HOW THE
DISTURBANCE NEAR THE MARIANAS WILL INTERACT WITH THE MONSOON TROUGH
AS IT MOVES WESTWARD INTO THE PHILIPPINE SEA.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS/EAST AND WEST MICRONESIA UPDATE: STANKO
MARIANAS: BOWSHER
EAST MICRONESIA: DOLL
WEST MICRONESIA: SCHANK
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