538
FXPQ50 PGUM 221959
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
559 AM CHST WED JUL 23 2025
MARIANAS UPDATE
AS TROPICAL DISTURBANCE INVEST 98W CONTINUES TO CLOSE ITS GAP WITH
THE MARIANAS, CONVECTIVE ACTIVITY HAS SPIKED AHEAD OF THE STORM
CENTER, ALLOWING HEAVIER SHOWERS AND SOME THUNDERSTORMS TO BUILD IN.
AS SUCH, HIGHER POPS WERE BROUGHT IN EARLIER TO TODAY AND TONIGHT,
INTRODUCING FLASH FLOODING RISKS AS WELL. GENTLE TO MODERATE EAST TO
SOUTHEAST WINDS LOOK TO CONTINUE THROUGH TONIGHT, BEFORE
STRENGTHENING THEREAFTER AS 98W APPROACHES. THERE COULD BE TIMES WHEN
WINDS WILL BE LIGHT AND VARIABLE OVER GUAM AS A BRIEF COL SETTLES
OVERHEAD, WHICH MAY INDUCE ISLAND CONVECTION AHEAD OF 98W'S APPROACH,
BUT UNCERTAINTY REMAINS HIGH FOR THIS DEVELOPMENT AT THIS TIME.
TROPICAL SYSTEMS UPDATE
TROPICAL DEPRESSION (TD) 10W HAS QUICKLY MOVED NORTH-NORTHWEST AND
IS NOW LOCATED NEAR 20.1N, 131.3E. THE WIND FIELD IS STILL
ASYMMETRICAL WITH MAX SUSTAINED WINDS OF 30 MPH WITHIN THE EASTERN
PERIPHERY. 10W IS FORECAST TO INTENSIFY FURTHER POSSIBLY BECOMING A
TROPICAL STORM TONIGHT, WHILE MOVING WEST OF THE AREA OF
RESPONSIBILITY (AOR). FOR MORE INFORMATION ON TD 10W, PLEASE REFER TO
BULLETINS ISSUED BY JTWC UNDER WMO HEADERS WTPN32 PGTW AND FROM THE
NATIONAL WEATHER SERVICE UNDER WMO HEADER WTPQ32 PGUM.
A LOOSELY ORGANIZED TROPICAL DISTURBANCE, KNOWN AS JTWC'S INVEST 98W,
IS CENTERED NEAR 11N149E EAST-SOUTHEAST OF GUAM WATERS. MODERATE TO
OCCASIONALLY DEEP CONVECTION IS ASSOCIATED WITH THIS FEATURE,
PRODUCING NUMEROUS TO WIDESPREAD HEAVY SHOWERS WITHIN THE EASTERN AND
SOUTHERN PERIPHERIES, AS A LARGE CONVERGENCE ZONE SPANNING FROM
EASTERN YAP STATE TO CHUUK STATE CONTINUES TO FUNNEL MOISTURE INTO
THE FEATURE. EVEN SO, THE SYSTEM STRUGGLES TO CONSOLIDATE,
OCCASIONALLY DETACHING FROM THE SURFACE AS AN OPEN WAVE TO THEN
RE-SURFACE CLOSER TO THE MARIANAS IN THE LATTER HOURS OF THE NIGHT.
98W WILL CONTINUE TO CONSOLIDATE IN THE GENERAL VICINITY, SLOWLY
MOVING NORTH-NORTHWEST ACROSS THE MARIANAS OVERNIGHT INTO THURSDAY,
POTENTIALLY BECOMING A TROPICAL DEPRESSION BY THEN. AT THIS TIME,
MODEL GUIDANCE CONTINUES TO STRUGGLE WITH THE DEVELOPMENT OF SAID
SYSTEM, SO STORM INTENSITY WILL CONTINUE TO BE HIGHLY UNCERTAIN UNTIL
A MORE STABLE CIRCULATION IS OBSERVED.
FOR MORE INFORMATION REGARDING INVEST 98W, SEE BULLETINS ISSUED BY
JTWC UNDER WMO HEADER ABPW10 PGTW.
HYDROLOGY UPDATE
A GENERALLY UNSETTLED PATTERN CAUSED BY LOCALIZED CONVERGENCE AND
NEARBY TROPICAL DISTURBANCE INVEST 98W SOUTHEAST OF THE MARIANAS, IS
LIKELY TO PRODUCE HEAVY SHOWERS AND THUNDERSTORMS TO BRING LOCALIZED
FLASH FLOODING RISK TO THE REGION FOR AT LEAST THE NEXT SEVERAL DAYS.
AS SUCH, A FLASH FLOOD WATCH HAS BEEN ISSUED, VALID THROUGH FRIDAY
AFTERNOON. AS 98W CONTINUES TO CONSOLIDATE AND MOVE CLOSER TO THE
MARIANAS, SHOWER AND THUNDERSTORM COVERAGE WILL CONTINUE TO INCREASE,
WITH RAINFALL AMOUNTS OF 6 TO 10 INCHES POSSIBLE THROUGH LATE FRIDAY
AFTERNOON. DUE TO THE LACKLUSTER NATURE IN 98W'S DEVELOPMENT CYCLE,
HIGH UNCERTAINTY REMAINS FOR PROBABLE RAINFALL AMOUNTS, BUT WILL
BECOME MORE CLEAR AS THE SYSTEM COMES CLOSER TO THE MARIANAS.
EASTERN MICRONESIA UPDATE
INFRARED SATELLITE IMAGERY SHOW PATCHY SHOWERS OVER ACROSS THE REGION
AND SHOWERS TRENDING DOWN OVER THE NORTHERN MARSHALLS FROM A
WEAKENING CONVERGENCE ZONE. SHOWERS NEAR KOSRAE DID INCREASE TO
SCATTERED OVERNIGHT, BUT KEPT THE TREND FOR ISOLATED LIGHT SHOWERS
TODAY. NO MAJOR CHANGES FOR POHNPEI AND MAJURO, WHERE FAIRLY QUIET
CONDITIONS ARE EXPECTED AS WELL TODAY.
WESTERN MICRONESIA UPDATE
A TROPICAL DISTURBANCE, REFERRED TO AS INVEST 98W, REMAINS POORLY
ORGANIZED, BUT IS SLOWLY DEVELOPING NORTHWEST OF CHUUK, NEAR
11N149E. THE BULK OF DEEPER CONVECTION ASSOCIATED WITH 98W REMAINS TO
THE NORTH AND EAST OF CHUUK AS THIS DISTURBANCE CONTINUES TO MOVE
NORTHWEST TOWARDS THE MARIANAS. HOWEVER, SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS ARE STILL EXPECTED THROUGH TONIGHT, WITH
UPPER-LEVEL CLOUD LAYERS TRAVELING FROM UNSETTLED WEATHER TO THE
NORTH.
YAP AND PALAU REMAIN UNDER A REGION OF RELATIVELY QUIET CONDITIONS,
BETWEEN THE TWO MAJOR WEATHER PATTERNS IN THE REGION, THE MONSOON
PATTERN TO THE NORTHWEST, FEEDING INTO TROPICAL DEPRESSION 10W AND
98W TO THE NORTHEAST. NO MAJOR CHANGES IN THE FORECAST. KEPT
SCATTERED SHOWERS IN FOR PALAU DUE TO AN AREA OF WEAK CONVERGENCE IN
THE SOUTHWESTERLY FLOW. SCATTERED SHOWERS MAY BUILD IN FOR YAP
THIS AFTERNOON AND TONIGHT, DEPENDING ON THE DEVELOPMENT OF 98W AND
CONVERGENT FLOW OVER YAP STATE.
PREV DISCUSSION
/ISSUED 1122 PM CHST TUE JUL 22 2025/
UPDATE...
THE TROPICAL DISTURBANCE, FORMERLY KNOWN AS JOINT TYPHOON WARNING
CENTER'S (JTWC) INVEST 97W, HAS BEEN UPGRADED TO A TROPICAL
DEPRESSION (TD) 10W. 10W HAS CONTINUED TO MOVE NORTHWEST AND IS NOW
LOCATED NEAR 18.8N, 132.3E. ALTHOUGH SLOW TO DEVELOP AND AN
ASYMMETRICAL WIND FIELD REMAINS, 10W HAS BECOME BETTER ORGANIZED
TODAY WITH A LOW TO MID LEVEL CIRCULATION PRESENT AND CONVECTION
INCREASING AROUND ITS EASTERN AND NORTHERN FLANKS. MAX SUSTAINED
WINDS ARE 25 KT AROUND THE EASTERN PERIPHERY. 10W LOOKS TO REMAIN IN
OUR AREA OF RESPONSIBILITY (AOR) FOR THE NEXT DAY OR TWO AS IT MOVES
NORTHWEST OUT OF OUR REGION.
FOR MORE INFORMATION ON TD 10W, PLEASE REFER TO BULLETINS ISSUED BY
JTWC UNDER WMO HEADERS WTPN32 PGTW AND FROM THE NATIONAL WEATHER
SERVICE UNDER WMO HEADER WTPQ32 PGUM.
PREV DISCUSSION... /ISSUED 635 PM CHST TUE JUL 22 2025/
MARIANAS SYNOPSIS...
SATELLITE AND RADAR OBSERVATIONS SHOW MOSTLY CLOUDY SKIES AND
SCATTERED SHOWERS ACROSS THE MARIANAS. RITIDIAN AND IPAN BUOYS SHOW
COMBINED SEAS BETWEEN 3 AND 5 FEET, WITH SEA HEIGHTS PEAKING NEAR 7
FEET EAST OF THE MARIANAS ACCORDING TO ALTIMETRY AND SOFAR DRIFTING
BUOY DATA.
DISCUSSION...
THERE WAS LITTLE CHANGE TO THE FORECAST IN THE SHORT TERM. THE
MARIANAS ARE LOCATED BETWEEN A BROAD DEVELOPING MONSOON DISTURBANCE,
JOINT TYPHOON WARNING CENTER'S (JTWC) INVEST 97W OVER THE PHILIPPINE
SEA, AND A DISORGANIZED TROPICAL DISTURBANCE TO THE NORTH OF CHUUK,
JTWC'S INVEST 98W. CONVERGENCE BETWEEN THE EAST TO SOUTHEAST TRADE
WINDS AND THE MONSOON PATTERN TO THE WEST WILL MAINTAIN SCATTERED
SHOWERS AND ISOLATED THUNDERSTORMS IN THE AREA THROUGH TOMORROW, AS
98W MOVES NORTH-NORTHWEST AND BEGINS TO CONSOLIDATE SOUTHEAST OF THE
ISLANDS. CURRENTLY, INVEST 98W REMAINS HIGHLY DISORGANIZED WITH
ALMOST NO SIGN OF A SURFACE CIRCULATION SEEN IN THE ASCAT PASS FROM
THIS MORNING. HOWEVER, THERE IS A DISTINCT MID-LEVEL CIRCULATION
SEEN ON VISIBLE SATELLITE CO-LOCATED WITH A MAXIMUM IN 850-700 MB-
LEVEL VORTICITY, CENTERED NEAR 10.7N 151.1E. THE MAIN WEATHER
CONCERN REMAINS IN THE POSSIBILITY FOR HEAVY RAIN OVER THE ISLANDS
AS 98W MOVES INTO THE AREA, FROM THURSDAY THROUGH THE WEEKEND, WITH
LESS CERTAINTY AS TO THE STRENGTH OF 98W. PLEASE SEE THE HYDROLOGY
AND TROPICAL SYSTEMS SECTIONS FOR MORE DETAILS.
MARINE...
GENTLE TO MODERATE EAST TO SOUTHEAST WINDS LOOK TO PERSIST THROUGH
WEDNESDAY. WINDS THEN LOOK TO STRENGTHEN THROUGH THE LATTER HALF OF
THE WEEK AS TROPICAL DISTURBANCE INVEST 98W APPROACHES THE REGION.
COMBINED SEAS OF 4 TO 6 FEET, COMPRISED MAINLY OF A PRIMARY EASTERLY
SWELL AND SECONDARY WESTERLY SWELL, ARE EXPECTED TO INCREASE THROUGH
THE SECOND HALF OF THE WEEK AS 98W DEVELOPS NEAR THE REGION, LEADING
TO HAZARDOUS SEA CONDITIONS. UNCERTAINTY REMAINS HIGH AT THIS TIME
REGARDING WINDS AND SEA STATE DEVELOPMENT TOWARD THE WEEKEND, AS
CONDITIONS MAY CHANGE SIGNIFICANTLY OVER THE COMING DAYS.
HYDROLOGY...
A TROPICAL DISTURBANCE, JOINT TYPHOON WARNING CENTER'S INVEST 98W,
IS CENTERED SOUTHEAST OF GUAM NEAR 10.7N 151.1E. THIS SYSTEM REMAINS
HIGHLY DISORGANIZED, BUT IS EXPECTED TO MOVE TOWARD THE MARIANAS OVER
THE NEXT DAY OR TWO AS IT CONSOLIDATES. 98W IS EXPECTED TO BRING
HEAVY RAINFALL TO THE MARIANAS THURSDAY EVENING THROUGH SUNDAY WITH
AN EXPECTED PEAK ON FRIDAY THROUGH SATURDAY. MODELS DIFFER ON
LOCATION AND STRENGTH OF 98W AS IT MOVES NEAR THE MARIANAS, HOWEVER,
THERE IS STRONG AGREEMENT THAT HEAVY RAINFALL IS LIKELY AS EARLY AS
THURSDAY, CONTINUING THROUGH THE WEEKEND.
THERE WAS STRONG CONSIDERATION TO ISSUE A FLASH FLOOD WATCH THIS
FORECAST CYCLE, CONSIDERING THE POSSIBILITY OF FLASH FLOODING WITHIN
THE NEXT 48 HOURS, BUT THE TIMING AND PROBABILITY REMAINS UNCERTAIN.
LATEST ENSEMBLE MODEL GUIDANCE SHOWS BETTER AGREEMENT FOR HEAVY RAIN
POTENTIAL FROM FRIDAY THROUGH SATURDAY, AND GIVEN THE CURRENT POOR
ORGANIZATION OF 98W AND WIDE RANGE OF POSSIBLE MODEL OUTCOMES, THERE
IS SUFFICIENT UNCERTAINTY IN THE 48-HOUR FLASH FLOODING POTENTIAL TO
MAINTAIN THE HYDROLOGIC OUTLOOK THIS FORECAST CYCLE. THERE IS LITTLE
UNCERTAINTY AS TO WHETHER HEAVY RAIN WILL OCCUR OVER THE MARIANAS,
WITH MORE UNCERTAINTY AS TO WHERE AND WHEN.
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. MODEL ENSEMBLE GUIDANCE INDICATES A 10 PERCENT PROBABILITY
SOME LOCATIONS COULD SEE ACCUMULATIONS AS HIGH AS 10 TO 15 INCHES BY
12Z SUNDAY. THIS RAINFALL IS EXPECTED TO PRODUCE MINOR FLOODING
ACROSS THE ISLANDS, WITH FLASH FLOODING POSSIBLE IN AREAS OF HEAVIER
RAINFALL, ESPECIALLY OVER SOUTHERN GUAM.
RESIDENTS ON THE MARIANAS NEED TO CLOSELY MONITOR THIS DEVELOPING
SITUATION AS WATCHES, ADVISORIES AND WARNINGS COULD BE ISSUED LATER.
IF LIVING NEAR STREAMS AND RIVERS, PREPARE TO MOVE ITEMS AWAY FROM
STREAM AND RIVER BANKS. MAKE SURE STORM DRAINS NEARBY ARE NOT
CLOGGED, ESPECIALLY IF LIVING IN LOW-LYING AREAS.
TROPICAL SYSTEMS...
INVEST 98W REMAINS NNW OF CHUUK, CURRENTLY NEAR 10.7N 151.1E. WHILE A
LARGE CLUSTER OF CLOUDS HAS BEEN PERSISTENT NEAR THIS AREA, DEEP
CONVECTION HAS BEEN MORE CYCLICAL THE PAST 24 HRS. JTWC HAS RAISED
ITS DEVELOPMENT POTENTIAL TO MEDIUM, MEANING DEVELOPMENT INTO A TC
IS LIKELY, BUT BEYOND 24 HRS. SCATTEROMETRY DATA TODAY FAILED TO
DEPICT A COHERENT, WELL-DEFINED LOW-LEVEL CIRCULATION, BUT CIMSS
VORTICITY FIELDS REVEALED A VORTICITY MAXIMA AT 850MB, 700MB, AND
500MB JUST SOUTH OF THE LOW-LEVEL POSITION. ADDITIONALLY, UPPER-LEVEL
HIMAWARI DERIVED MOTION WINDS SHOW A POCKET OF WEAKER WINDS ALOFT
THAT WOULD BE SUPPORTIVE OF IMPROVED ORGANIZATION OF 98W IN THE
COMING DAY OR TWO. MODEL GUIDANCE CONTINUES TO SHOW A QUICK
CONSOLIDATION OF 98W WED-THU AS IT HEADS TOWARD THE MARIANAS. THE
UPCOMING OVERNIGHT AND NOON PASSES OF SCATTEROMETRY IN THE NEXT 24 HR
WILL BE CRUCIAL IN DISCERNING ORGANIZATIONAL CHANGE IN SUPPORT OF
CURRENT MODELED IMPROVEMENT. FOR LATER WEEK, ENSEMBLE GUIDANCE STILL
SUPPORTS A DEVELOPING CIRCULATION TO APPROACH AND PASS OVER OR ALONG
THE MARIANAS ISLAND CHAIN ON A NNW-NNE HEADING AS EARLY AS THURSDAY.
WHILE A VERY WET PERIOD IS MORE CERTAIN, LESS CERTAIN IS THE OVERALL
WIND THREAT TO THE ISLANDS. ENSEMBLE GUIDANCE LENDS STRONG SUPPORT
TOWARD A TROPICAL DEPRESSION (SUSTAINED WINDS UP TO 38 MPH). HOWEVER,
THERE IS A SMALL CHANCE FOR TROPICAL STORM WINDS (AT LEAST 39 MPH),
ABOUT 10-20 PERCENT FOR GUAM AND THE LOWER CNMI AND SLIGHTLY HIGHER
FOR THE FAR NORTHERN ISLANDS. FOR MORE INFORMATION ON 98W, SEE
BULLETINS ISSUED BY THE JTWC UNDER WMO HEADER ABPW10 PGTW.
INVEST 97W IS LOCATED NEAR 17N133E, WELL WEST OF THE MARIANAS AND
HAS BEEN UPGRADED TO A TROPICAL CYCLONE FORMATION ALERT (TCFA). THIS
MEANS THIS SYSTEM IS EXPECTED TO BECOME A SIGNIFICANT TROPICAL
CYCLONE WITHIN THE NEXT 12-24 HOURS. 97W IS HEADING IN A NORTH-
NORTHWEST DIRECTION. THIS SYSTEM POSES NO DIRECT THREAT TO PALAU,
YAP, AND THE MARIANAS, BUT WILL GENERATE A WESTERLY SWELL THROUGH THE
REGION. FOR MORE INFORMATION ON 97W, SEE BULLETINS ISSUED BY THE
JTWC UNDER WMO HEADER ABPW10 PGTW.
EASTERN MICRONESIA...
SATELLITE OBSERVATIONS SHOW ISOLATED SHOWERS ACROSS KOSRAE WITH
SCATTERED SHOWERS ACROSS POHNPEI AND THE RMI. BUOY AND ALTIMETRY
DATA SHOW COMBINED SEAS BETWEEN 5 AND 7 FEET ACROSS EASTERN
MICRONESIA.
THE ACTIVE ITCZ AND TRADE-WIND CONVERGENCE PATTERN HAS SHIFTED NORTH
OF POHNPEI, KOSRAE, AND MAJURO, LEADING TO A DECREASE IN SHOWER
ACTIVITY. GUIDANCE CONTINUES TO TREND SHOWER COVERAGE DOWNWARD
TOMORROW WITH ISOLATED SHOWERS EXPECTED ACROSS POHNPEI, KOSRAE, AND
MAJURO. THIS TEMPORARY DECREASE IN SHOWERS LOOKS TO CONTINUE THROUGH
THURSDAY FOR KOSRAE AND MAJURO AND FRIDAY FOR POHNPEI. TOWARDS THE
END OF THE WEEK, THE ITCZ WILL FRAGMENT AND ALLOW FOR TRADE-WIND
CONVERGENCE TO INCREASE ACROSS THE REGION. THIS WILL ALLOW FOR THE
RETURN OF SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS BY THIS
WEEKEND.
LITTLE TO NO CHANGES WERE MADE TO THE MARINE FORECAST. BENIGN SEAS
OF 5 TO 7 FEET LOOK TO DECREASE TO 4 TO 6 FEET FOR MAJURO AND 3 TO 5
FEET FOR KOSRAE AND POHNPEI. WE MAY SEE ANOTHER INCREASE IN SEAS
ACROSS THE RMI LATE THIS WEEKEND AS TRADE-WINDS BEGIN TO INCREASE.
WESTERN MICRONESIA...
THERE ARE TWO INVEST AREAS IN THE REGION, 97W AND 98W (MORE ON THESE
IN THE TROPICAL SYSTEMS SECTION). THESE TWO SYSTEMS ARE THE MAJOR
FEATURES PROVIDING WEATHER ACROSS THE AREA. BOTH ARE GENERALLY
MOVING AWAY FROM WESTERN MICRONESIA. OVER THE NEXT 24 HOURS, THE
REMAINING SHOWERS OVER PALAU AND YAP ARE EXPECTED TO DIMINISH,
LEADING TO A DRIER PATTERN FOR THE REMAINDER OF THE FORECAST PERIOD.
THIS DRIER PATTERN IS DUE TO 97W MOVING FURTHER AWAY, AND A
STABILIZING HIGH PRESSURE RIDGE OVER THE SOUTHERN PORTION OF YAP
STATE AND PALAU.
NEAR CHUUK, 98W IS ALSO MOVING NORTHWEST, AWAY FROM WENO. THE
SHOWERS ASSOCIATED WITH THIS SYSTEM WILL ALSO BEGIN TO DIMINISH OVER
CHUUK WITHIN THE NEXT 48 HOURS OR SO. DURING THIS TIME, WINDS ARE
EXPECTED BE LIGHT TO MODERATE DROPPING TO LIGHT TO GENTLE. FOR
TONIGHT, THOUGH, SOME SHORT-LIVED GUSTS COULD APPROACH 20 KT. BY
FRIDAY, PLEASANT WEATHER IS ANTICIPATED TO RETURN.
ALTIMETRY SHOWS COMBINED SEAS OVER THE REGION OF 3 TO 4 FEET. SEAS
EXPECTED FOR WEDNESDAY AT CHUUK WILL BE 3 TO 5 FEET. WINDS OVER
PALAU AND YAP ARE EXPECTED TO REMAIN LIGHT TO MODERATE, FALLING TO
LIGHT TO GENTLE BY THE WEEKEND. OVERALL SEAS ARE EXPECTED BECOME
MORE PLEASANT AS THE FORECAST PERIOD PROGRESSES.
GUM WATCHES/WARNINGS/ADVISORIES
GU...FLOOD WATCH THROUGH FRIDAY AFTERNOON FOR GUZ001.
MP...FLOOD WATCH THROUGH FRIDAY AFTERNOON FOR MPZ001>003.
MARIANAS WATERS...NONE.
MARIANAS/TROPICAL/HYDROLOGY UPDATE: MONTVILA
EAST/WEST MICRONESIA/TROPICAL UPDATE: CRUZ
MARIANAS: DECOU
TROPICAL SYSTEMS: AYDLETT/BOWSHER
EAST MICRONESIA: WILLIAMS
WEST MICRONESIA: BOWSHER
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