813
FXPQ50 PGUM 300921
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
721 PM CHST MON JUN 30 2025
MARIANAS SYNOPSIS
MODERATE SOUTHWESTERLIES PREVAIL ACROSS THE REGION, WITH SATELLITE
IMAGERY AND DOPPLER RADAR INDICATING NUMEROUS SHOWERS AND SOME
THUNDERSTORMS IN GUAM AND ROTA COASTAL WATERS. MOST RECENT ALTIMETRY
PASSES SHOW SEAS ARE 3 TO 5 FEET, AND THE BUOYS SHOW SEA HEIGHTS OF
2 TO 4 FEET.
DISCUSSION
AN UNSETTLED WEATHER PATTERN CONTINUES OVER THE MARIANAS AS THE
MONSOON TROUGH BUILDS TO THE NORTH OF THE REGION. A FEW EMBEDDED
DISTURBANCES WITHIN THE MONSOON TROUGH ARE EXPECTED TO FURTHER
DESTABILIZE THE LOCAL ATMOSPHERE, ALLOWING SOME SPORADIC DEVELOPMENT
OF SHOWERS AND THUNDERSTORMS (ALBEIT LESS IN COVERAGE) THE NEXT FEW
DAYS AS SAID FEATURES CONTINUES TO TRY TO CONSOLIDATE TO THE
NORTH. THESE SHOWERS WILL CONTINUE TO BUILD OVERNIGHT, WITH A PEAK
EXPECTED JUST NEAR MIDNIGHT. AS THESE DISTURBANCES MOVE TO THE NORTH,
SHOWERS WILL DIMINISH SIGNIFICANTLY, WITH ONLY ISOLATED TO LOW-END
SCATTERED SHOWERS AND SOME THUNDERSTORMS EXPECTED FOR THE REST OF
THE WEEK.
MARINE
COMBINED SEAS OF 3 TO 5 FEET ARE EXPECTED TO BUILD TO 4 TO 6 FEET BY
TUESDAY, PERSISTING AS SUCH IN CNMI WATERS FOR THE REST OF THE WEEK.
HOWEVER, SEAS IN GUAM COASTAL WATERS ARE EXPECTED TO GRADUALLY
DIMINISH AS THE MONSOON TROUGH PULLS NORTHWARD, ALLOWING
SOUTHWESTERLY SWELL AND LOCALIZED WIND WAVES TO DIMINISH. A MODERATE
RISK OF RIP CURRENTS IS EXPECTED ALONG EAST FACING REEFS OF THE
MARIANAS FOR MUCH OF THE WEEK, WITH RIP CURRENT RISK POTENTIALLY
BECOMING MODERATE ALONG WEST REEFS BY THE WEEKEND AS WELL, ONCE
ANOTHER PULSE OF SOUTHWESTERLY SWELL MAKES ITS WAY ACROSS THE REGION.
HYDROLOGY
A LULL IN SHOWERS EARLIER TODAY HAS BROUGHT ONLY 0.05 INCHES OF RAIN
AT THE AIRPORT, AS MOST OF THE CONVECTION STAYED JUST SOUTHWEST OF
GUAM WATERS, AND CONCENTRATED WITHIN AND ALONG THE EASTERN FLANKS OF
INVEST 90W NORTHEAST OF SAIPAN. ONCE THESE NEARBY DISTURBANCES
DESTABILIZED THE ATMOSPHERE LATER IN THE AFTERNOON, SHOWERS HAVE
DEVELOPED THAT ARE EXPECTED TO BRING WETTING RAINS AND FLASH
FLOODING TO GUAM OVERNIGHT. AS SUCH, A FLASH FLOOD WATCH REMAINS IN
EFFECT FOR GUAM THROUGH TUESDAY AFTERNOON, AS THIS WET MONSOON
PATTERN LINGERS. HIGH UNCERTAINTY REMAINS AT THIS TIME IF THESE
CONVECTIVE CELLS WILL PERSIST DOWNSTREAM OVER THE CNMI, BUT IF
CONDITIONS WARRANT, A FLASH FLOOD WATCH MAY BE EXTENDED TO THE CNMI
AS WELL.
TROPICAL SYSTEMS
THERE ARE THREE TROPICAL DISTURBANCES ACROSS THE REGION THIS
AFTERNOON. TWO OF THESE DISTURBANCES ARE ASSOCIATED WITH THE
NORTHEASTERN END OF THE MONSOON TROUGH. NEWLY FORMED INVEST 90W IS
FOUND WEST-NORTHWEST OF SAIPAN, NORTHWEST OF GUAM, AT 16.0N 149.5E.
INVEST 99W IS SEEN FARTHER NORTH, NEAR 19.2N 145.6N. NEITHER OF THESE
HAVE A DISTINCT CIRCULATION CENTER, BUT ARE LOCATED ALONG A MORE
PRONOUNCED KINK IN THE TROUGHING. THESE DISTURBANCES ARE EXPECTED TO
DRIFT NORTH OVER THE NEXT FEW DAYS, WITH 90W BEING THE MORE FAVORED
DISTURBANCE FOR ANY DEVELOPMENT. LATEST MODEL GUIDANCE SHOWS A
CIRCULATION DEVELOPING WITH 90W AS MOVES NORTH, STAYING EAST OF THE
NORTHERN CNMI. AS IS THE NORM, THE GFS IS MORE AGGRESSIVE WITH THE
DEVELOPMENT, THOUGH ALL MODELS SHOW THE SYSTEM DEVELOPING WELL NORTH
OF THE MARIANAS AND EAST OF THE NORTHERN CNMI.
INVEST 98W IS SEEN WELL WEST OF THE MARIANAS, CENTERED NEAR 15.2N
132.6E, MOVING SLOWLY WEST. 98W HAS BEEN UPGRADED TO LOW, MEANING
DEVELOPMENT IS EXPECTED, BUT NOT WITHIN THE NEXT 24 HOURS. INVEST 98W
WILL CONTINUE TO DRIFT WEST, WITH MODEL GUIDANCE IN PRETTY GOOD
AGREEMENT THAT IT WILL STRENGTHEN WEST OF 130E. INVEST 98W IS NOT
EXPECTED TO HAVE ANY DIRECT IMPACTS WITH THE MARIANAS, PALAU OR YAP
AS IT MOVES FARTHER AWAY. FOR MORE INFORMATION ON INVEST 98W, SEE
BULLETINS ISSUED BY THE JOINT TYPHOON WARNING CENTER UNDER WMO HEADER
ABPW10 PGTW.
EASTERN MICRONESIA
A TRADE-WIND TROUGH IS APPROACHING MAJURO, INTERACTING WITH A
DEVELOPING INTER-TROPICAL CONVERGENCE ZONE (ITCZ). NUMEROUS SHOWERS
AND ISOLATED THUNDERSTORMS ARE NOW MOVING INTO MAJURO WATERS. THE
TROUGH IS EXPECTED TO MOVE WEST OVERNIGHT, APPROACHING KOSRAE TUESDAY
MORNING. LATEST GUIDANCE SHOWS THE TROUGH LOSING A LITTLE STEAM BY
TUESDAY, THEREFORE, SET POPS (PROBABILITY OF PRECIPITATION) AT 40
PERCENT. IF THE ITCZ REMAINS AS STRONG AND THE TROUGH MAINTAINS ITS
INTENSITY AS WELL, POPS MAY NEED TO BE INCREASED. THE SAME IS
EXPECTED FOR POHNPEI TUESDAY NIGHT. THE ITCZ LOOKS TO DROP A BIT
FARTHER SOUTH, ALLOWING A DRIER PATTERN TO MOVE INTO MAJURO TUESDAY
NIGHT. OTHER THAN A BRIEF PERIOD OF SCATTERED SHOWERS AS A TRADE-
WIND TROUGH MOVES THROUGH THE MARSHALLS ON WEDNESDAY NIGHT, THE DRY
PATTERN IS EXPECTED TO CONTINUE INTO THE WEEKEND. THE ITCZ, HOWEVER,
DOES LOOK TO REMAIN OVER POHNPEI AND KOSRAE THROUGH MOST OF THE WEEK,
MAINTAINING INCREASED CONVECTION THROUGH THURSDAY FOR POHNPEI AND
THROUGH FRIDAY FOR KOSRAE.
LITTLE CHANGES EXPECTED FOR THE MARINE FORECAST, WITH COMBINED SEAS
OF 5 TO 7 FEET AT MAJURO SLOWLY SUBSIDING OVER THE NEXT FEW DAYS,
DROPPING TO BETWEEN 3 AND 5 FEET FOR THE WEEKEND. SEAS OF 4 TO 6 FEET
FOR KOSRAE AND POHNPEI COULD REACH 5 TO 7 FEET TUESDAY NIGHT INTO
WEDNESDAY BEFORE SUBSIDING TO BETWEEN 3 AND 4 FEET FOR THE WEEKEND.
GENTLE TO MODERATE WINDS ARE EXPECTED THROUGH THE WEEK FOR KOSRAE AND
MAJURO, WITH GENTLE TO MODERATE WINDS BECOMING LIGHT TO GENTLE FOR
POHNPEI AROUND MIDWEEK.
WESTERN MICRONESIA
A SOUTHWESTERLY MONSOON PATTERN WILL CONTINUE TO BRING HIGHER-END
SCATTERED SHOWERS TO PALAU AND YAP TONIGHT, FOLLOWED BY ANOTHER
UPTICK IN SHOWERS ALONG WITH GUSTIER WINDS AND THUNDERSTORMS NEAR
PALAU TUESDAY. AS VARIOUS MONSOON DISTURBANCES MOVE NORTHWARD AND
SHIFT THE MONSOON TROUGH ALONG WITH IT, A RIDGE WILL FORM ALONG THE
EASTERN SIDE OF SAID TROUGH, ALLOWING CONDITIONS TO IMPROVE
SIGNIFICANTLY WITH ONLY ISOLATED SHOWERS EXPECTED AT YAP BEGINNING
TUESDAY AND AT PALAU TUESDAY NIGHT. MEANWHILE AT CHUUK, LOCALLY
DIFFUSIVE FLOW INDUCED BY THE MONSOON TROUGH AND EMBEDDED
DISTURBANCES MENTIONED PREVIOUSLY, IS ALLOWING A DRIER REGIME TO
ENSUE THROUGH TUESDAY. AFTER, SOME PATCHY SHOWERS WILL RETURN,
FOLLOWED BY A LEADING EDGE OF A SHARP TROUGH OR VORTICITY FEATURE
BRINGING VERY STRONG GUSTS AND HEAVIER SHOWERS AROUND MIDWEEK.
COMBINED SEAS OF 4 TO 5 FEET ARE EXPECTED TO TAPER DOWN A FOOT OR
TWO AT PALAU AND YAP IN THE NEXT FEW DAYS, AS VARIOUS SWELLS AND
LOCALIZED WIND WAVES DIMINISH. THEN, A NORTHWEST SWELL GENERATED BY A
DISTANT TC WELL NORTHEAST OF THE PHILIPPINES WILL START TO TRICKLE IN
BY THE NEXT WEEKEND. AT CHUUK. COMBINED SEAS OF 3 TO 5 FEET, WILL
MOMENTARILY INCREASE ANOTHER FOOT OR TWO AROUND MIDWEEK AS THE
AFOREMENTIONED TROPICAL FEATURE WILL MAKE ITS WAY ACROSS AND PRODUCE
CHOPPIER SEAS DURING PASSAGE.
GUM WATCHES/WARNINGS/ADVISORIES
GU...FLOOD WATCH THROUGH TUESDAY AFTERNOON FOR GUZ001.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS/HYDROLOGY: BOWSHER/MONTVILA
EAST MICRONESIA/TROPICAL: KLEESCHULTE
WEST MICRONESIA: MONTVILA
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