831
FXPQ50 PGUM 160839
AFDGUM
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TIYAN GU
639 PM CHST WED SEP 16 2026
MARIANAS SYNOPSIS
VISIBLE SATELLITE AND RADAR IMAGERY DEPICTS SCATTERED TO NUMEROUS
SHOWERS ACROSS THE REGION WITH ISOLATED THUNDERSTORMS MAINLY IN THE
EASTERN COASTAL WATERS OF ROTA. ALTIMETRY DATA INDICATES COMBINED
SEAS OF 6 TO 8 FT ACROSS THE REGION.
DISCUSSION
ANOTHER ROUND OF HIGH-END SCATTERED SHOWERS FOR GUAM AND ROTA AND
NUMEROUS SHOWERS FOR SAIPAN AND TINIAN IS EXPECTED THIS EVENING,
SLIGHTLY TAPERING DOWN AFTER MIDNIGHT. THE BACK-END OF TROPICAL
DEPRESSION 24W (DISCUSSED IN THE TROPICAL SYSTEMS SECTION BELOW) IS
THE CULPRIT FOR WIDESPREAD CLOUD COVER AND TOWERING CONVECTION THIS
EVENING. ISOLATED THUNDERSTORMS AND LOCALLY HEAVY RAINFALL ARE STILL
POSSIBLE TONIGHT AS TD 24W CONTINUES MOVING NORTH AND AWAY FROM THE
ISLANDS. AS FOR THE LONG-TERM, SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS REMAIN A POSSIBILITY FOR ALL ISLANDS THROUGH THE ENTIRE
FORECAST PERIOD AS THE MONSOON TROUGH LIFTS NORTH, TRAILING BEHIND
TD 24W. THE ZONES REMAINED SPLIT TO CONVEY A SLIGHT DIFFERENCE IN
SHOWER COVERAGE OVER GUAM AND ROTA VERSUS SAIPAN AND TINIAN SHOULD
THE MONSOON TROUGH'S NORTHWARD MOTION BE RELATIVELY SLOW. IN
ADDITION, WIND DIRECTIONS APPEAR TO BE SLIGHTLY DIFFERENT BETWEEN THE
TWO ZONES. THERE IS STILL HIGH UNCERTAINTY REGARDING FLOODING
CONCERNS AS ENSEMBLES GREATLY DIFFER IN TERMS OF INTENSITY AND
DISTRIBUTION OF RAINFALL. THIS SITUATION WILL CONTINUE TO BE
MONITORED OVER THE COMING WEEK.
MARINE
COMBINED SEAS ARE EXPECTED TO BE 5 TO 8 FT THROUGHOUT THE ENTIRE
FORECAST PERIOD, WITH SAIPAN AND TINIAN MOST LIKELY ON THE HIGHER
END DUE TO THEIR PROXIMITY TO TD 24W'S NORTHEASTERLY SWELL. GENTLE TO
MODERATE WINDS OUT OF THE NORTH AND NORTHWEST WILL TURN TO THE WEST
FOR ALL ISLANDS ON THURSDAY. SURF IS NOT EXPECTED TO REACH ANY
HAZARDOUS CRITERIA AT THIS TIME, HOWEVER, A SMALLER, LONG-PERIOD
NORTHEASTERLY SWELL MAY CAUSE OCCASIONALLY ELEVATED WAVES ALONG NORTH
AND EAST FACING REEFS. IN ADDITION, THERE IS A MODERATE RISK OF RIP
CURRENTS ALONG ALL REEFS, SO REMAIN CAUTIOUS NEAR AND IN THE WATERS
OVER THE NEXT SEVERAL DAYS. AS TD 24W CONTINUES MOVING NORTH, ITS
CORRESPONDING SWELLS MAY SUBSIDE A FOOT OR TWO. SUBSEQUENTLY, THE
MONSOONAL SOUTH TO SOUTHWESTERLY SWELL MAY INCREASE THROUGHOUT THE
FORECAST PERIOD.
TROPICAL SYSTEMS
TROPICAL DEPRESSION 24W CONTINUES TO STRUGGLE (SOMEWHAT) TO
STRENGTHEN FURTHER, AS IT MEANDERS EAST OF SAIPAN. THE 1 PM CHST
POSITION PLACES THE LLCC (LOW-LEVEL CIRCULATION CENTER) AT 15.5N
148.8E, WITH ITS MOVEMENT BEING NORTH-NORTHEAST AT 2 MPH. 24W HAS
BEEN FIGHTING SOME DRY AIR ENTRAINMENT FROM THE NORTHWEST TO NORTH.
WITH THAT SAID, VERTICAL WIND SHEAR IS LOW (5-10 KNOTS) AND WARM SSTS
(SEA SURFACE TEMPERATURES) OF 29 TO 30 DEG. C, AND MODERATE POLEWARD
OUTFLOW DOES MAKE THE ENVIRONMENT MARGINALLY FAVORABLE. THE LATEST
FORECAST FROM JTWC (THE JOINT TYPHOON WARNING CENTER) DOES SHOW 24W
INCREASING FORWARD SPEED DRAMATICALLY AS IT LIFTS OFF TO THE NORTH
OVER THE NEXT 24 HOURS, BECOMING A TROPICAL STORM AS IT DOES SO. IT
CURRENTLY LOOKS TO REMAIN EAST OF EVEN THE NORTHERN CNMI, WITH THE
MAUG ISLANDS PERHAPS BEING THE CLOSEST AS THE LLCC PASSES NEAR.
EASTERN MICRONESIA
THE WEATHER PATTERN REMAINS FAIRLY QUIET ACROSS MOST OF EASTERN
MICRONESIA. THE REGION LIES SOUTH OF THE MONSOON TROUGH WHICH ENTERS
THE REGION NEAR BIKINI ATOLL, DIVING SOUTHEAST TO A WEAK CIRCULATION
NORTHEAST OF MAJURO. MEANWHILE, A SHARP RIDGE AXIS EXTENDS NORTH
ACROSS THE EQUATOR AND THROUGH WESTERN POHNPEI STATE. THE SURFACE
WIND FLOW ALLOWS FOR A RIBBON OF DECENT SURFACE CONVERGENCE TO BE
FOUND FROM NEAR KOSRAE TO THE MAJURO AND KWAJALEIN AREAS. HERE,
SCATTERED WEAK CONVECTION CONTINUES TO DEVELOP, WITH THE MAIN NOTABLE
FEATURE BEING MOSTLY CLOUDY SKIES. THE LACK OF GREATER RAINFALL IS
TIED TO THE DRY AIR DIVING SOUTH-SOUTHWEST FROM THE NORTH, DUE TO THE
SYNOPTIC PATTERN.
WITH THAT SAID, THE MODELS SHOW A SOUTHEAST INFLOW BAND EXTENDING
FROM TD 24W INTO THE KOSRAE AND POHNPEI REGION DEVELOPING THURSDAY,
POSSIBLY INCLUDING MAJURO AS WELL. GIVEN SOME SEMBLANCE OF THIS
VISIBLE ALREADY EVIDENT IN THE STREAMLINE ANALYSIS AND SATELLITE
DATA, WE KEPT THIS EXPECTATION IN THE FORECAST. HOWEVER, HEADING
TOWARDS THE WEEKEND, RAINFALL POTENTIAL LOOKS TO DECREASE.
AS FOR MARINE CONDITIONS, SEAS OF 4 TO 6 FEET LOOK TO PERSIST INTO
NEXT WEEK. LIGHT TO GENTLE WINDS WILL BE COMMON AT POHNPEI AND
MAJURO, BUT SLIGHTLY HIGHER IN THE GENTLE TO MODERATE RANGE AT KOSRAE
(AS KOSRAE WILL GET THE "SQUEEZE PLAY" BETWEEN THE RIDGE AXIS AND
LOW PRESSURE CIRCULATION). WITH THAT SAID, NO HAZARDOUS SURF IS
EXPECTED.
WESTERN MICRONESIA
THE BULK OF THE SHOWERS HAVE MOVED NORTH OF PALAU, AND WILL REMAIN
THERE THROUGH SATURDAY. SUNDAY AND MONDAY COULD SEE SCATTERED SHOWERS
RETURNING AS THE MONSOON BAND REORIENTS. WINDS WILL BE FROM THE WEST
OR SOUTHWEST AT 10 TO 15 KNOTS, WITH OCCASIONAL GUSTS TO 25 KNOTS
TONIGHT AND THURSDAY. SEAS OF 5 TO 7 FEET WILL DIMINISH SLIGHTLY TO
BETWEEN 4 AND 6 FEET BY MONDAY. THIS WILL RESULT IN MARINE CONDITIONS
THAT ARE CHALLENGING THROUGH THURSDAY THEN BENIGN THEREAFTER.
FOR YAP, THE BAND OF MONSOON WINDS HAS SHIFTED RIGHT OVER THEM, SO
LOOK FOR WEST WINDS OF 15 TO 25 KNOTS THROUGH FRIDAY, THEN THEY
SHOULD SETTLE DOWN TO BETWEEN 10 AND 15 KNOTS. SEAS WILL START
TONIGHT AT 6 TO 8 FEET AND MAINTAIN THROUGH FRIDAY, THEN BY MONDAY
DIMINISH TO BETWEEN 4 AND 6 FEET. SHOWERS WILL BE SCATTERED THROUGH
SUNDAY THEN COULD INCREASE TO WIDESPREAD MONDAY. THIS WILL LEAD TO
MARINE CONDITIONS THAT ARE HAZARDOUS THROUGH FRIDAY, THEN BENIGN FOR
THE WEEKEND AND MONDAY. A SMALL CRAFT ADVISORY HAS BEEN ISSUED FOR
THE WINDS TONIGHT THROUGH FRIDAY.
AT CHUUK, WINDS WILL BE SOUTHWEST AT 10 TO 15 KNOTS THROUGH FRIDAY
NIGHT, THEN SOUTHWEST AT 5 TO 10 KNOTS THROUGH MONDAY. SHOWERS WILL
BE ISOLATED THROUGH SATURDAY, THEN SCATTERED FOR SUNDAY AND MONDAY.
SEAS WILL BE 5 TO 7 FEET THROUGH THURSDAY NIGHT, THEN START TO
DIMINISH. THEY ARE EXPECTED TO REACH 3 TO 5 FEET ON SATURDAY AND STAY
NEAR THERE THROUGH MONDAY. THIS WILL RESULT IN MARINE CONDITIONS THAT
ARE BENIGN THROUGH THE FORECAST PERIOD.
GUM WATCHES/WARNINGS/ADVISORIES
GU...NONE.
MP...NONE.
MARIANAS WATERS...NONE.
MARIANAS: MESA
EAST MICRONESIA/TROPICAL SYSTEMS: DOLL
WEST MICRONESIA: STANKO
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