FXPQ50 PGUM 230917 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 717 PM ChST Sun Aug 23 2026 .Marianas Synopsis... Satellite imagery shows high-end scattered showers and isolated thunderstorms across the Marianas coastal waters. Model guidance suggests combined seas of 7 to 9 ft. && .Discussion... The Marianas is located within an unstable weather pattern due to Typhoon Saudel's (17W) monsoonal tail, cross-equatorial flow, and a col to the east-northeast. Scattered showers today have steadily increased to numerous across coastal waters, which will eventually make its way over the islands before midnight. Gentle to moderate southwesterlies with strong, convective gusts are expected at least through Monday afternoon. Isolated thunderstorms are occurring and will persist possibly through the rest of the week. There is uncertainty regarding timing and intensity of heavy showers on Monday, so the forecast reflects a 60 percent chance of showers. The previously anticipated wet period starting on Tuesday looks to be delayed a day or two, with flooding and mudslide potential most likely in the latter half of the week. The forecast reflects the slight lull in shower activity midweek before picking back up by the weekend. && .Marine... Combined seas of 7 to 9 ft tonight will increase to 8 to 10 ft tomorrow, so the previously issued Small Craft Advisory remains in effect through Monday afternoon. In addition, strong, convective gusts associated with heavy showers will give way to choppy seas. A long-period southwesterly swell from Tropical Depression 19W (located well to the west) will induce hazardous surf after tonight, and allow the high risk of rip currents to remain along west-facing reefs. There is a moderate risk of lightning throughout the week. && .Hydrology... A reinvigorated monsoon trough with a potential embedded tropical disturbance will likely develop in the area and bring periods of heavy rainfall to the Marianas starting midweek. Between Wednesday and Friday, early ensemble guidance suggests 4 to 6 inches with a low probability of up to 10 inches for Guam and Rota, and 3 to 5 inches with a low probability of up to 8 inches for Saipan and Tinian. This situation will continue && .Tropical Systems... Typhoon Saudel (17W) is centered near 22N140E, moving northwest at 18 mph. Maximum sustained winds are 125 mph near the center, with this intensity persisting through Monday. Saudel is expected to make a slight turn toward the west-northwest with little change in forward speed through Monday as it heads toward Okinawa. Tropical Depression 19W is centered near 14N136E, moving east- northeast at 9 mph. Maximum sustained winds remain at 35 mph near the center, with this intensity persisting through Monday, remaining a Tropical Depression. TD 19W is to expected to continue to move into a a favorable environment while pivoting toward departing TY Saudel well to the north-northeast, with some resemblance of a distant Fujiwara Effect. This is expected to help further prime the environment around the Marianas for another enhancement in the monsoon or potentially a weaker tropical disturbance to develop in the area, bringing in another period of heavy rainfall to the region later this week (more details in the Hydrology Section). For more information on Typhoon Saudel (17W), please refer to the Tropical Cyclone Warning from JTWC under WMO header WTPN31 PGTW, and the TC Public Advisory from the National Weather Service under WMO header WTPQ31 PGUM. For more information on Tropical Depression 19W, please refer to the Tropical Cyclone Warning from JTWC under WMO header WTPN33 PGTW, and the TC Public Advisory from the National Weather Service under WMO header WTPQ33 PGUM. && .Eastern Micronesia... Pohnpei and Kosrae - short term: The main forecast challenge was determining rainfall coverage/potential through the first 36 hours. Currently, fairly persistent shower activity has been ongoing for the past few hours. However, recent infrared satellite imager shows rapid warming of cloud top temperatures, while visible imager in addition to infrared overcast skies. This is often a "stratification effect" after a prolonged period of heavier showers and thunderstorm occurrence. Thus, it's highly likely that some high cloud-based light rainfall continues from the a modest directional and speed shear environment (based on the Pohnpei sounding and pattern recognition in the numerical forecast guidance). Typically, this "stable bubble" persists for 6 to 10 hours. With this in mind, we do expect increased shower activity to redevelop, and this was reflected in tonight's forecast. The thought of adding "after midnight wording" was strongly considered, for conveying when the best rainfall potential would occur. However, good speed convergence in the low-level streamline analysis, along with recent development south of Kosrae, precluded this decision. Majuro - short term: Majuro too involved a little bit of a tricky forecast tonight and tomorrow. as the best convergence and attendant convective development is south and east of you per satellite imagery. However, short term forecast guidance strongly suggests this backbuilding line of convection will slide north with time, potentially even before midnight. This is supported in the latest satellite imagery as well. Thus we precluded any more specific temporal refinement to the forecast. Extended outlook for all three locations (36 hours to day 7): Forecast guidance and pattern recognition all points to at least scattered shower potential continuing into next weekend, with a few thunderstorms mixed in from time to time. A tropical disturbance/circulation located east of Pohnpei/northeast of Kosrae will not only maintain respectable convergence in the cyclonically- enhanced monsoon flow, but there may be a few periods of locally increased shower coverage and intensity. Marine and surf conditions: A west to southwest primary and secondary swell will continue through the weekend at Pohnpei and Kosrae, with a slight backing of swell direction due to the previously-mentioned cyclonic flow in the monsoon environment, will allow for a southwest and south swell direction to continue at Majuro. This swell regime will continue to be augmented by the persistent cross-equatorial flow/swell pattern. The net result is elevated surf concerns will continue the next several days at Kosrae and Majuro as well. Otherwise, mainly gentle to light winds currently, may become light to gentle from time to time, especially for the latter half of next week. Seas of 5 to 7 feet are anticipated through the forecast period. && .Western Micronesia... As Typhoon Saudel well northwest of the Marianas and Tropical Depression 19W north-northwest of Yap continue to move away from the region, the last pulse of a fresh to strong southwesterly monsoon is expected to bring more patchy showers Yap tonight. While winds are expected to diminish slightly thereafter, a reigniting monsoon surge is still expected to pummel Yap and much of eastern Yap State the next several days, with a peak shower and thunder coverage expected Tuesday and Tuesday night. A tropical ridge anchored just north of the Equator is expected to bring a dry spell to Palau beginning Monday night, which is likely to last for the rest of the week. While moist westerlies are expected to continue at Chuuk for most of the forecast period, the aforementioned tropical ridge will allow Chuuk to fall in between the convectively active pattern hugging 10N and the much quieter sector of the Equator between Indonesia and Kapingamarangi. This will allow patchy showers and some thunderstorms to form at Chuuk for much of the forecast period, followed by a drying trend in the second half of next week as a larger buffer circulation induces more diffusive flow. The aforementioned monsoon pulse and remaining significant southwesterly swells and fresh to strong winds continues to produce sea conditions hazardous to small craft, which are expected to fall below hazardous criteria by sunrise on Monday. In the meantime, combined seas of 5 to 7 feet in Palau waters are expected to taper down a few feet over the next few days as the southwesterly swell diminishes, being slowly replenished by various smaller northerly swells building in. Combined seas of 5 to 7 feet are expected to slowly drop a foot or two toward the next weekend as swells and localized wind waves diminish. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...High Rip Current Risk through Monday afternoon for GUZ001. High Surf Advisory until 4 PM ChST Monday for GUZ001. MP...High Rip Current Risk through Monday afternoon for MPZ001>003. High Surf Advisory until 4 PM ChST Monday for MPZ001>003. Marianas Waters...Small Craft Advisory until 4 PM ChST Monday for PMZ151>154. && $$ Marianas/Hydrology: Mesa East Micronesia: Doll West Micronesia/Tropical: Montvila