FXPQ50 PGUM 180815 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 615 PM ChST Mon May 18 2026 .Marianas Synopsis... Mostly cloudy skies and scattered showers are forecasted for the Marianas tonight. A return to isolated showers and partly cloudy skies may continue until the weekend. Combined seas are between 6 to 8 feet, and a high risk of rip currents remains along east-facing reefs at least through Tuesday afternoon. && .Discussion... Satellite and model trends show low-end scattered showers as the trade-wind surge and surface troughs continue to produce showers moving downstream towards the Marianas, so Probability of Precipitation (PoPs) values will remain at 30 percent for the entire region tonight. Isolated showers are expected to remain through the rest of the week, but moisture trends will continue to be monitored as the troughs south of Wake Island progress downstream over the next several days. Scatterometry shows some pockets of elevated trade winds over open waters to the far east, but models (validated by today's observations) indicate winds remaining 15 to 20 miles per hour with gusts up to 25 miles per hour for the region until the latter half of the week when the trades potentially downtrend through the weekend. A slight chance of thunderstorms is introduced in the last few periods of the forecast, but confidence is low as models struggle to agree on the source, whether it be a weak trough or a potential disturbance passing south of Guam. Regardless, this will continue to be monitored and communicated as the week progresses. && .Marine... Buoy and altimetry data indicated combined seas of 6 to 8 feet across the region and is expected to continue until midweek when sea heights may drop a foot or two through the rest of the week. The easterly trade winds are expected to remain moderate to fresh through the rest of the week before potentially dropping to just moderate over the weekend. Surf will remain at 9 feet tonight along east-facing reefs, so a high risk of rip currents remains at least through Tuesday afternoon. Surf conditions will continue to be monitored as the rip current risk will need to be re-evaluated tomorrow afternoon. && .Tropical Systems... A weak subtropical disturbance on the northwestern edge of Guam's Area of Responsibility (AOR), centered near 22N131E, has been dubbed Invest 98W by the Joint Typhoon Warning Center. 98W exists on the southwestern edge of a decaying stationary frontal boundary to the east of Taiwan, which extends east-northeast to a low-pressure system well beyond 25N137E. Numerous showers and scattered thunderstorms are seen within the moist convergent flow wrapping into 98W along its eastern flank, just south of the nearly-dissipated baroclinic zone extending from it. Invest 98W looks to remain within the region only briefly, with model guidance depicting a north to north-northwest movement along 130E, exiting north and west of the AOR by late Tuesday morning. 98W then looks to merge with an eastward-propagating mid-latitude system around midweek. && .Eastern Micronesia... A disorganized, unsettled pattern continues across the region. Pohnpei saw numerous showers and isolated thunderstorms throughout the morning hours, due to strong convergence along a nearby trade- wind trough. This was supported by an area of divergence aloft, associated with a broad upper-level low centered well northeast of Pohnpei, to the south of Wake Island. Himawari visible satellite this afternoon shows lingering widespread cloud cover across Pohnpei State, with patchy stratiform showers nearby. Much drier conditions prevail over Kosrae and Majuro, with most of the showers focused well to the north of Kosrae, and over the northeastern Marshall Islands. Numerous showers are also seen to the southwest of Kosrae, near and south of the Equator, associated with an Intertropical Convergence Zone (ITCZ) fragment over far southern Pohnpei State. The upper-level low to the south of Wake Island looks to shift slightly westward over the next day or so as it extends toward the west-southwest. The change in orientation of the low, in addition to the development of an upper-level high to its west near the Marianas, looks to focus strong upper-level divergence over Chuuk and Pohnpei States over the next few days. This looks to interact with a trough and convergence boundary at the surface, to support numerous showers and isolated thunderstorms for Pohnpei around Tuesday and Tuesday night. The upper-level low looks to gradually open into a trough, which will linger and weaken throughout the rest of the week. Model guidance indicates an unsettled pattern will continue through the week, with numerous to widespread showers focusing farther to the west, mainly over central Micronesia. Meanwhile, an ITCZ looks to develop near the Marshalls by the end of the week, with a series of weak troughs propagating westward along it. These features will maintain a wet pattern across the region, with at least scattered showers expected into the end of the week for all three forecast points. Benign marine conditions are expected through the week. Latest altimetry data indicates combined seas between 5 and 7 feet across much of the region. Models depict a gradual relaxation of the east to northeast trade swell throughout the week, allowing seas to diminish by a foot or two over the next several days. && .Western Micronesia... The main weather makers for the region are a band of convergence between Chuuk and Pohnpei, a second band of convergence near Yap Proper, and a trough over southern Palau. Over southern Palau, an east-to-west oriented trough has set up and is producing scattered showers. Model guidance suggests this trough will move through the region over the next day or so. Looking further out, a trade-wind regime is expected to return through next week. Near Yap, the convergence band is interacting with a subtle trough and is producing scattered showers that is expected to build into widspread showers with isolated thunderstorms over the evening and early morning hours. This interaction is anticipated to remain in the Yap Proper area for a day or two. Then, a series of troughs is expected to pass through the beginning of next week, anticipated to bring scattered showers to Yap. Meanwhile in Chuuk, satellite imagery shows trade-wind convergence showers to the south of Weno. Showers are expected to build into Chuuk's coastal waters overnight. Then over the week, the showers and troughs that are affecting Pohnpei now are expected to move into Weno making for a fairly wet week. Benign marine conditions are expected to continue through much of the upcoming week. Altimetry shows 2 to 4 foot seas at Palau and Yap, and 4 to 6 feet at Chuuk. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...High Rip Current Risk through Tuesday afternoon for GUZ001. MP...High Rip Current Risk through Tuesday afternoon for MPZ001>003. Marianas Waters...None. && $$ Marianas: Mesa East Micronesia: DeCou West Micronesia: Bowsher