Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
070 FXPQ50 PGUM 210839 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 639 PM ChST Tue May 21 2024 .Marianas Synopsis... Afternoon satellite imagery shows fair weather across the Marianas with a few trade-wind showers passing through bringing short-lived showers and brief gusts to around 30 mph. Both the 00Z RAOB and MIMIC TPW indicated about 2in of precipitable water near Guam, with MIMIC TPW showing drier conditions over the CNMI. The higher TPW over Guam is connected to the increased moisture field extending northward from the West Micronesia circulation, Invest 93W. 93W remains broad and weak, centered south of Yap near 5N138E. Seas of about 6-8 feet are found around the Marianas with highest seas to the east of the islands. && .Discussion... Made a few short-term forecast changes, mainly to reduce showers for Guam and remove the thunderstorms for the short term since deeper moisture had not pushed as far north into the area as models had previously shown. Had considered a slight chance of thunderstorms for the Guam area into Wednesday, but despite the near 2in TPW, the mid and upper atmosphere remain quite dry. NUCAPS profiles from the afternoon METOP-B/C passes show very shallow moisture in the region between Guam and Chuuk with much drier conditions aloft, as noted in Himawari low and mid-level water vapor imagery. Model data shows the moist layer limited to around 700mb then significant drying aloft. Later in the week, though, around Thursday into Friday, deeper moisture will push through the Guam area and is expected to bring an uptick in showers for Guam and Rota as well as a small chance for thunderstorms. Trade winds will hold steady through the week before decreasing over the weekend with next week seeing gentle easterly winds. && .Marine... A high risk of rip currents will continue through Wednesday afternoon with 6-8 ft seas resulting in surf along east facing reefs up to 9 ft. Trade winds and swell will begin to subside late week and into the weekend with seas eventually falling to the 4-5 ft range by Sunday. && .Tropical systems... The broad and weak circulation, 93W, south of Yap near 5N138E remains the tropical feature of interest. It is currently rated as a LOW for development potential by the JTWC, meaning TC development is unlikely within the next 24 hours. Afternoon visible satellite imagery shows an exposed low-level circulation center with deep convection displaced to the east. West of the center, clusters of showers and thunderstorms are developing amid a swath of drier air feeding in from the west, as noted in recent MIMIC TPW imagery. Deterministic and ensemble model guidance largely keeps 93W as a broad circulation with a large center of weak winds as it potentially passes near Palau Wednesday or Thursday. The most likely effects from 93W to the Palau and Yap areas will be the heavier shower and stronger easterly wind gust potential as 93W heads WNW. Areas south of 93W`s center will experience much lighter winds out of the west. Once 93W heads into the PI Sea, models show a slow organization of the circulation with GFS still being more aggressive in the 5-10 day time frame, as has been the case for quite some time. && .Eastern Micronesia... Shallow cumulus clouds are seen across the three forecast points and for most of the region. Isolated showers are expected for Pohnpei tonight, there are some residual clouds and showers seen nearby with the cold front to the northeast. This is expected to move out of Pohnpei`s waters by tomorrow. Expect a slightly drier pattern for Kosrae and Pohnpei for a day or two then showers increasing by the week`s end as a TUTT and upper low enter the region, most likely being on and off scattered showers for Kosrae and Pohnpei. Beginning of next week, a moderate ITCZ pattern develops, bringing scattered showers again, and possibly locally heavy showers as the models indicate pockets of deeper convection. Gentle to moderate trade winds and trade-wind swell continues across the region. An afternoon altimetry pass near Kosrae indicates combined seas around 5 to 6 feet and the Majuro buoy recently reported seas around 5 feet. && .Western Micronesia... A broad circulation, Invest 93W, was located south of Yap and southeast of Palau, near 5N138E. Convergence along the northern peripheries is bringing widespread showers and isolated thunderstorms to most of Yap State. An upper-level trough to the northwest, helped maintain clusters of thunderstorms around Ulithi today. As this upper-level feature moves west and weakens overnight, it will gradually decouple from surface features, losing out on that supplementary support for thunderstorms. Although, 93W, alone, will undoubtedly supply a busy weather pattern for far western Micronesia. Currently, the favored trajectory of this circulation is to pass just south of the main islands of Palau late Wednesday night, then continuing northwest into the Philippine Sea. Although models show little development until it is well to the northwest of the region, periods of heavy rainfall and gusty winds are still possible Wed-Thu for Yap and Wed night-Thu for Palau. As 93W continues its departure from the region, a trade-wind pattern looks to return next week. Farther east, Chuuk is outside the grip of 93W and a relatively quiet pattern is expected. Subtle troughing in the trade flow will bring periodic upticks in clouds and showers, then next week an ITCZ pattern begins to build back in from eastern Micronesia. Winds will be mainly moderate for Yap and Chuuk and gentle to moderate for Palau through the week. Fresh to strong gusts are possible near heavier showers for Yap and Palau Wed-Thu, due to the passage of 93W. Seas are 4 to 6 feet and up to 7 feet at Chuuk and are expected to subside a foot or two over the weekend as trade swell weakens. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...High Rip Current Risk through Wednesday afternoon for GUZ001. MP...High Rip Current Risk through Wednesday afternoon for MPZ001>003. Marianas Waters...None. && $$ Marianas: Aydlett Tropical: Aydlett Micronesia: Hong/Cruz