Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
392 FXPQ50 PGUM 201939 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 539 AM ChST Tue May 21 2024 .Marianas Synopsis... Satellite imagery showing partly cloudy skies and isolated trade showers. Latest altimetry data is indicating seas of 6 to 9 feet to the east of the coastal waters. && .Discussion... A tropical disturbance (Invest 93W) well south of the Marianas, continues to drift west over the coming days. Moisture and convergent flow along the northern flank of 93W looks to remain a bit further south then originally through. Have lowered shower potential and only bring scattered showers and isolated thunderstorms into Guam through Wednesday. Mainly isolated showers expected for the CNMI, even with a trade trough/surge moving through the Marianas. Another disturbance passing to the south on Thursday and Friday, along with a couple of trade troughs, will bring an slightly elevated chance of showers, mainly for Guam which will be closer to the convergent flow along the north side of the disturbance. && .Marine/Surf... A trade-wind surge and an elevated trade swell will move through the Marianas through Wednesday night. Winds will be moderate to fresh, with occasional strong gusts, and combined seas of 6 to 9 feet expected. Winds and seas may approach Small Craft criteria through Wednesday afternoon, but for now look to remain just below thresholds. Scattered showers and isolated thunderstorms will be possible at times over the Guam coastal water, especially the southern waters. Winds and seas diminish Thursday onward. A high risk for rip currents will persist through Wednesday along east facing reefs, with moderate risk along north facing reefs. Surf looks to lower beginning Wednesday night as the trade swell diminishes, with the rip risk possibly decreasing along north and east facing reefs. && .Fire Weather... Only a trace of rain fell yesterday at the Guam airport, and the KBDI rose to 569; well into the high category. Shower potential looks be a bit higher through the week, and a chance for a wetting rain will be possible, especially through Wednesday. Gusty winds expected through Wednesday as well, but with the elevated clouds and moisture, the fire threat looks to remain below Red Flag criteria. && .Tropical Systems... Invest 93W, centered near 3n146e, still remains broad and disorganized. Latest model guidance shows 93W moving to the west- northwest and slowly approaching Palau and Yap over the coming days. A robust trade trough, currently near Pohnpei, looks to move into western Micronesia and begin to interact with 93W around Wednesday or Thursday. If the trough remains as strong as it is now, this could be the kicker that 93W needs to begin development into a tropical cyclone, but this is unlikely to occur before passing through Yap and Palau by late this week. This system will continue to be watched carefully over the next few days. Residents of Yap State and the Republic of Palau should continue to monitor forecasts for any changes in expected conditions. && .Eastern Micronesia... A robust trough that is located between Pohnpei and Kosrae continues to march westward. This is expected to bring numerous AM showers to Pohnpei. These showers are anticipated to diminish in the afternoon to evening hours. Once this trough passes, a drier pattern will begin to set up for the rest of the work week. For Majuro and Kosrae, a fairly dry day is expected with isolated showers that may pop up as weak convergence may ignite some quick- moving and spotty convection. The Inter-Tropical Convergence Zone (ITCZ), has continued to farther degrade and has became hard to distinguish from the increased trade- winds converging in the area. Altimetry shows combined seas of 6 to 9 feet east of Majuro, 3 to 5 feet near Kosrae, and 6 to 8 feet near Pohnpei. Winds are expected to be gentle to moderate this week. && .Western Micronesia... Update: Satellite imagery and the ASCAT show that model guidance were lacking. Luckily, the forecast was very robust, so not a lot was needed to be changed. Currently, the main concern is how a incoming robust trough will interact with Invest 93W later in the week, if it interacts at all. Previous Discussion: The Near-Equatorial Trough and Invest 93W remain the focus across western Micronesia. Over the past several days, model guidance has gone through numerous changes in the expectations on development and movement of Invest 93W. Latest GFS and ECMWF, along with GFS ensembles, show little development of 93W until well northwest of Palau over the Philippine Sea. However, models have been moving 93W through Palau way too fast for the past several days. Therefore, model guidance was delayed by about 48 hours on expected movement. This would delay the increase in convection over both Yap and Palau until at least Thursday. Based on the very low confidence in the forecast for the area, decided to introduce 50 percent POPs (Probability of Precipitation) for Yap and Palau on Thursday, continuing at 50 percent through the weekend. As the actual movement and any possible intensification become more clear, the forecast will be corrected to account for any changes. For Chuuk, the slower movement of 93W will also affect how long scattered showers and isolated thunderstorms will remain over Weno. Based on the uncertainty of the movement of 93W, maintained scattered showers and isolated thunderstorms over Weno through Thursday, with POPs decreasing to around 20 percent at the end of the week. For the weekend, a broad, weak band of convergence developing over eastern Micronesia looks to drift into Chuuk State, bringing increased convection to Weno. Winds will be mostly gentle to moderate for Yap and Chuuk through the week, with short periods of fresh winds possible. Light to gentle winds will continue at palau through the week. Seas of 3 to 5 feet are seen on altimetry for Palau, with 6 to 7 feet at Chuuk and Yap. Seas are expected to remain at these levels through the week. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...High Rip Current Risk through Wednesday afternoon for Guz005. MP...High Rip Current Risk through Wednesday afternoon for Mobs>003. Marianas Waters...None. && $$ Marianas: Slagle Tropical: Slagle Eastern Micronesia: Bowsher Western Micronesia: Bowsher/Kleeschulte