Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
158 FXPQ50 PGUM 180750 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 550 PM ChST Wed Sep 18 2024 .Marianas Synopsis... Satellite imagery shows the cloud cover this morning is starting to break up, but more showers and thunderstorms are developing to the east of the coastal waters. The airports across the Marianas are reporting are reporting light to gentle winds, while combined seas are 5 to 7 feet according to latest buoy and altimetry data. && .Discussion... An approaching Tropical Upper Tropospheric Trough (TUTT) and associated upper-level low will increase instability over the Marianas, helping to support the development of showers and thunderstorms along any weak troughs passing through. This will keep the risk for periods of scattered showers and isolated thunderstorms in the forecast through Thursday night. For Friday and Saturday, TUTT is expected be well northwest of the Marianas, allowing cloud cover and the potential for showers and thunderstorms to decrease. Potential for showers will start to increase again Sunday and Monday as models shows the next series of troughs moving in from the east. && .Marine... Mainly gentle trades will are expected through the weekend. The southwest to west swell will continue to decrease through the week allowing seas around 5 to 7 feet, to drop to 4 to 6 feet by Friday, and maybe 3 to 5 feet early next week. The decreasing swell will also allow surf to slowly drop along west facing reefs. Weak troughs and an approaching TUTT will keep scattered showers and isolated thunderstorms going at times through Thursday night, with the potential for showers and thunderstorms decreasing for Friday and Saturday. && .Tropical systems... Pulasan (15W) briefly became a typhoon earlier today but has since weakened back down to a tropical storm. Tropical Storm Pulasan is located near 24N130E this evening, just on the edge of WFO Guam`s area of responsibility and is expected to exit the region over the next few hours. Once Pulasan exits the region, tropical cyclone activity is not expected through the weekend and into early next week. && .Eastern Micronesia... A dry pattern is currently in place across Kosrae and Pohnpei, with wetter conditions seen across Majuro. A weak, but developing, ITCZ (Inter-tropical Convergence Zone) is seen over the Marshalls, stretching southeast from near Enewetak to southeast of Majuro. Scattered showers and isolated thunderstorms look to continue across Majuro as the ITCZ slowly meanders around the region. Majuro should see a drier pattern later this evening that will be fairly short- lived, as a trade-wind trough approaches from just east of the Date Line and interacts with the tail of the ITCZ, bringing increased showers back to the atoll for Thursday. There are a couple more troughs east of the Date Line that will move into the Marshalls by the end of the week. The ITCZ should also be a bit more developed by then. This should bring numerous/likely showers and a few thunderstorms to the atoll into Saturday. Convection should begin to wind down Saturday afternoon or evening, followed by a drier pattern. For Kosrae, a more moist pattern will move in by Thursday morning as the aforementioned ITCZ approaches. Along with the ITCZ, the series of troughs moving through the Marshalls look to interact with the ITCZ, resulting in increased convection there as well. However, POPs look to remain a bit lower for Kosrae. Farther west, Pohnpei will see similar conditions as Kosrae, though this will occur about 12 hours later than Kosrae. Over the weekend, all three locations will see drier conditions that could last through the middle of next week. Benign marine conditions look to continue through the next several days, with combined seas of 3 to 4 feet at Pohnpei and Kosrae, while seas could reach as high as 5 feet for Majuro. Light to gentle winds are expected through the beginning of next week. && .Western Micronesia... Satellite trends this afternoon show a gradual clearing of heavy showers and thunderstorms near Palau and Yap as ridging builds over the area behind the weakening monsoon trough, which is shifting further northwest with the movement of TS Pulasan. Still, areas of thunderstorm development are seen to the southeast and just west of Palau, maintained by low-level convergence within the southeasterly flow and strong divergence aloft. POPs were bumped up slightly for tonight given persistent convection and scattered showers across the area. Weak troughing and convergence in the easterly trades will keep scattered showers in the forecast over the next few days, with increasingly spotty coverage heading into the weekend. By early next week, a broad ridge will set up over far western Micronesia, leading to a drier pattern for the first half of next week. Farther to the east, satellite shows scattered to numerous showers developing to the north of Chuuk State along the axis of a broad trade wind trough. Thick upper-level cloud debris from these showers blankets the skies over Chuuk, but showers are isolated across the local area. Little change was made to Chuuk`s forecast. Isolated to scattered showers are expected for the next few days until a trough at the leading edge of the ITCZ increases showers come Friday. Little change needed for the marine forecast. The Babeldaob buoy off Palau`s east coast is showing 2 to 4 ft seas as it captures a south- southeast swell, which is expected to become a 3 ft east swell by the weekend. 3 to 5 ft seas near Chuuk will come down another foot or so by next week as winds become light to gentle. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: Schank Eastern Micronesia: Kleeschulte Western Micronesia: DeCou