Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
631 FXPQ50 PGUM 150808 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 608 PM ChST Mon Jul 15 2024 .Marianas Synopsis... Trade winds and isolated showers are seen across the Marianas waters. Further east, showers and isolated thunderstorms are seen approaching the waters. Marianas buoys are reporting 4 to 5 feet. && .Discussion... No major changes to the forecast. A few showers are seen near the Marianas which is expected to pass through tonight with only isolated showers. A gradual increase in shower potential is expected trending from scattered to numerous showers Tuesday through Wednesday night as the monsoon trough lifts north and interacts with a descending Tropical Upper Tropospheric Trough (TUTT). Thursday through Sunday`s forecast is the same as yesterday`s with a decreasing trend of showers, but possible we may see slightly higher potential for showers for the islands through the weekend, and higher gusts for Saipan and Tinian on Thursday, based off of GFS. For now, keeping closer to the ECMWF solution for the longer term. .Marine... Gentle to moderate trades are expected across the Marianas through Friday, becoming fresh at times Tuesday night and with occasional strong gusts starting Thursday. Seas of 4 to 5 feet today will become 5 to 7 feet by end of the week. A west swell is expected to start building Tuesday evening as a result of the monsoon activity near Palau. && .Eastern Micronesia... The Inter-Tropical Convergence Zone (ITCZ) remains fairly active across the Marshall Islands and Kosrae, generating scattered to numerous showers and isolated thunderstorms. Over by Pohnpei, most of the shower and thunderstorm activity is north of Pohnpei as the greatest convergence is associated with the trade-wind surge north Pohnpei near 10N, but some scattered showers are still developing around Pohnpei, and more showers and thunderstorms will move in from Kosrae later tonight. As showers and thunderstorms near Kosrae and Majuro continue to push westward, expect numerous, locally heavy showers to develop across Kosrae and Pohnpei Tuesday, while showers are expected to be more scattered in coverage across Majuro and the northern Marshall Islands. The trade- wind surge however, will strengthen over the Marshall islands later tonight and Tuesday, bringing breezy conditions of 20 to 25 mph across the Marshall Islands. Winds will then start to weaken Tuesday night. The ITCZ is still expected to slowly fragment into a series of troughs later this week as models show the ITCZ losing its southeast winds. This pattern will still support showers and thunderstorms across the region, with some being locally heavy showers possible, but showers will start to cluster more around the troughs allowing for greater variation in shower coverage from day to day. Altimetry data shows seas are 4 to 5 feet around Pohnpei and Kosrae, and pushing up to 6 to 8 feet around the Marshall Islands, where winds are slightly stronger due to the trade-wind surge. As the trade-wind surge rebuilds across the Marshalls tonight and Tuesday, seas are expected to build up to 9 feet with sustained winds around 20 knots. This is just below small craft conditions of 10 feet and/or winds of 22 knots or greater, so held off on an advisory for Majuro, but kept the headline in the forecast for Majuro`s coastal waters to highlight the potential for choppy seas tonight and Tuesday, with winds then decreasing for the second half of the week. Otherwise, seas are expected to build a foot at Pohnpei and Kosrae over the next few days with some slightly stronger winds near heavier showers and thunderstorms. && .Western Micronesia... The monsoon trough, extending across the Philippines, passes through JTWC`s Invest 91W near 8N129E, and continues southeast, across the main islands of Palau and south of Yap State, ending near EQ152E. The region is divided between monsoon west-southwesterly flow, over Palau, colliding with the trade-wind pattern across Yap and Chuuk. Showers are slow to build back into the main islands of Palau, so delayed the onset of numerous showers to late tonight. 91W is diverting some of the monsoon flow, making it difficult for showers to reach Palau lately. 91W is currently rated "low" for development and will not have any impact to the region since it is off to the west. Overall, a fairly active monsoon pattern will persist, creating unsettled weather for the region. Model guidance shows a gradual north to northwest shift of the monsoon trough, maintaining showery monsoon weather in far western Micronesia, with locally heavy rain and gusty winds possible for Palau and Yap through Wednesday night. Between the monsoon and trade-wind patterns, a broad trough is traversing Chuuk tonight and is generating widespread showers and a few thunderstorms. Showery weather persists through the week with the ITCZ pattern remaining across eastern and central Micronesia, possibly fragmenting near Chuuk late week. Even so, trade-winds troughs passing through the ITCZ are still expected to heighten shower and thunderstorm activity at times. Altimetry and scatterometry data show seas of 3 to 6 feet and light to moderate winds. However, localized effects of heavier showers will still lead to choppy seas across Palau and Yap over the next few days. Southwest to west winds and swell in far western Micronesia will further develop, allowing for seas to build up to 7 feet around Wednesday or Thursday, especially for Palau, while seas near Chuuk look to stay between 4 and 6 feet. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: Hong Micronesia: Schank/Cruz