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Area Forecast Discussion
Issued by NWS Tiyan, Guam
Issued by NWS Tiyan, Guam
948 FXPQ50 PGUM 150846 AFDGUM Area Forecast Discussion National Weather Service Tiyan GU 646 PM ChST Thu Aug 15 2024 .Marianas Synopsis... Satellite imagery shows an overcast stratus and cirrus deck across the Marianas. Showers have remained isolated today with a few showers passing through the Northern Marianas. Ritidian and Tanapag buoys show seas between 3 and 5 feet. && .KEY MESSAGES... * Heavy rain, flooding, and gusty winds will be possible this weekend. * Uncertainty in timing and location of the heaviest rains still exist and will be worked out with future forecasts. * A Special Weather Statement has been issued and gives more detail on the upcoming heavy rain event. .Discussion... Satellite observations have shown robust convection to the east of the Marianas persisting throughout the day. This region of convection is where a Tropical Upper Tropospheric Trough (TUTT) is interacting with an Upper Level Anticyclone (ULAC) and a trade-wind trough. These features will generally move west-northwestward into the Marianas and will line up with an enhanced area of surface convergence that extends southward from Typhoon Ampil. These features should interact to produce an atmosphere favorable for numerous showers and isolated thunderstorms this weekend. Very heavy rainfall of 3 to 5 inches with locally heavier amounts will be possible for the Northern Marianas. For Guam there is quite a bit more uncertainty in the rainfall forecast, but locally heavy rain will still be possible. The GFS and ECMWF has flipped their position on the location of the heavy rain. The GFS keeps the bulk of the rain over the northeastern coastal waters of Saipan, while the high resolution ECMWF has the axis of heavy rain over Tinian and Saipan. The updated forecast package leans closer to hi-res ECMWF, which is more aligned with the ECMWF ensembles. Stay tuned as future forecasts continue to refine the timing and location of heavy rain. && .Marine... Combined seas of 3 to 5 feet could build to 4 to 6 feet early next week. Winds will generally be gentle around 10 knots, however, gustier winds of 20 knots will be possible near showers this weekend. A moderate risk of rip currents will continue to be expected along east facing reefs this weekend and into next week. && .Eastern Micronesia... Relatively quiet weather prevails over this region. ASCAT Analysis indicates weak surface troughing over the Marshall Islands, while weak surface ridging is between Pohnpei and Kosrae. A few lightning strikes were noted recently just north and northeast of Majuro, and a few lightning strikes were southeast of Kosrae. Models indicate the surface trough will continue to move westward and cause a few showers for Kosrae and Pohnpei in the coming days. Otherwise, just spotty to patchy showers are expected for Eastern Micronesia the next few days. The southeast swell from south of the Equator persists, especially for Kosrae, and Pohnpei and Majuro to a lesser extent. Utwe/Kosrae Buoy, just off the southeast coast of Kosrae, shows combined seas around 5 feet, with an occasional bigger wave, mainly from the southeast. This buoy should be monitored in case some waves arrive much bigger than model data suggests. && .Western Micronesia... After tonight`s showers, Palau and Yap are expected to see a much more pleasant weather regime through the weekend, followed by a series of trade-wind troughs bringing on-and-off showers during the first half of next week. Meanwhile, Chuuk will observe dryer, diffusive trades through Friday, before a broad surface trough moves in to bring patchy showers and some thunderstorms during the weekend. Shortly after, a wetter trade-wind pattern will ensue. Combined seas of 2 to 4 feet near Palau and Yap, in addition to seas of 3 to 5 feet near Chuuk, are expected to persist into next week. && .GUM WATCHES/WARNINGS/ADVISORIES... GU...None. MP...None. Marianas Waters...None. && $$ Marianas: Williams Micronesia: Simpson/Montvila