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Marianas Synopsis

Radar imagery shows isolated showers west of the Marianas and over southern Guam coastal waters. There is also a broad area of isolated to scattered showers further upstream, associated with trade-wind convergence along a shallow trough. Buoy and altimetry data show combined seas of 8 to 10 feet.

Discussion

A trade-wind surge continues across the region, reinforced by the pressure gradient between Tropical Storm Hagupit (05W) to the south and high pressure to the north. Scatterometry data from this morning depicts a broad area of fresh to strong trade winds, extending east of 145E to roughly 155E, from around 10N to 20N. Wind speed convergence in the trades produced isolated to scattered showers across the Marianas through the morning hours, and mostly quiet conditions prevail this evening. As TS Hagupit moves west-northwest, gradually strengthening over the next few days, periods of low-end scattered showers are expected for Guam and Rota, associated with increased convergence into the far northern periphery of the system. A fairly dry trade-wind pattern is then expected through the weekend, before the regional pressure gradient and wind speeds diminish early next week.

Marine/Surf

High pressure north of the region and strengthening Tropical Storm Hagupit (05W) to the south will bring mostly fresh trade winds tonight through Thursday afternoon, before easing slightly to moderate to fresh. This will maintain combined seas around 8 to 10 feet, and a Small Craft Advisory remains in effect through Thursday afternoon. Additionally, light south-southwest swell will build by a foot or two through Thursday night as TS Hagupit strengthens south of the Marianas. There is a high risk of rip currents along east- facing reefs due to the elevated trade swell and associated surf, which is expected to persist into the weekend. There is a moderate risk along north and south-facing reefs, in part due to the effects of island wrap-around.

Tropical Systems

At 400 PM ChST, 0600 UTC, the center of Tropical Storm Hagupit (05W) was located near Latitude 6.9 degrees North and Longitude 145.8 degrees East. Himawari visible satellite reveals a very disorganized, lopsided storm, with a broad convective complex located along its northeastern to eastern flank where it interacts with the upstream trade-wind flow. Although it strengthened slightly this afternoon, scatterometry this morning depicted a very similar structure, with near-gale winds seen within its northeast quadrant with a narrow embedded area of tropical storm force winds. Hagupit is moving west- southwest at 13 mph. It is expected to make a slight turn toward the west tonight, before shifting toward the west-northwest Thursday with a slight decrease in forward speed. Maximum sustained winds remain at 40 mph. Hagupit is forecast to maintain this intensity tonight, then strengthen Thursday and Thursday night, possibly becoming a strong tropical storm by Friday. Tropical storm force winds extend outward from the center up to 100 miles to the northeast.

A circulation is also seen over eastern Micronesia to the south of Pohnpei and Kosrae, dubbed Invest 94W by the Joint Typhoon Warning Center (JTWC). 94W remains categorized as a sub-low, meaning it is not expected to develop into a significant tropical cyclone over the next few days. Moderate to deep convection is seen within the westerlies to its south, with nothing organized about the center. Invest 94W remains weak, and may be closed later this evening.

For more information on Tropical Storm Hagupit, please refer to bulletins from the Joint Typhoon Warning Center under WMO header WTPN31 PGTW and from the National Weather Service under WMO header WTPQ31 PGUM.

Eastern Micronesia

Invest 94W and an enhanced Intertropical Convergence Zone (ITCZ) continue to bring an unstable weather pattern to the region. Scattered showers and isolated thunderstorms are visible on satellite over Pohnpei, Kosrae, and Majuro. Showers are expected to become heavier and more widespread by late tonight. Gusty winds of 25 to 30 kts are possible during thunderstorms for Pohnpei and Kosrae due to their proximity to Invest 94W. The widespread showers and isolated thunderstorms are expected to persist through at least Friday as Invest 94W appears likely to merge with the ITCZ and dissipate. By the weekend, the ITCZ is expected to drift northward away from the islands and weaken, so rainfall coverage is expected to decrease, bringing a drier regime to Pohnpei, Kosrae, and Majuro early next week. A persistent trade-wind pattern will continue to produce elevated seas and surf along east and northeast waters and exposures, but not quite reach hazardous surf levels at Pohnpei and Kosrae. The seas are expected to slowly diminish as the trade swell and localized wind waves weaken over the weekend.

Western Micronesia

A busy pattern is now in place across western Micronesia. At 1 PM, Tropical Storm Hagupit was centered near 7N146E, moving west- southwest through eastern Yap State. For more information on TS Hagupit, see the Tropical Systems section above. Latest satellite imagery shows partly cloudy skies with isolated showers across Yap and Palau. For Weno, Chuuk, cloudy skies with numerous locally heavy showers and isolated thunderstorms prevail.

For Chuuk, numerous locally heavy showers are expected to continue across Weno through tonight. This convection is found to the east of TS 05W. As 05W moves west, convection will decrease overnight, becoming scattered showers and isolated thunderstorms by Thursday morning. POPs (Probability of Precipitation) look to remain around 50 percent through Thursday, with isolated showers expected Thursday night as 05W moves far enough west. Late in the week and over the weekend, convection looks to again increase, with POPs reaching up to 40 percent as the trailing Near-Equatorial Trough (NET) and Invest 94W move closer to Weno.

For Yap and Palau, TS Hagupit will be the main weather producer for the next few days. Latest forecast track for TS Hagupit has the storm moving through just south of Yap Friday. Tropical storm force winds are possible at Yap Proper as early as Friday morning. As Hagupit approaches Yap Proper, convection will gradually increase, with showers becoming scattered at Yap Thursday and Palau Thursday night. Widespread locally heavy showers are expected at Yap Proper Friday night into Saturday. For Palau, the latest track for Hagupit keeps the storm to the north, with increased convection remaining over Palau through the weekend, with POPs of around 50 percent through Saturday. Both Yap Proper and Palau should see POPs begin to decrease Sunday and falling to around 20 percent Monday.

For the marine forecast, Palau and Chuuk should see seas below Small Craft Advisory levels, however, choppy seas will be possible for Chuuk, especially near heavier showers and thunderstorms due to gusty winds. Seas look to be around 6 to 8 feet through the weekend for Chuuk and between 4 and 7 feet at Palau. Light to gentle winds will prevail at Palau, with mostly gentle to moderate winds with occasional fresh winds at Weno. For Yap Proper, seas ware expected to build as Hagupit approaches, possibly reaching as high as 11 to 16 feet. Tropical storm force winds are possible as early as Friday morning, and could reach as high as strong tropical storm force Friday night.

GUM WATCHES/WARNINGS/ADVISORIES

GU...High Rip Current Risk through Sunday afternoon for GUZ001.

MP...High Rip Current Risk through Sunday afternoon for MPZ001>003.

Marianas Waters...Small Craft Advisory until 5 PM ChST Thursday for PMZ151>154.