Area Forecast Discussion

Marianas Synopsis

Mostly mid and upper-level clouds are seen across the Marianas this evening. Gentle to moderate winds prevail over Tinian and Saipan, with mostly gentle winds at Rota and Guam. Altimetry shows seas between 5 and 7 feet west of the coastal waters, with 11 to 17 feet seen east of 150E.

Discussion

A very dry pattern is now entrenched across the Marianas, with only spotty light showers evident on the latest radar data. Typhoon In-fa is seen to the east-northeast of Saipan, moving steadily toward the northern CNMI. However, convection and the wind field remain close to the center. Therefore, not expecting to see much activity south of Alamagan. Over the next week, POPs will range from 0 to 20 percent across the Marianas. For Sunday and Monday, light and variable winds will allow island convection to develop over Guam and possibly the CNMI. By Tuesday, winds look to be a bit stronger, decreasing the likelihood of island effect showers.

Marine/Surf

For the marine forecast, large, long-period northeast swell emanating from TY In-fa will result in a quick increase in seas over the region, with Tinian and Saipan waters seeing seas up to 14 feet, especially over the northeast waters, and Rota and Guam seeing seas of 10 feet. Altimetry data confirms seas up to 16 feet well east of the waters. Seas are expected to exceed 10 feet across the region shortly after midnight tonight, peaking during the evening Friday. Seas will then begin to subside, likely dropping below 10 feet Saturday morning. As the seas build, expect increasing surf heights as well. Surf could build as high as 18 feet along north and east facing reefs of Tinian and Saipan, and up to 14 feet for Rota and Guam. This surf will result in a high risk of dangerous rip currents Friday through Saturday. Minor coastal inundation is possible along north and east facing reefs of Tinian and Saipan as well. Gentle to moderate winds will continue for Tinian and Saipan, with mostly gentle winds for Guam and Rota.

Tropical Systems

As of 4:00 PM ChST earlier today, Typhoon In-fa (30W) was located near 15.9N 154.6E and moving west at 18 MPH. In-fa is expected to make a slight turn to the west-northwest through Friday before turning northwest early Saturday morning. Maximum sustained winds decreased to 130 MPH, still remaining a Category 4 typhoon, and will maintain this intensity as it passes near or over Agrihan of the northern CNMI Friday evening. Once In-fa emerges west of the Marianas, maximum sustained winds may gradually decrease, potentially becoming a Category 3 typhoon Saturday afternoon as it makes a sharp turn to the west towards the Philippine Sea. Damaging winds of at least 39 MPH are expected within the next 24 hours at Agrihan, Pagan, and Alamagan, with typhoon conditions of 74 MPH winds or more also expected. As such, the previously issued Typhoon Watch was upgraded to a Typhoon Warning for these three islands. As for Guam, Rota, Tinian, and Saipan, a southward shift in In-fa's track is always possible, but direct impacts remain highly unlikely at this time.

For more information on Typhoon In-fa, please refer to bulletins from the Joint Typhoon Warning Center under WMO header WTPN31 PGTW and from the National Weather Service under WMO headers WTPQ31 PGUM and WTPQ81 PGUM.

Eastern Micronesia

Pohnpei will see a short period of scattered showers and light to gentle southwesterlies tonight with limited convection as cross- equatorial winds flowing into Typhoon In-fa moves through the area, mainly concentrated to the west. Kosrae sits within broad, anticyclonic flow, bringing isolated showers and light to gentle variable winds tonight. This pattern will encompass Pohnpei, so light and variable winds and isolated showers are to be expected Friday. Majuro is situated within northwesterly convergence, so scattered showers and isolated thunderstorms are forecast tonight through Friday before this feature eventually moves west and dissipates. Then, ridging will occur over all three forecast points, so isolated showers and light to gentle winds of differing directions will prevail through the rest of the forecast period, with the possible exception of Kosrae from Saturday night through Tuesday as a fragment of the monsoon trough potentially re-establishes itself nearby.

Benign seas will continue through this weekend with swells out of the northwest and southeast for Pohnpei and Kosrae, and out of the east for Majuro. Then, distant mid-latitude features may send a long- period, slightly elevated north swell to the region next week. Combined seas could stay below hazardous criteria, but surf heights along north and east facing reefs may warrant advisories. Those with marine interests should remain updated with the forecast over the next several days.

Western Micronesia

Typhoon In-fa is well north of Chuuk, with only spotty showers extending into northern Chuuk State due to modest convergence along the gentle to moderate southwesterly flow. As In-fa moves northwest, farther away from the region, a broad area of high pressure looks to set up across the region, leading to drier conditions Friday and Saturday for Chuuk, extending westward towards Yap and Palau over the weekend. A trough builds in over Chuuk soon after, resulting in a period of scattered showers around Sunday afternoon, through Sunday night or early Monday morning. Showers then shift across southwest Chuuk State and eastern Yap State, trending down to isolated across Chuuk Lagoon early next week, Fairly quiet conditions persist at Palau and Yap this evening, while an increase in convergence southeast of weak circulation brings about scattered showers Friday and Friday night. The potential of isolated thunderstorms looks to be closer to Palau, but have also included isolated thunderstorms for Yap on Friday. The aforementioned area of high pressure is expected to maintain light to gentle winds and drier weather over the weekend and early next week. The exception will be during the afternoon times at Palau, where weak winds and daytime heating may lead to island- effect showers across parts of Babeldaob.

Combined seas are around 3 to 4 feet at Yap and up to 4 to 6 feet at Chuuk. North to northeast swell emanating from In-fa is expected to build seas to 4 to 6 feet across Yap and Palau over the weekend and next week, possibly reaching 7 feet at Yap early next week. Additonally, the primary swell period of up to 13 to 14 seconds will also support elevated surf along north-facing reefs early next week. Meanwhile, seas and surf at Chuuk gradually trend down over the next few days as In-fa moves farther away, with seas decreasing to 3 to 5 feet over the weekend and elevated surf up to 8 feet along north- facing reefs looks to trend down to around 5 feet over the weekend.

GUM WATCHES / WARNINGS / ADVISORIES

GU...High Rip Current Risk from late tonight through Saturday afternoon for GUZ001.

High Surf Advisory from 2 AM Friday to 4 PM ChST Saturday for GUZ001.

MP...High Rip Current Risk from late tonight through Saturday afternoon for MPZ001>003.

High Surf Advisory from 2 AM Friday to 4 PM ChST Saturday for MPZ001.

High Surf Warning from 1 AM to 4 PM ChST Friday for MPZ002-003.

High Surf Advisory from 4 PM Friday to 4 PM ChST Saturday for MPZ002-003.

Marianas Waters...Small Craft Advisory from 2 AM Friday to 4 PM ChST Saturday for PMZ151-152.

Small Craft Advisory from 1 AM Friday to 4 PM ChST Saturday for PMZ153-154.