Area Forecast Discussion
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Area Forecast Discussion
National Weather Service Tiyan GU
747 PM ChST Sat Oct 21 2017

.Update...
The Tanapag buoy near Saipan has been showing increasing combined
seas the past several hours with combined seas touching around 15 ft,
most of that comprised of a 13 second west swell. The max wave plot
has also shown waves topping out around 25 ft since 2 PM this
afternoon. As a result, have issued a high surf warning for west
facing reefs of Tinian and Saipan through Monday afternoon. Dangerous
surf will persist along west facing shores the next couple of days
before slowly subsiding Monday night. Some coastal inundation and
increased erosion is expected, especially near times of high tide as
water piles along the reefs. Hazardous surf is still occurring along
western reefs of Guam and Rota, as well as south facing reefs of the
Marianas. Surf will soon rise along north facing reefs of the islands
as the long-period west swell turns to the northwest in the coming
days.

&&

.Prev discussion... /issued 645 PM CHST Sat Oct 21 2017/

Marianas Synopsis...
A very nice day across the Marianas with a ridge, between Super
Typhoon Lan near 21N131E and Tropical Depression 27W southeast of
the Marianas near 12N149E. Combined seas have risen to between 10
and 14 feet.

Discussion...
It was a spectacular Saturday across the region today, but this
was probably the nicest day out of the next several. There is
still some uncertainty with regard to Tropical Depression 27W.
Most of the guidance is now moving the system south of Guam early
next week. Currently it is very slow moving. Some guidance is a
bit faster than others. A few outliers remain in the track
guidance. Latest model data also suggests that there will be some
showers by morning tomorrow, but differs on coverage. For now
have gone with scattered. Rain will ramp up during Monday and into
Tuesday, depending on the speed and path. There will be at least
a few period of widespread and heavy rain somewhere in the Monday
to Wednesday time frame, but will hold off introducing those into
the forecast for now, until the timing can be better pinpointed.
After midweek, conditions will improve as the tropical depression
moves away.

Marine...
Super Typhoon Lan continues to move northeastward toward Japan.
Its only impacts for our region is long period swells, mainly on
west facing reefs. A rise on north facing reefs is possible as
well, and will have to be monitored for a possible high surf
advisory. South facing reefs could be impacted by the path of TD
27W. A high surf warning may be needed for west facing reefs. Small
craft advisories remain in effect. Wind will increase on Monday
through midweek, based on the current track of Tropical Depression
27W, though weak, will likely hit small craft criteria. Surf
looks to remain hazardous and rip currents high through at least
midweek.

Tropical systems...
Tropical Depression 27W was near 12N and 149E, southeast of the
Marianas. It is moving west very slowly. The current forecast
keeps it fairly weak, as a depression until it is west of the
Marianas when it could strengthen. The latest forecast also brings
it south of Guam on Monday or Monday night. There is still some
uncertainty in the models regarding the track and speed and we
would like to see model consistency remain before overall
confidence increases. Given the proximity to the area and the lack
of continuity, the system will be monitored for changes.

Eastern Micronesia...
The axis of a surface trough remains stretched along 8N-10N from
northwest Pohnpei State to the northeast Marshall Islands. Also, a
tropical disturbance with locally heavy showers is centered near
Enewetak Atoll. Areas of showers and isolated thunderstorms will
continue developing near the trough, especially over much of Pohnpei
State. So far, heavier showers are remaining north of Kosrae. GFS
Model indicates the trough will move slowly westward in the coming
days, so that showers will continue to affect Majuro through Sunday,
and persist at Pohnpei through Monday or Tuesday. Improving weather
is expected across eastern Micronesia for most of next week.

Long-period west-northwest swell generated by distant Typhoon Lan
will begin to arrive at Pohnpei and Kosrae waters around Sunday
night. This swell will cause surf to build slowly Mon-Wed, but
hazardous surf is not expected for now. Majuro/Kalo Buoy currently
shows combined seas near 6 feet.

Western Micronesia...
Tropical Depression 27W is northwest of Chuuk near 12N149E and
moving away slowly west-northwest. Although 27W is moving away from
Chuuk, models indicate unstable weather will develop over Chuuk
State through the next few days as a pattern of surface troughing
arrives from the east. Weather should be improving for Chuuk by
later in the coming work week. Long-period west to west-northwest
swell generated by distant Typhoon Lan will gradually build across
Chuuk State in the coming days and surf could approach advisory
criteria late Monday or Tuesday.

Palau and Yap will be in a fairly dry southwest flow the next couple
of days as distant Typhoon Lan moves toward Japan. This pattern will
change by Monday as Tropical Depression 27W currently southeast of
Guam moves farther west. A moist flow is expected to develop over
western Micronesia, giving Yap scattered showers and thunderstorms
Tue and Wed. Koror forecast remains isolated for now, but showers
could be scattered at times for Koror next week.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Surf Advisory until 6 PM CHST Wednesday for GUZ001-002-005.

     High Surf Advisory from 6 PM Monday to 6 PM CHST Wednesday for
     GUZ003-004.

     High Surf Warning until 6 PM CHST Monday for GUZ003-004.

Marianas Waters...Small Craft Advisory until 6 PM CHST Tuesday for
 PMZ151>154.

&&

$$

W. Aydlett



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