Area Forecast Discussion
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FXPQ60 PGUM 222136

Area Forecast Discussion
National Weather Service Tiyan GU
730 AM ChST Mon Oct 23 2017

.Marianas Synopsis...
Tropical Depression 27W is about 390 miles southeast of Guam this
morning. Only a few showers and thunderstorms prevail over the local
area this morning.


Still some debate on what Tropical Depression 27W will do. It did
move to the southeast overnight and resumed a more westward track
early this morning. The convection is not that close to the center
and so the system will be slow to increase in intensity which is
what the models and JTWC forecast show. Models do seem in fair
agreement with 27W passing south and southwest of Guam sometime on
Tuesday. It may become a tropical storm as it passes on Tuesday.
It is not certain if it will reach this intensity since the system
still is undergoing some wind shear. One thought to consider is
that after looking at the surface analysis is that despite the
small insignificant size of 27W it is embedded inside a larger
circulation pattern. Also looking at the satellite imagery there
are hints of this larger circulation pattern. Could the
depression quickly gel into a better organized system. At this
time it does not appear to be ready to do this and models suggest
if it does, it will be passed the local area. At the very least
27W will bring some showery weather with the possibility of gusty
winds tonight through Wednesday night.


Observations from Ritidian show sea heights between 10 to 15 feet
and between 11 and 17 feet at Tanapag with seas over 20 feet earlier
this morning. With Tanapag near High Sea Warning criteria left
the warning out through the day.

Surf on north facing reefs was 10 to 12 feet and 12 to 15 feet on
west facing reefs. Swells have been decreasing but not a comfortable
enough margin exist to cancel the high surf warning on west facing
reefs and so extended that through the day.

The swell from distant Typhoon Lan will decrease slowly and allow
the hazardous seas and surf to persist through Wednesday. Depending
on what Tropical Depression 27W does...seas and surf could remain
hazardous for a longer period.


.Tropical systems...
Tropical Depression 27W is southeast of Guam this morning and is
poorly organized. It is expected to slowly develop and may become a
tropical storm as it passes southwest of Guam on Tuesday.


.Eastern Micronesia...
A surface trough extends northeastward from Tropical Depression 27W
northwest of Chuuk near 9N148E to end north of Wake Island. Modest
converging south winds south of the trough axis will keep some
showers and a thunderstorm or two near Pohnpei today. By this
evening, a surface ridge building in from the east along with a mid-
level dry air mass should spread a drying trend already over Kosrae
and Majuro westward into Pohnpei. Farther east of the Date Line near
170W, the leading element of trade-wind convergence zone is present.
This feature could be the next weather maker for the region after

Long-period northwest swell generated by distant Typhoon Lan is
beginning to arrive at Pohnpei and Kosrae this morning. It will
cause surf to build over the next couple of days. Altimetry trend
reveals seas of 9 to 12 feet stalling near 150E. They have been
advancing more southward than eastward. Therefore decided to hold
off on a high surf advisory for Pohnpei and Kosrae for now.
Majuro/Kalo Buoy currently shows combined seas of 5 to 7 feet.


.Western Micronesia...
Tropical Depression 27W is just northwest of Chuuk near 9N148E and
is gradually turning west this morning. It is beginning to show
signs of a monsoon depression, with convergent bands of northwest
and southwest monsoonal winds forming. The band of converging
southwest winds over Chuuk will continue to trigger periodic heavy
showers and isolated thunderstorms there thru Tuesday. As 27W moves
west-northwestward over the next few days, the band of converging
northwest winds will also shift westward over Yap by late this
evening and Koror by Tuesday. For Chuuk, this will allow a surface
ridge to build westward from near Pohnpei and introduce more stable
conditions by midweek. Once 27W has tracked past 140E and becoming
better organized near midweek, it will probably enhance a westerly
to southwesterly monsoon surge across Palau and western Yap State.
This will sustain inclement weather near Koror and Yap during the
second half of this week.

Large Long-period northwest swell generated by Distant Typhoon Lan
will maintain hazardous surf across Palau and western Yap State
through mid-week. Surf across Chuuk State will also rise and reach
hazardous levels by this afternoon. Sea conditions from Palau
eastward to Chuuk will be hazardous for small craft operation for
the next few days.


GU...High Surf Advisory from 6 PM this evening to 6 PM CHST Wednesday
     for GUZ001>005.

     High Surf Warning until 6 PM CHST this evening for GUZ001>004.

Marianas Waters...Small Craft Advisory from 6 PM this evening to 6 PM CHST
     Wednesday for PMZ152>154.

     Hazardous Seas Warning until 6 PM CHST this evening for

     Small Craft Advisory until 6 PM CHST Wednesday for PMZ151.



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