Small Craft Advisory 19, NWS Guam

Area Forecast Discussion
National Weather Service Tiyan GU
327 PM ChST Tue Sep 27 2016

.Synopsis...
Tropical disturbance 98W is centered about 550 miles east of Guam
and 470 miles east of Saipan near 15N153E, moving west-southwest
at 16 MPH. It is expected to move through the Marianas Wednesday.
This afternoon, however, the weather is fairly quiet with mainly
isolated showers, gentle north winds and seas near 4 ft.

&&

.Discussion...
Have re-worked the forecast to account for the passage of 98W
across the Marianas Wed and Wed night. The models are not showing
any development as 98W moves by, but it is currently producing
winds of 20-30 MPH north of the center and lots of rain and
lightning. Upper-level convergence may stifle 98W somewhat if the
models are correct, but 98W as it is now is enough to warrant a
forecast of locally heavy rain and moderate to strong winds. If
98W moves between Rota and Tinian as forecast, Guam and Rota will
likely see only moderate to fresh winds, but Saipan might see
winds reaching 25 mph at times Wed or Wed evening. Rainfall is
highly uncertain, but several inches is possible in the heaviest
rain.

The uncertainty factor is high in general in this forecast, as the
models insist on keeping 98W weak when it is currently looking a
lot like a tropical depression. The forecast was made to be a
little pessimistic, and if the models are correct there may be less
wind and rain than forecast. On the other hand, if 98W maintains
its present status as it moves by tomorrow, the forecast would
turn out to be pretty close. Island residents should monitor this
situation closely, as indeed we will be as well.

&&

.Marine...
As noted above, the forecast is based on 98W coming through in
much the same condition it is in now. Seas will rise in step with
the winds, and may reach near 9 ft in northern Saipan waters. For
now, however, high surf is not expected. A Small Craft Advisory
may be needed Wed-Thu, but probably only for Saipan waters. Anyone
planning boat trips in the Marianas waters should check the latest
forecasts and statements.

&&

.Tropical Systems...
98W has shown a marked daily convective cycle the last couple of
days, with increased deep convection and a lot of lightning during
the morning,then decreasing by late afternoon and evening. But
since Monday afternoon the overall condition of 98W has been
fairly steady with little change in its wind field. Upper level
winds are not especially favorable for development the next couple
of days with an upper low northeast of 98W. As a result, it
appears likely to move through the Marianas with little change in
intensity and possibly producing quite a bit of rain and lightning.

&&

.Eastern Micronesia...
Surface troughs continue to generate showers and thunderstorms
across the region. The first is near Pohnpei and the second is
near Majuro. Fair weather conditions at Chuuk will come to an end
overnight as the trough near Pohnpei approaches. Scattered showers
will reach Chuuk by Wednesday morning. Expect better weather
conditions at Kosrae to build in at Pohnpei on Wednesday in the
wake of the surface trough. The second trough will bring showers
and thunderstorms to Majuro tonight, continuing into Thursday.
This trough will slowly head west bringing more showers to Kosrae
by Wednesday night, and Pohnpei by Thursday night. A quieter
pattern is expected over the Marshalls by the weekend, but models
hint at another disturbance early next week.

Benign sea conditions expected the next several days. However,
PACIOOS Wave Run-Up Forecast shows higher seas late this week and
through the weekend with the New Moon phase. Water levels will be
high during high tides from September 29 through October 3.
Inundation is not expected at this time as combined seas will be
between 3 and 5 feet.

&&

.Western Micronesia...
Reasoning for the Chuuk forecast is included in the Eastern
Micronesia section above.

A surface trough and an upper-level trough are pairing together to
generate showers and isolated thunderstorms across Palau. Much of
this weather is just south and west of Yap now. Isolated showers and
thunderstorms will persist through tonight at Yap before drier
weather builds in. Scattered showers and isolated thunderstorms will
decrease by Wednesday, with fair weather by Thursday.

Fair weather and light/variable winds are expected Thursday at both
locations as the weather becomes influenced by a disturbance near
the Marianas. By Friday, this disturbance will reinvigorate the
monsoon flow over the islands. For now, models show little moisture
with the moderate southwest winds.

A moderate long-period northwest swell produced by distant Typhoon
Megi, now over Taiwan, will cause higher seas and surf across The
Republic of Palau and Western Yap State over the next couple of
days. However, both seas and surf should remain below hazardous
levels.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
Marianas Waters...None.

&&

Middlebrooke/M. Aydlett