FXPQ60 PGUM 240753

Area Forecast Discussion
National Weather Service Tiyan GU
553 PM ChST Mon Sep 24 2018

.Marianas Synopsis...Isolated showers and southwest winds prevail
over the Marianas this afternoon.


A few changes made to the forecast. Radar and satellite imagery
show only a few showers over the Marianas, and while lightning was
not detected over the local area it was fairly close, so kept
isolated showers and thunderstorms in the forecast through Tuesday
night. Models bring in drier conditions mid-week and expect only
isolated showers Wednesday through Thursday night. Models all suggest
that this dry period will be short lived. A circulation north of
Pohnpei this afternoon may bring back the showers by next weekend.
All of the models keep this circulation weak as it passes south of
Guam next Saturday. The models portray a large area of rain showers
north of the circulation. This would suggest scattered showers and
isolated thunderstorms Saturday and Sunday possibly persisting
through Tuesday. Not certain on the scattered showers but did
introduce mostly cloudy skies, isolated showers and thunderstorms
Friday through Monday.


Observations from Tanapag and Ipan buoys this afternoon show sea
heights between 3 to 4 feet. Tanapag has a southwest swell and a
northeast swell and Ipan shows a northeast swell. Altimetry shows
sea heights of 5 to 6 feet just west of the local islands so feel
4 to 6 feet is Representative.

Expect surf of 5 to 7 feet on west and 4 to 6 feet on south facing
reefs through Wednesday. The west swell increases in the second half
of the week. The increase in swell is likely from Typhoon Trami over
the Philippine Sea. Models continue to show swells for the typhoon
and the monsoon flow causing this increase in the the west swell.
The track forecast for Trami indicted that it may slow down in
forward motion, therefore remaining in the area longer. Predictions
are that it will increase in intensity and may become a super typhoon
for a short time. All this will equal an increased chance for Trami
to send large swells in the direction of the Marianas which will
increase the surf. This increase may be enough that surf may become
hazardous on west facing reefs by Thursday.


.Eastern Micronesia and Chuuk...

A surface trough runs eastward through a broad circulation, Invest
Area 94W, centered north of Pohnpei at 10N158E, across the northern
Marshall Islands through a weak circulation northeast of Majuro near
12N177E. Patchy showers persist over Chuuk State and the Marshall
Islands, while skies are partly cloudy at Pohnpei and Kosrae. The
GFS model indicates Invest 94W will drift toward south or southwest
during the next day or two, then move more westward later this week.
Weather conditions will remain unsettled across the region while the
circulations are nearby, and the situation will be monitored in the
coming days to see if the circulations or any areas of convection
become better organized.


.Western Micronesia...

Typhoon Trami continues to move WNW which is helping to pull the main
channel of moist monsoon flow away from Yap and Koror. Showers
quickly diminished overnight but another burst of showers and
thunderstorms is expected later today and tonight. Scatterometry data
indicate weaker winds in the region so have brought winds down to
below 20 kt for the week. Southwest winds will remain into late week,
but drier weather is expected to return by Tuesday. Perhaps as early
as this weekend, clouds and showers will return to Yap and Koror
areas as 94W approaches from Chuuk State. Long range models show it
passing between Yap and Guam over the weekend.

Surf remains hazardous along south and west facing reefs, but will
decrease tonight. Surf will rise along north facing reefs Wednesday
as long period swell arrives from the north from TY Trami.


Marianas Waters...None.


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