High Surf in Guam, January 17 to 18, 2017

Area Forecast Discussion
National Weather Service Tiyan GU
549 PM ChST Mon Jan 16 2017

.Synopsis...
A weak stationary front transitioning into a shear line is present
over the Marianas. Mostly cloudy skies are seen on latest visible
satellite imagery over all the islands and waters. Isolated showers
and thunderstorms are evident on latest PGUA WSR-88D data over Guam
and Rota, while scattered showers and isolated thunderstorms are seen
on satellite over Tinian and Saipan.

&&

.Discussion...
The weak front, now quasi-stationary, over the CNMI is almost
transitioned into a shear line this afternoon. Scattered showers and
an occasional thunderstorm have occurred across the Marianas through
the day. Ahead of the front/shear line, light to gentle winds are
present over Guam due to the proximity of the neutral point. Weak
winds and a PWAT of 1.96 inches have allowed island convection to
spawn a couple of thunderstorms this afternoon. Island thunder will
diminish this evening. With the shear line, scattered showers
continue across the CNMI and are gradually inching their way toward
Guam. It appears that the shear line will continue to drift southward
over the next couple of days before stalling near of just south of
Guam Wednesday. Based on this, decided to increase showers for Guam
to scattered for Tuesday night and Wednesday. Also added
thunderstorms to all locations, as PWAT looks to remain at or above 2
inches for the foreseeable future, combining with strong converging
winds associated with the shear line.

Light winds over Guam and moderate winds over Rota, Tinian and
Saipan will gradually increase over the next couple days as well as
the shear line drifts south, with breezy to windy conditions likely
for Tinian and Saipan and breezy conditions likely for Guam. Overall,
an unsettled pattern is likely for the Marianas through the end of
the week.

&&

.Marine...
Largest changes to the forecast and grids lies in the Marine Zones.
Latest WW3 model guidance is trending 2 to 3 feet lower than what is
indicated at the Tanapag and Ritidian buoys this afternoon. Model
guidance shows seas of 6 to 7 feet, while Tanapag is currently
showing sea heights of 10.3 feet. Ritidian model guidance shows 5 to
6 feet while the actual buoy readings are currently at 8.1 feet and
rising. Therefore, made major changes to wave and swell grids over
the next few days, increasing seas to a peak of 10 to 12 feet
Wednesday afternoon. Based on this, issued a Small Craft Advisory,
effective now for Tinian and Saipan and effective Tuesday at 1 am for
Guam and Rota. Initial Small Craft Advisory criteria is for sea
heights, but as the shear line drifts further south, winds are also
expected to increase to advisory levels Tuesday. Current advisories
are through Friday morning, but based on wave model guidance, may
need to be extended.

A High Surf Advisory has also been issued, for north facing reefs,
valid at the same times as the Small Craft Advisory. As the north
swell produced by the approaching front gradually veer toward the
northeast over the next couple of days, the High Surf Advisory may be
extended to east facing reefs as well. Surf is expected to peak at 14
feet on Wednesday, so will need to be monitored in case it reaching
warning levels of 15 feet.

&&

.Eastern Micronesia...
The ITCZ remains active near and east of Kosrae, passing just south
of Majuro and extending to east of the date line. Visible satellite
imagery shows scattered showers and isolated thunderstorms within
this band. Relatively dry weather is seen at Kosrae and Majuro this
afternoon, while fair weather is seen across Pohnpei State. A trade-
wind disturbance within this band of showers is near the date line
and will bring more showers to Majuro and Kosrae on Tuesday. This
disturbance will head WSW with fewer showers expected at Pohnpei as
it passes to the south late in the week. Weekend forecast looks much
drier across the region.

WW3 model indicates a north swell will build at all locations and
combine with increasing trade-wind waves this week. High Surf
advisories may be needed by Tue night or Wed for Pohnpei and Kosrae,
but will probably not be needed at Majuro due to higher thresholds
for hazardous surf along north and east facing reefs.

&&

.Western Micronesia...
At Chuuk...A surface trough will head west across the state keeping
skies mostly cloudy tonight, but with isolated showers. Drier
weather is expected the next several days across the state as gentle
trades build thru the week.

Yap and Koror will keep fairly dry the next few days with a shear
line to the north and trade wind showers generally to the south.
Models do show a little more moisture later in the week.

WW3 Model shows a north swell building into Western Micronesia
Monday night and Tuesday. Marine conditions will need to be
monitored for possible advisories later in the week.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Surf Advisory from 1 AM Tuesday to 6 AM CHST Friday for
     GUZ001-002.

     High Surf Advisory until 6 AM CHST Friday for GUZ003-004.

Marianas Waters...Small Craft Advisory until 6 AM CHST Friday for
     PMZ153-154.

     Small Craft Advisory from 1 AM Tuesday to 6 AM CHST Friday for
     PMZ151-152.

&&

Kleeschulte/M. Aydlett