Area Forecast Discussion
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FXPQ60 PGUM 212148

Area Forecast Discussion
National Weather Service Tiyan GU
619 AM ChST Sun Oct 22 2017

.Marianas Synopsis...
Clouds have spread in over the area from heavy showers and
thunderstorms that developed overnight near TD 27W. A significant
amount of lightning was observed during a large flare up after
midnight and now another plume is producing lightning about 250
miles southeast of Guam. TD 27W is holding steady at 35 mph and
moving WSW at 8 mph. Seas are still up with buoys showing 11 to 17
ft seas across the region, mainly from a large west swell from
distant Super Typhoon Lan.


TD 27W is still expected to slow down today before turning NW on
Monday and pass by south of Guam Tuesday. This will keep the
Marianas under a wet and convectively unstable pattern. While
scattered showers are expected at times through Monday night, the
wettest part of the week is still shaping up to be Tuesday through
Wednesday as winds strengthen over the Marianas and turn
southeast. For a couple of days now, models have held that trend,
showing the most rainfall for the islands being on the eastern
side of 27W. Showers will decrease Wednesday night into Thursday.
Decided to increase the winds for Monday night through Wednesday,
including stronger wind gusts, but they may need to be increased
further depending on how quick 27W decides to strengthen.


Sea conditions, primarily the west swell, have been consistently
under-forecast by the models and are still averaging 3 to 6 feet
above guidance. Because of the lag time in altimetry data, am
relying on Ritidian and Tanapag buoys to assess the current size
of the west swell. Seas staying between 15 and 18 feet this
morning prompted the elevation of small craft advisory to a
hazardous seas warning for the CNMI waters that will end Monday
morning. This will likely come down only with favorable downward
trends in the buoy data. Ritidian buoy will need to be closely
watched today, but for now, it has been showing 11 to 14 foot
seas. The small craft advisory following the hazardous sea
warning will end Tuesday evening, but that will likely need to be
extended until around Thursday as the depression will cause an ESE
swell and wind waves to build on Tuesday. With that said, west
swell has increased, thus, high surf warning is in effect for all
west- facing reefs of the Marianas. Surf will increase on north
reefs the next couple days as swell shifts to northwest.

Boaters, especially in smaller boats, should refrain from heading
out. Conditions will be hazardous for the next several days. Surf
and rip currents will be dangerous, particularly on west-facing
beaches and reefs. Be safe and stay out of the water and away from
the edges of reefs.


.Tropical systems...
Super Typhoon Lan has moved north of 25N, therefore NWS Guam will
no longer be issuing public advisories on it. But the massive wind
field of Lan will still cause long-period west and northwest swell
to affect the Marianas until around Thursday.

Tropical Depression 27W will be in the area for a few days before
moving along to the west Tuesday. Its passage to the south of Guam
will bring increasing east to southeast winds and moisture Monday
through Wednesday. TD 27W is expected to strengthen some, possibly
becoming a tropical storm sometime Monday. Still, changes with the
forecast track and intensity are expected before it moves away
from the area, so it is very important to keep a close eye on all
the latest information.


.Eastern Micronesia...
A surface trough stretches across eastern Micronesia just north of
10N. Low-level convergence south of the trough will produce scattered
showers and isolated thunderstorms at Pohnpei through Monday night
and today at Majuro. Kosrae is too far south of the convergence zone
and only isolated showers are expected there. Southerly winds prevail
across most locations south of the trough. The trough will wither as
time goes on and trade-winds will push into eastern Micronesia with
east winds returning to Majuro tonight and on Tuesday at Pohnpei and


.Western Micronesia...
The monsoon trough stretches across the Western Pacific well north
the Palau and Yap although it passes through Tropical Depression 27W
just northwest of Chuuk at 11N148E and continues north of that
location at around 11N. Low-level convergence south of the trough
will bring scattered showers and isolated thunderstorms to Chuuk this
afternoon through Tuesday.

Tropical Depression 27W will move west along the trough and pass
north of Yap and Palau on either Tuesday or Wednesday. At that time
27W may be a tropical storm. 27W will increase the winds in the
monsoon flow and 10 to 20 knot winds are expected at Palau tonight
through Thursday and Tuesday through Thursday at Yap. 27W will also
increase convergence and deep layer moisture over these two locations
with scattered showers and isolated thunderstorms expected at Palau
Tuesday and at Yap Tuesday and Wednesday.

Long-period swell from Distant Super Typhoon Lan will produce
hazardous surf across Palau and Yap through at least mid-week.
The swell direction will be mainly from the west today but will
gradually shift to the northwest by tonight. Expect hazardous surf on
west facing reefs of Palau and Yap today shifting to north by tonight
at both locations. Surf heights of 7 to 9 feet at Palau today will
increase to 9 to 11 feet tonight and at Yap surf of 8 to 11 feet will
build to 10 to 12 feet Monday.


GU...High Surf Advisory from 6 PM Monday to 6 PM CHST Wednesday for

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

Marianas Waters...Small Craft Advisory from 5 AM Monday to 6 PM CHST
     Tuesday for PMZ152>154.

     Hazardous Seas Warning until 5 AM CHST Monday for PMZ152>154.

     Small Craft Advisory until 6 PM CHST Tuesday for PMZ151.



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