Area Forecast Discussion
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FXPQ60 PGUM 182059
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Area Forecast Discussion
National Weather Service Tiyan GU
546 AM ChST Thu Oct 19 2017

.Marianas Synopsis...
Radar shows a general decrease in showers across the Marianas, but
satellite shows the circulation east of the islands, 93W, apparently
sinking southward along longitude 147-148E. Combined seas have
varied greatly at the buoys between 7 and 11 feet overnight, with
Ritidian currently at 6.7 feet, down from 10.7 feet around 11 PM,
while Tanapag spiked at 11 feet around 430 AM.

&&

.Discussion...
Have refreshed the wind and wave grids from the 12Z GFS, but the
overall forecast is little changed on that account. But I did
extend mostly cloudy skies beyond tonight based on the GFS, so
that mostly cloudy with isolated showers and thunderstorms is the
forecast through the next week.

&&

.Marine/Surf...
The marine forecast is little changed, with large swell from Typhoon
Lan expected to persist for several days. As the swell veers from
southwest to northwest over the next few days, high surf should
diminish for south facing shore by Friday, but remain hazardous for
west facing reefs for several days, possibly reaching 15 feet early
next week. In addition, surf will likely become hazardous on north
facing shores at some point early next week as the swell becomes
more northwest.

&&

.Tropical systems...
This morning, JTWC Invest 93W is a major source of uncertainty in
the forecast. The GFS still takes it out to the east to about 152E,
then takes it northwestward, to north of Agrihan, on Sunday. But
the ECMWF now takes it ESE to near 13N152E, then brings it back
WNW through the Marianas between Tinian and Rota on Sunday. This
seems unlikely, but we will keep 93W under close watch. To further
increase uncertainty, satellite seems to show 93W moving southward
along about 147.5E, but it also appears to be weakening with warming
cloud tops. And now, JTWC has just issued a Tropical Cyclone
Formation Alert on this system. Stay tuned for further developments!

&&

.Eastern Micronesia...
An unsettled weather pattern remains across the eastern region this
morning with weak circulations possible near Pohnpei, Enewetak and
east of Majuro. Patchy showers, isolated thunderstorms and gentle
winds are seen near Chuuk, Pohnpei, Kosrae and Majuro. Conditions
will remain unsettled the next few days as the weak disturbances
persist across the region. Models keep these disturbances weak with
little or no development through the weekend, but we will keep a
close watch on these. GFS shows a relatively drier trade wind pattern
building in from the east by early next week.

Majuro/Kalo Buoy shows combined seas around 4 to 5 feet this morning.
Wave models show a building northeast swell the next few days that
will generate higher surf along north and east facing reefs of
Majuro. Hazardous surf is not expected, but this wave event will
occur during the New Moon phase and its associated higher high tides.
Will watch closely for the threat of any minor coastal inundation
this weekend.

A long period WNW swell will build at Chuuk, Pohnpei and Kosrae this
weekend and into early next week. This will cause surf to rise to
near hazardous levels along west facing reefs of Chuuk on Sunday and
on Monday at Pohnpei.

&&

.Western Micronesia...
Typhoon Lan continues northward away from Yap and Koror. Infrared
satellite shows weather conditions are rapidly improving at both
locations. Showers and thunderstorms have persisted near Yap
overnight with heavy showers and thunderstorms just north. Showers
and thunderstorms will shift northward with a drier southwest wind
building in at Yap and Koror today. Isolated showers could still
generate brief gusty winds today. Fresh southwest winds will
become moderate by Friday. A wetter monsoon flow could develop
over the region this weekend, possibly in response to forecast
intensification of 93W centered east of Saipan.

Seas will remain hazardous for small craft through the week with
hazardous surf spreading to north facing reefs over the weekend.
Surf will gradually fall below advisory levels along south facing
shores of Yap and Koror tonight and Friday, but become hazardous
along north facing shores by Saturday. Large swell from Typhoon Lan
will affect both Yap and Koror much of next week.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Surf Advisory until 6 PM CHST Tuesday for GUZ001>005.

Marianas Waters...Small Craft Advisory until 4 PM CHST Sunday for
     PMZ151>154.

&&

$$

Middlebrooke/M. Aydlett



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