Area Forecast Discussion
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FXPQ60 PGUM 180938
AFDPQ

Area Forecast Discussion
National Weather Service Tiyan GU
608 PM ChST Wed Oct 18 2017

.Marianas Synopsis...
Showers, thunderstorms, and gusty winds are present across the
Marianas and local airport observations still indicate some wind
gusts between 30 and 35 mph. These gusts are down from today`s
gusts that reached up to 45 mph. A poorly-defined circulation is
near Saipan close to 16N146E. A circulation seen in recent
composite radar imagery to the southeast of Tinian is likely a
midlevel circulation as surface wind observations across the CNMI
still reflect fresh southwest winds. Farther southwest, satellite
imagery shows surges of heavy showers and thunderstorms heading
northeast out of Yap State toward the Marianas.

&&

.Discussion...
Short term weather will be driven by the eastward-moving circulation
near Saipan with periodic surges of strong monsoonal southwest
winds, showers and thunderstorms with occasional gusts to 35 mph.
Models forecast quick development of the circulation to the east
of the Marianas, likely too quick. Showers and thunderstorms with
gusts up to 35 mph will persist across the area tonight, slowly
decrease through the morning and afternoon hours Thursday, and
become isolated by Thursday night. The question is just how much
will convection consolidate around the developing circulation.
Models open up a ridge and really dry the Marianas by Thursday
night-- perhaps too optimistically. With that in mind, have
maintained the previous forecast of isolated showers and
thunderstorms through the weekend.

&&

.Marine...
Strong winds and their resultant large wind waves this morning
prompted the issuance of a small craft advisory for Tinian and
Saipan. Winds in the CNMI will still be gusty tonight, but not as
strong as the past 12 to 24 hours, and the short-period wind waves
will subside. However, the long-period SW swell will rapidly
increase tonight and Thursday, bringing about large sea conditions
hazardous to small craft. Wave models show seas remaining
hazardous above 10 feet into next week as slow-moving Typhoon Lan
lifts northward across the Philippine Sea. As Lan passes well to
our west, the swell will swing to the west and then the northwest
early next week. Hazardous surf will transition from south and
west facing reefs in the short term to west and north facing
reefs over the weekend.

&&

.Tropical systems...
Typhoon Lan will gradually move north across the Philippine Sea
the next several days, passing well to our west. The primary
effects we will see from Lan will be the southwest monsoon flow in
the short term and a long-duration long-period southwest to west
swell that will affect seas and surf into next week.

Closer to home, a circulation near Saipan is expected to move east
tonight, becoming better organized Thursday. Models seem to move
the circulation and develop it rather quickly, but this could
possibly be the models dissolving the current circulation and
forming a new one just a little farther east within the monsoon
trough. Model consensus keeps the circulation nearly stationary
east of the islands through Friday. By the weekend, models diverge
in outlooks, often differing each run. GFS and ECMWF are becoming
more consistent on the circulation moving NNW, crossing near the
far northern Marianas over the weekend as it gradually gets
absorbed in the expansive wind field of TY Lan. NAVGEM remains the
big outlier, keeping the circulation drifting east of the
Marianas into next week.

&&

.Eastern Micronesia...
Satellite and ASCAT Analysis show a tropical disturbance developing
near Enewetak at 11N163E. Also, northwest winds have already arrived
at Chuuk and westerly winds at Pohnpei. Surface winds were light and
variable at Kosrae and Majuro. Showers and thunderstorms are north
of Kosrae in an area centered near 9N164E, otherwise only patchy
showers were near Weno Chuuk, Pohnpei, Kosrae and Majuro today.
Conditions will be unstable in the coming days. GFS Model indicates
a weak circulation could develop near the disturbance and showers
may increase across eastern Micronesia during this weekend.

Majuro/Kalo Buoy shows combined seas near 5 feet. Wave models show a
building northeast swell later in the week 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.

At Chuuk, a long period WNW swell will build into the area over the
weekend and into early next week. This will cause surf to rise to
near hazardous levels along west facing reefs around Monday.

&&

.Western Micronesia...
High Surf Advisory remains in effect for Yap and Koror, and it will
need to be monitored since surf conditions could approach Warning
criteria at some locations. The Special Weather Statement was re-
issued and Small Craft Advisory remains in effect. Typhoon Tan is
near 13N132E and moving slowly northward, so that showery weather
and gusty winds will persist tonight and probably through Thursday,
then, weather should improve significantly by this weekend.

Seas will remain hazardous for small craft through the week with
hazardous surf spreading to north facing reefs over the weekend.
Large north swell produced by Typhoon Lan could still 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.

&&

$$

W. Aydlett/Simpson



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