Area Forecast Discussion
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899
FXPQ60 PGUM 200857
AFDPQ

Area Forecast Discussion
National Weather Service Tiyan GU
657 PM ChST Thu Sep 20 2018

.Marianas Synopsis...
Andersen Air Force Base Doppler weather radar shows frequent showers
over Guam and Rota and southeastward, with scattered showers over
Tinian and Saipan and eastward. The VAD wind profile indicates
northeast winds of 13 to 18 knots through the lowest 10 thousand
feet of the air. The buoys reveal east-northeast trade-wind swell and
wind waves. At the Tanapag buoy these are refracting around the
coast, so they seem to be coming from the north-northeast.

&&

.Discussion...
Showers are starting in earnest now. While ScatSat showed a trough
south of Guam that was rather elongated, the radar is showing a weak
circulation as part of this tropical disturbance near 12.9N and
145.9E. Likely within the next few hours this disturbance will be the
subject of a Tropical Cyclone Formation Alert (TCFA) from the Joint
Typhoon Warning Center (JTWC). For the foreseeable future we will
continue to handle it with the flash flood watch and small craft
advisory that are already issued. Only if it is looking likely to
become a tropical storm (looks like a dim possibility at this point)
would we make a radical shift in how we approach it. Curiously, the
worst weather and sea conditions could come after it passes if we get
into a monsoon tail feeding into it. Will have to keep an eye on how
it starts developing.

&&

.Marine...
The GFS model shows stronger winds north of the center, and local
observations are backing this up. Guam shows winds of 10 knots with
gusts up to 20 knots, while Saipan is showing winds of 15 to 22 knots
with peak gusts up to 35 knots. Handled this for now with a small
craft advisory for winds, as seas are not responding yet. Seas could
diminish a bit Sunday, then will come back with a vengeance around
Monday as the monsoon flow sets in.

&&

.Hydrology...
The next couple of days look rainy indeed. This has been expected for
a day or two already. Went ahead and worded Guam and Rota forecast
with frequent showers, which will only increase as we head into
tonight and then Friday. Issued a flash flood watch through Saturday
night at this time. It may need to be extended, and at some point it
may well need to be upgraded to a warning.

&&

.Tropical systems...
The weak circulation near Guam is listed as Invest area 92W at this
time. A center is showing up on Guam radar near 12.9N and 145.9E.
ScatSat indicates that an area of troughing extends to the west or
west-southwest. It is expected to move west-northwest, which should
bring the radar center very close to Guam overnight. It will likely
become the subject of a Tropical Cyclone Formation Alert within the
next few hours.

&&

.Eastern Micronesia...
Satellite shows a sharp surface trough and showery weather moving
across the Marshall Islands today, while fair weather prevails over
Pohnpei and Kosrae. Models indicate the trough and showers will move
into Kosrae State overnight and to Pohnpei by Saturday. Unsettled
weather with trailing low-level convergence and westerly winds at
times are expected over the region through early next week.

&&

.Western Micronesia...
A Special Weather Statement was issued for western Micronesia. The
tropical disturbance near Guam is expected to move west-northwest
and further develop over the Philippine Sea. This will cause an
increase in the westerly monsoon flow across Yap State and Palau.
Winds and showers will increase for Yap and Koror the next few days,
so that Small Craft and High Surf Advisories may be needed over the
weekend. Then, slowly improving weather is expected for Koror and
Yap next week.

Chuuk; Fairly dry weather and a ridge of high pressure is expected
over Chuuk the next couple of days. A surface trough arriving from
the east could bring clouds and a few showers to Chuuk by Sunday.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...Flash Flood Watch through late Saturday night for GUZ001>004.

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

&&

$$

Stanko/Simpson



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