NWS Guam wrote its last area forecast discussion before the first event at Dec 13, 10:01 AM UTC, 3 hours 59 minutes earlier.
Area forecast discussion, Dec 13, 10:01 AM UTC, 3 hours 59 minutes before
Area Forecast Discussion
National Weather Service Tiyan GU
801 PM ChST Mon Dec 13 2021
.Marianas Synopsis...
Partly cloudy skies and fresh trade winds are present across the
Marianas this evening. Satellite shows an old shear line fragment
pushing some light showers through the islands, mainly across Rota.
Seas at Ipan buoy are between 8 and 9 feet. Altimetry data is not
available, but surf observations indicate that combined seas are
between 8 and 10 feet, as north swell is steadily diminishing,
bringing an end to high surf, but leaving a high rip risk along east
facing reefs.
&&
.Discussion...
Made some minor changes to weather/POP/sky grids, leaving scattered
showers for Thursday through Friday in convergent trade winds, but
removed mention of thunderstorms due to lack of enough moisture.
Will be watching the shear line fragment, but for now it appears
showers will be light and mainly isolated, with rainfall through
Wednesday pegged by the models at less than a quarter inch.
&&
.Marine/Surf...
Based on surf reports and latest wave guidance, have reduced north
swell to 4 feet tonight, and 2 feet by Wednesday night. Also
refreshed some of the total seas grids from the ECMWF, keeping seas
a little higher the next 2-3 days. The net result is to end high
surf on north facing reefs, and to boost east swell a little, ending
the high surf (12 ft or greater) along east facing reefs but
keeping the rip risk high through at least Wednesday. Meanwhile,
winds are generally 15-20 kt the next few days, just below Small
Craft Advisory levels. So both winds and seas are forecast to be
below SCA levels, but not by much. Anyone planning marine or beach
activities this week should keep up-to-date on the latest forecasts
and advisories in case winds or seas happen to increase enough to
trigger an advisory.
&&
.Tropical systems...
Tropical Depression 28W has become better organized today, and
will continue to strengthen over the next few days. For more
information, please refer to the Western Micronesia section below.
&&
.Eastern Micronesia...
A series of weak trade-wind troughs are moving through eastern
Micronesia this evening. These features along with a broad, weak ITCZ
pattern across the area will bring periods of scattered showers
to all 3 locations. Majuro will see scattered showers tonight and
showers will increase later tonight over Kosrae. Both locations
will then see drier conditions begin to move in Tuesday morning.
These showers will then move into Pohnpei by Tuesday morning,
remaining there a little bit longer, with drier conditions expected
by Wednesday morning. Then, a dry trade-wind pattern is expected
to continue into the weekend.
&&
.Western Micronesia...
The main feature over western Micronesia is newly formed Tropical
Depression 28W. 28W will continue to move toward the west-northwest
over the next few days, strengthening as it moves toward western Yap
State and Palau. As the system continues to move away from Chuuk, a
much drier pattern is expected to continue across Weno until
Wednesday morning. On Wednesday, scattered showers associated with a
series of weak trade-wind troughs, currently over eastern Micronesia,
are expected. By Thursday, these troughs are expected to weaken as
they move a little farther westward into TD 28W.
For Palau and Yap, TD 28W is expected to continue strengthen as it
moves toward the west-northwest over the next few days. 28W is
expected to become a Tropical Storm within the next 12 hours,
bringing Tropical Storm conditions to Ngulu late tonight or early
Tuesday morning. Yap proper is expected to see increasing winds and
convection, though the latest forecast track for TD 28W keeps Yap
proper at the far northern periphery of tropical storm force winds.
Gusty conditions along with heavy rainfall are likely for Yap, though
winds should remain below tropical storm force. The earlier forecast
for 28W indicated the system could reach Typhoon strength near
Kayangel by Tuesday evening. This led to a Typhoon Watch being
issued for both Kayangel and Koror. The latest forecast, however,
keeps the system as a strong Tropical Storm until farther west, past
Kayangel and Koror. Either way, Koror and Kayangel, along with Ngulu,
will see the brunt of this system. Ngulu should see improving
conditions Tuesday evening, though this could change if the speed of
movement changes. Koror and Kayangel could remain in tropical storm
conditions as late as Wednesday afternoon.
Seas and surf will become very dangerous over the next several
hours. A long period north swell will continue to produce hazardous
surf conditions along north facing reefs of Palau and Yap tonight. As
TD 28W continues to approach and intensify, seas and surf will
deteriorate. Hazardous surf conditions are expected to become
dangerous, with surf in excess of 15 feet possible. Coastal
inundation and extensive coastal erosion due to the combination of
higher seas produced by the ongoing La Nina pattern, the long period
north swell, and developing storm surge are expected. Inundation
of up to 2 feet is expected for Yap, 2 to 3 feet for Ngulu, and 2
to 4 feet for Koror and Kayangel. Inundation will be most pronounced
around times of high tide.
Residents of Yap, Ngulu, Koror, and Kayangel should continue to
monitor forecasts for any updates, watches, or warnings as this
system progresses. For more details about TD 28W, see bulletins
issued by the Joint Typhoon Warning Center under WMO header WTPN31
PGTW, and by the National Weather Service under WMO headers WTPQ31
PGUM and WTPQ81 PGUM.
&&
.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Rip Current Risk through Wednesday for GUZ001>004.
MP...None.
Marianas Waters...Small Craft Advisory until 6 PM ChST Tuesday
for PMZ151>154.
&&
Marianas: Middlebrooke
Micronesia: Kleeschulte
Tropical: Doll