NWS Guam wrote its last area forecast discussion before the first event at Dec 31, 10:38 AM UTC, 3 hours 22 minutes earlier.
Area forecast discussion, Dec 31, 10:38 AM UTC, 3 hours 22 minutes before
Area Forecast Discussion
National Weather Service Tiyan GU
838 PM ChST Thu Dec 31 2020
.Marianas Synopsis...
A few showers are crossing the Marianas this evening, as a weak low
appears to be forming 100 miles east of the region. Otherwise, the
latest buoy observations from Ritidian and Tanapag show seas
currently running in the 4 to 6 feet range, with a water temperature
of 83 degrees. Altimetry shows seas of 8 to 11 feet to the northwest
of the CNMI.
&&
.Discussion...
The main forecast challenge continues to be timing of showers and
coverage. The previous forecast package delayed the onset of
scattered showers and this worked out good. The latest hi-res ARW
and NMM models, along with the GFS and ECMWF models continue to show
shower coverage increasing after midnight (10Z), with the latest
cross section data showing low-level moisture depth increasing to
above 80% in the lowest 12-14,000 feet, with some low-level omega
(lift) also present.
The latest infra-red imagery also shows a broad area of low pressure
developing about 100 miles east of the marianas. The net result,
shower coverage could get deflected more to our north depending on
if the circulation intensifies, or remains weak enough for a slightly
intensifying surface trough (currently located from 10N148E to
18N149E) can remain the "dominant" feature to act as focus for shower
development.
Otherwise, the next item of interest is a high surf that rolls in
Friday evening through Saturday, as a large northwest swell arrives.
A High Surf Advisory has been issued to address this, and is talked
about in the marine discussion. Heading into next week, a typical
dry-season pattern continues with occasional surface troughs
approaching/crossing the region, potentially leading to periodic
spikes in low-level moisture depth and potential shower coverage.
&&
.Marine...
Hazardous conditions for small craft will likely develop over the
weekend, with a high likelihood of a small craft advisory being
issued with later forecasts. Initially, a large swell of 6 to 7 feet
will roll in from the northwest, causing combined sea heights to
rise into the 8 to 10 feet range, beginning Friday night or Saturday
morning. This will also allow coastal surf to increase rapidly into
the 8 to 11 feet range for north and west-facing reefs during this
time, for which a High Surf Advisory was issued earlier.
Then as we head into the Saturday night/Sunday time frame, seas will
remain in the 8 to 10 feet range while winds increase into the 15 to
25 knot range with higher gusts likely. It's possible that the surf
could end up being a tad higher, but with warning criteria being
15 feet, and current forecasts showing it topping out at 11 (perhaps
12 feet), a warning is not anticipated.
&&
.Eastern Micronesia and Chuuk...
An ITCZ stretches east-southeastward from near Chuuk at 7N150E to
near Kosrae at 5N164E, then continues east-northeastward across the
southern Marshall Islands to beyond 180 at 9N. A fresh to strong
trade-wind surge can be seen north of this feature over the northern
Marshall Islands.
The weaker western portion of the ITCZ will keep patchy showers near
Chuuk thru Friday evening and Pohnpei thru tonight. Then drier
trades should dominate Chuuk by this weekend, and Pohnpei on Friday.
As the whole ITCZ progresses westward, the more robust eastern
portion of the ITCZ is going to bring showery conditions to Kosrae
by later this evening, Pohnpei by Friday night and also prolong them
for Majuro thru Friday evening. Meanwhile, the surging trades are
expected to displace the ITCZ farther southward so that it will pass
south of Chuuk. Under the same reasoning, this should also allow
drier trades to return to Majuro and Kosrae this weekend, Pohnpei by
Saturday night. Afterward, an upper-level trough extending
southwestward over the region from a pronounced mid-latitude trough
near Midway Island will increase the chance of spotty convection
early next week.
A north swell near 4 feet at 14 to 16 seconds is registered by the
Kalo Bay buoy at Majuro this afternoon. WW3 model guidance suggests
this swell at near 3 feet is also impacting Kosrae. This swell will
peak at 3 to 4 feet at Kosrae, 4 to 5 feet at Majuro from Friday
evening thru Saturday evening. This will generate surf heights at 5
to 7 feet on Kosrae, and 7 to 9 feet at Majuro. With the Spring Tide
cycle peaking from Friday evening thru Sunday evening, this
combination will create minor inundation at Kosrae, and moderate
inundation at Majuro inside the lagoon. Persistent trade-wind swell
and wind waves will keep seas hazardous for small craft across the
entire region at least thru this weekend. Surf on Kosrae will also
remain hazardous at least thru Monday.
&&
.Western Micronesia for Palau and Yap...
A surface trough extends northeastward from a weak circulation
southwest of Koror at 4N130E to end west of the Mariana Islands at
15N143E. An upper-level high northeast of Yap near 14N142E should
provide modest divergent flow over far western Micronesia thru this
weekend.
The surface trough will continue to trigger sporadic convection near
Koror and Yap tonight. After it has drifted west of the area, modest
surface ridging should provide fair weather for Yap thru Saturday,
and Koror thru early Friday evening. Then another surface trough
currently upstream southwest of Chuuk near 5N147E will approach
Koror, while passing south of Yap. Therefore, anticipate increasing
clouds and showers for Koror by late Friday evening but partly
cloudy skies for Yap thru Saturday night. By Sunday, drier moderate
to fresh trades should filter across the entire area.
Long-period northwest swell produced by strong to near-gale winds
associated with a cold front south of Japan is going to arrive at
far western Micronesia starting Friday evening. This will cause surf
along north and west facing reefs on Koror and Yap to rise and reach
hazardous levels late Friday night, and a High Surf Advisory will
likely be issued by Friday morning. Seas will also build and become
hazardous for small craft on Saturday or Sunday.
&&
.GUM WATCHES/WARNINGS/ADVISORIES...
GU...Beach Hazards Statement through Tuesday afternoon for GUZ001>004.
High Surf Advisory from 6 PM Friday to 6 PM ChST Sunday for
GUZ001>004.
Marianas Waters...None.
&&
Marianas: Doll
Micronesia: Chan