NWS Guam wrote its last area forecast discussion before the first event at Nov 30, 8:48 AM UTC, 5 hours 12 minutes earlier.
Area forecast discussion, Nov 30, 8:48 AM UTC, 5 hours 12 minutes before
Area Forecast Discussion
National Weather Service Tiyan GU
648 PM ChST Mon Nov 30 2020
.Marianas Synopsis...
Scattered showers are seen on the latest radar and satellite imagery,
as they continue to move in a mainly east to west direction. The
latest observations from Ritidian and Tanapag buoys continue to show
easterly swells of 8 to 9 feet.
&&
.Discussion...
The main weather story looks to be a heavy rain event for late
Wednesday night into Thursday. Until then, the latest numerical
guidance shows a mid-level wedge of drier air moving into the
Marianas Tuesday, which would normally help decrease shower coverage
compared to today. The fly in the ointment is increasing moisture
and lift in the upper-levels, which may help compensate, if
sufficient instability exists, for any updraft to punch through this
dry layer. Otherwise, look for showers to cross the region from time
to time, with breezy conditions persisting through tomorrow
afternoon.
Then our attention turns to a trade wind disturbance, currently
over the Marshall Islands between Pohnpei and Kosrae. This area of
disturbed weather will make a northwestward turn as we head into
the Wednesday into Thursday time frame, accompanied by a surface
trough. Increased convergence and moisture through the column,
combined with good omega (lift) aloft, all points to a potential
heavy rainfall event for all of the Mariana Islands. Some of the
guidance also points to a weak circulation developing, which would
add to this convergence, leading to possibly higher totals for Guam
and Rota. An extended hydrologic outlook has been issued to cover
this concern, and it's possible a flash flood watch may need to be
issued in the next forecast package or two if model trends continue
to point to this heavy rain event.
Heading into the end of the week, as the disturbance passes by to
the west, lift will decrease some, but a moist profile will remain.
Given good moisture content and afternoon heating helping to enhance
the instability, some heavier showers remain possible on Friday, but
not with as much coverage compared to the day before. Drier air then
advects into the region for Saturday, with just isolated showers
expected.
&&
.Marine...
A small craft advisory remains in effect as frequent wind gusts
of 25 to 30 knots are expected through Tuesday. Hazardous surf
conditions are also expected through at least Tuesday night, with
surf heights of 8 to 12 feet likely along east-facing reefs.
Lightning is also a concern for tonight and Tuesday.
As we head into the the middle of the week, a trade-wind
disturbance will bring periods of heavy showers and scattered
thunderstorms, which will create reduced visibilities at times,
along with increased lightning potential. At this time, late
Wednesday night into Thursday looks to be the favored time for
this to occur.
&&
.Eastern Micronesia and Chuuk...
An active ITCZ remains entrenched across Micronesia from Chuuk to
across the Date Line. A weak surface trough stretches southeastward
from a weak circulation south of guam near 10N146E. This trough is
interacting with the western end of the ITCZ to produce showers and
thunderstorms across Chuuk State. Activity has decreased a bit, for
now, at Weno, though scattered showers are expected to move back in
later this evening. Farther east, the ITCZ continues to produce
scattered to numerous showers across Pohnpei and Kosrae and across
the southern Marshall Islands. Models indicate the ITCZ will drift
south and begin to weaken around midweek, resulting in a drier
pattern for Majuro by Thursday and continuing into the weekend.
Although weakening, the ITCZ will continue to produce scattered to
numerous from Kosrae to Chuuk through the beginning of the weekend.
A strong high pressure system to the north will maintain elevated
winds for Majuro, with conditions remaining hazardous for small craft
operations through Tuesday night and possibly again Thursday into the
weekend.
&&
.Western Micronesia for Yap and Koror...
A surface trough remains to the west of Koror and will slowly drift
westward through the next couple of days. This trough generated a
small area of scattered showers across Koror this evening. These
showers will continue to move out of the area, with isolated showers
likely by midnight. A little farther east, a weak circulation is seen
south of Guam near 10N146E, with a trough extending southeast to end
west of Chuuk. This system will drift westward, bringing increasing
showers to Yap later this evening. These showers will approach Koror
by Tuesday afternoon. As this trough moves off to the west, drier
conditions will push into Yap Tuesday. Time/height products continue
to show a very moist profile across Koror Tuesday night and Yap
Wednesday night, continuing into the weekend. This is due to another
trough developing near the region, with a couple of weak
circulations, one northeast of Yap, the other southeast of Yap.
Although time/height products maintain a moist pattern for both
locations, models are showing much less rainfall than previous runs.
Therefore, expect scattered showers to move back into Yap early
Thursday morning and continue across Koror through at least Thursday,
though unsettled conditions will remain over Koror into the weekend.
&&
.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Rip Current Risk through Friday afternoon for GUZ001>004.
High Surf Advisory until 6 AM CHST Wednesday for GUZ001-002.
High Surf Advisory until 6 AM CHST Wednesday for GUZ003-004.
Marianas Waters...Small Craft Advisory until 6 PM CHST Tuesday for
PMZ151>154.
&&
Doll/Kleeschulte