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Hawaiian Islands Synoptic Discussion and Guidance

996
FXHW60 PHFO 270322 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
522 PM HST Fri Feb 26 2021

.SYNOPSIS...
Strong and gusty trade winds will continue into early next week,
while a trough aloft keeps the island atmosphere unstable. Clouds
and showers will focus over windward areas, with some occasional
heavy downpours. Increasing moisture near Maui and the Big Island
will bring a more widespread threat of heavy windward showers, and a
slight chance of thunderstorms.

&&

.DISCUSSION...
Very strong (1046 mb) surface high pressure to the distant NE will
gradually weaken the next couple of days, leading to a short-term
reduction in trade wind speeds. A new strong high (near 1044 mb)
will build far N of the islands Saturday night and Sunday, leading
to an increase in wind speeds Sunday and Monday. Overall, strong and
gusty trade winds will persist over the islands through early next
week, potentially easing around mid-week as a large low pressure in
the NE Pacific pushes the high westward, leading to a relaxation of
the local pressure gradient. A Wind Advisory that was in effect for
parts of the state has been cancelled, but the increased winds
Sunday and Monday could warrant a re-issuance.

The island atmosphere will remain unstable into next week, due to
the presence of a persistent trough aloft, and its associated cold
pool. Moisture riding in on the trade winds will be enhanced,
leading to higher than normal cloud tops (and larger-than-normal
rain drops), delivering heavier windward showers than might
otherwise be expected with trade winds in place.

While these showers will be progressive and will not linger over any
one area for a prolonged period, high-resolution and global model
guidance anticipates their repeated development over the windward
slopes of Maui and the Big Island over the next couple of days. This
persistent rainfall is expected to lead to increasing ground
saturation and rising levels in streams and rivers, with additional
rainfall potentially resulting in flash flooding as the weekend
progresses. A Flash Flood Watch (FFA) is in effect for windward
portions of Maui and the Big Island. An expansion of the FFA to
windward portions of the other islands is not anticipated as
guidance indicates the deepest moisture (and instability) over the
Big Island occasionally spreading to windward portions of Maui.

The cold temperatures aloft and deeper-than-normal moisture means
that the Big Island Summits will receive periods of wintry weather,
with the primary threats being freezing rain leading to icy
roadways, although snow showers will mix in as well. A Winter
Weather Advisory is in effect through Monday morning, and may need
to be extended as the trough aloft lingers well into next week.

&&

.AVIATION...
Strong easterly trade winds will continue for the next several days,
sustained by an area of very strong high pressure located north-
northeast of the main Hawaiian Islands. AIRMET Tango remains in
effect for low-level mechanical turbulence over and immediately
south through west of mountains. Additionally, since especially
strong winds are forecast to persist in the Alenuihaha Channel and
south of the Big Island, AIRMET Tango is also in effect for surface
winds greater than 30 kt. Low-level wind shear could also be a
concern at a few locations into this evening, especially PHOG.

As an upper-level low moves into the area into tonight, an uptick in
shower activity is expected, particularly over eastern sections of
the islands. MVFR conditions are likely in heavier showers,
especially for the Big Island and Maui tonight through Saturday,
though other islands could be included as well. There are currently
no AIRMETs for mountain obscuration, but are likely tonight as
showers increase further. Isolated thunderstorms are also possible
on leeward slopes of the Big Island this afternoon. Otherwise, VFR
conditions will prevail throughout the period. Instability from the
upper low will generate a chance for thunderstorms over most of the
Big Island and parts of Maui Saturday and Saturday night.

&&

.MARINE...
Large, strong high pressure located off the mainland west coast has
and will continue to maintain a tight enough pressure gradient
across the eastern Pacific to produce strong trades over the local
Hawaiian Island waters. An early week eastern migration of a
secondary northern Pacific high will keep a taut pressure gradient
back across the islands and sustain strong trades through Tuesday. A
mid week trough of low pressure developing over the Northeast
Pacific will push northerly high pressure southeastward. This will
slightly weaken the late week northeast to southwest pressure
gradient and responding trades will fall back to more moderate
magnitudes.

Rough sea conditions will remain elevated under persistent strong
trades over the next several days. The most recent ASCAT pass
verified that trades were locally strong through the windier inter-
island channels with only occasional gusts to gale force within the
Alenuihaha Channel. Thus, to account for these ongoing widespread
strong trades and high seas, an all coastal water Small Craft
Advisory is still in effect through the weekend.

Satellite-derived winds verify a large belt of strong to near gale
trades focused over and upstream of the state. This very long fetch
of robust trades will sustain rough, elevated seas and high surf
along most east-facing shorelines. Some of this trade wind swell
will also wrap around into adjacent northern and southern exposures.
The High Surf Advisory will therefore remain in effect the next
several days for the east-facing shorelines of Kauai, Oahu, Molokai,
Maui and the Big Island. As current northwest swells fade through
the day, a backing northwest swell will fill in this weekend and
peak near or slightly above head high surf late Saturday into early
Sunday. The next scheduled northwest swell will be larger than this
weekend's and peak surf to near advisory levels along northern and
western exposed shores late Wednesday into early Thursday. Small,
Southern Hemispheric background swell will keep small southern-
facing shore surf around through early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for East facing shores of
Kauai Oahu Molokai Maui and the Big Island.

Flash Flood Watch through Sunday afternoon for Maui Windward West-
Windward Haleakala-Haleakala Summit-South Big Island-Big Island
North and East-Big Island Interior.

Winter Weather Advisory until noon HST Monday for Big Island Summits.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian waters.

&&

$$

DISCUSSION...Birchard
AVIATION...TS
MARINE...Blood

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office