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Hawaiian Islands Synoptic Discussion and Guidance

774
FXHW60 PHFO 272003
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1003 AM HST Sat Feb 27 2021

.SYNOPSIS...
Strong and gusty trade winds will continue for the next several
days, while a trough aloft keeps the island atmosphere unstable.
While showers, some briefly heavy, will focus over windward areas,
the strong trades will push a few leeward. Increased moisture near
Maui and the Big Island will bring a more widespread threat of heavy
windward showers, and a slight chance of thunderstorms.

&&

.DISCUSSION...
Wet and windy trade wind weather is expected for at least the next
couple of days as a strong surface high builds N of the islands, and
a trough aloft lingers overhead. Some decrease in wind speed is
anticipated by the middle of the upcoming week, but the trough aloft
will be slow to depart.

Strong high pressure to the distant NE is in the process of
weakening, but a new strong high to the NW will soon replace it, and
wind speeds are expected to trend back up Sunday and Sunday night,
with windy conditions persisting into Tuesday. A Wind Advisory will
likely be required for parts of the state as winds increase. A large
low pressure system is expected to develop in the NE Pacific by mid-
week, shunting the high center to a position NW of the islands,
leading to a reduction in wind speeds, with the direction backing
slightly to the NE.

A trough aloft is destabilizing the island atmosphere, and moisture
arriving on the trade wind flow is becoming enhanced to the point
that thunderstorms have been observed over windward portions of Maui
and the Big Island this morning. Additionally, heavy showers moving
rapidly along with the trade winds have been strafing windward
portions of all islands but Kauai. Currently, heavy showers and
thunderstorms are somewhat anchored along the windward slopes of
Haleakala. A Flash Flood Watch remains posted for windward portions
of Maui and the Big Island. As the ground becomes increasingly
saturated due to the persistent rainfall, runoff will increase,
leading to rising water levels in streams and rivers. Additional
heavy rain bursts or temporary terrain-anchored heavy rain cells
could lead to flash flooding.

Model guidance indicates that some decrease in moisture may lead to
a reduction in trade showers early next week, but the trough aloft
will be lingering overhead, and thus we are somewhat reluctant to
anticipate a significant drying trend at this time.

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 or drizzle leading
to icy roadways, although snow showers will mix in as well. A
Winter Weather Advisory remains in effect through Monday morning,
and may need to be extended as the trough aloft lingers.

&&

.AVIATION...
Widespread shower activity along with MVFR level clouds and
visibility (brief IFR at times) will continue along north through
east sections of most islands today, especially Maui and the Big
Island where occasional heavy showers and even a few thunderstorms
are likely. AIRMET Sierra remains in effect for tempo mountain
obscuration for Maui and the Big Island, and now includes Oahu and
Molokai as well. The remainder of the state should remain under VFR
conditions, but will continue to monitor shower coverage for
changes.

Additionally, strong high pressure remains northeast of the main
Hawaiian Islands, keeping winds breezy to locally strong today,
though not quite as strong as the previous couple of days. AIRMET
Tango remains in effect for low-level mechanical turbulence over and
immediately south through west of the mountains of all islands.
Wind speeds are expected to increase again Sunday as a new, stronger
surface high builds in north of the state.

&&

.MARINE...
Central East Pacific large, strong high pressure will continue to
produce a very tight pressure gradient across the Hawaiian Island
waters to sustain these strong trade winds. The early week eastern
migration of a secondary northern Pacific high will keep the
pressure gradient back across the islands tight enough to maintain
strong trades through Tuesday. A mid week trough of low pressure
moving across the Gulf of Alaska will weaken the gradient enough to
allow trades to fall back to more moderate magnitudes. Late week
high pressure to the northwest of the islands will ensure moderate
to locally strong trades through next weekend.

Sea conditions will remain rough under persistent strong trades the
next few days. A Small Craft Advisory (SCA) is in effect for all of
the Hawaiian coastal waters this weekend to account for the
widespread strong trades and associated high seas. It appears likely
that the SCA will need to be extended through early next week. A
Gale Watch may be needed later today for the Alenuihaha Channel as
trades near gale force over these waters Monday and Tuesday.

Satellite-derived winds verify a large belt of strong trades focused
over and upstream of the state. This very long fetch of robust
trades will sustain turbulent, fully developed rough 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
into early next week for the east-facing shorelines of Kauai, Oahu,
Molokai, Maui and the Big Island. Small to moderate northwest swells
will fill in today and peak surf from head high to a couple of feet
above head high Sunday. The next scheduled northwest swell will be
larger than this weekend's and push surf to near advisory levels
along northern and western exposed shores Thursday. Along with the
trade wind wave and swell wrap, small Southern Hemispheric
background swells will keep small southern-facing shoreline 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



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