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Hawaiian Islands Synoptic Discussion and Guidance

919
FXHW60 PHFO 260151
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
351 PM HST Thu Feb 25 2021

.SYNOPSIS...
Breezy to locally windy trade winds will continue into the first
half of next week. Rainfall will remain focused over windward
slopes, but a disturbance aloft will enhance shower activity and
may produce a few thunderstorms Friday through the weekend.

&&

.DISCUSSION...
An exceptionally strong 1047 mb surface high centered nearly 1600
miles north of the state is maintaining locally strong and gusty
easterly trade winds. A Wind Advisory remains posted for portions
of the Big Island and Maui, where sustained winds of around 30
mph have been measured at a handful of gages. A narrow upper
level trough is building over the area from the east, eroding the
inversion and leading to some instability. As a result, isolated
heavy showers are flaring within a pocket of moisture currently
affecting windward Oahu and Kauai, and although moisture is
temporarily suppressed over the Big Island, a few showers are
developing along leeward slopes along with a couple of freezing
rain showers observed on the high summits.

Winds will hold tonight and Friday as the atmosphere gradually
destabilizes. The surface high will begin to drift eastward, but
since winds will likely persist around current strength, we have
extended the Wind Advisory through the day tomorrow. The upper
level trough will settle over the islands, wiping out the
inversion and producing instability. The GFS and ECMWF do not
show sources of deep moisture that could help to trigger
convection, but thunderstorms are a good bet for leeward Big
Island slopes in the afternoon. Otherwise, showers will be focused
over windward areas, with a few briefly passing over leeward areas
on the strong trades.

A breezy and somewhat wet and unstable trade wind pattern will
prevail this weekend. Expect pockets of enhanced moisture moving
in the trade wind flow to remain focused over windward areas, and
deeper moisture developing near the Big Island and Maui could
produce more widespread heavy showers and a few thunderstorms. The
area with the highest risk of sustained heavy rainfall will likely
be windward Big Island, though a Flash Flood Watch does not
appear warranted at this time.

The current run of breezy trades will persist into early next
week. High pressure will dominate far north of the state, but the
GFS and ECMWF continue to differ on some important details. The
GFS shows the upper level trough overhead weakening, while the
ECMWF suggests that this feature will be reinvigorated, leading to
weaker trades and wetter conditions. The forecast favors the GFS,
but confidence is low.

&&

.AVIATION...
Strong high pressure far north of the state is producing strong
easterly trade winds across the area. AIRMET Tango remains in
effect for low level mechanical turbulence for areas over and
immediately south through west of mountains. This AIRMET will
continue into the early morning hours. The strong winds are also
causing low level wind shear near Kahului Airport.

VFR conditions prevail across most of the area with brief periods
of MVFR conditions over some windward locations in low clouds and
showers. Expect these conditions to continue on through the
overnight hours.

&&

.MARINE...
Strong high pressure NE and N of the islands will maintain strong
to near gale force trade winds over and upstream of the islands
into next week. A Small Craft Advisory (SCA) for all waters through
the weekend will likely need to be extended further in time as
winds and/or seas will continue to meet criteria early next week.
Where winds are most accelerated, an ongoing Gale Warning for the
Alenuihaha Channel has been extended through Friday, but a subtle
decrease in wind speeds will likely allow the Gale Warning to be
dropped over the weekend. Winds may increase again early next week
as a new high builds N of the area.

The strong trade winds extend well upstream of the islands, and are
driving a well-developed (10-11 second period) wind swell toward
the islands from the E. A High Surf Advisory for E facing shores
will remain posted for the next several days. Elsewhere, surf will
be below advisory levels, but several long-period NW swells are
expected over the next week. Peaks in these swells can generally
be expected tonight into Friday, Saturday night/Sunday, and
Wednesday/Thursday, with the last swell expected to be the largest
of the three.

Some trade wind swell will wrap into exposed shores/reefs along N
and S facing shores, and a small long-period S swell is also
expected the next couple of days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST Friday for Molokai-Kahoolawe-Maui
Windward West-Maui Leeward West-Maui Central Valley-Windward
Haleakala-Leeward Haleakala-South Big Island-Big Island North
and East-Kohala-Big Island Interior.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Maalaea Bay-
Pailolo Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

Gale Warning until 6 PM HST Friday for Alenuihaha Channel.

&&

$$

DISCUSSION...Wroe
AVIATION...Burke
MARINE...Birchard

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