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Hawaiian Islands Synoptic Discussion and Guidance

690
FXHW60 PHFO 280634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Sat Feb 27 2021

.SYNOPSIS...
Trade winds will strengthen on Sunday, with locally windy conditions
expected to persist for the next several days, while a trough aloft
keeps the island atmosphere unstable. While fast-moving heavy
showers will focus over windward areas, the strong trades will also
push a few showers 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...
Currently at the surface, a 1038 mb high is centered around 1400
miles northeast of Honolulu, while another 1042 mb high is
centered around 1250 miles north-northwest of Kauai. The resulting
gradient is producing breezy trade winds across the island chain
this evening. Infrared satellite imagery shows mostly cloudy
conditions in windward areas, with partly cloudy skies prevailing
in most leeward locales. Radar imagery shows scattered to numerous
showers moving into windward and mauka sections of the smaller
islands, with widespread rainfall over windward Big Island. A few
showers are occasionally making it into leeward sections of the
smaller islands, and a few diminishing showers are noted along the
leeward Big Island slopes. Main short term concerns continue to
revolve around the ongoing Flash Flood and Winter Weather
headlines, as well as the potential need for Wind Advisories.

High pressure northeast of the state will settle southward and
weaken tonight and Sunday, while the high north-northwest of the
state builds eastward and strengthens to around 1044-45 mb, due
north of the island chain by Sunday afternoon. This high will
then settle southward and weaken slightly Monday and Tuesday,
followed by a new strong high building northwest of the state
Wednesday through Friday, then shifting eastward far to the north
of the island chain next weekend. Overall, breezy to windy trades
will continue through early next week, with the trades peaking
late Sunday through early Tuesday. Wind Advisories will likely be
required once again for portions of the state during this time.
The trades should ease a bit for the middle and latter part of
next week as the gradient relaxes, but will likely hold in the
moderate to breezy range.

As for the remaining weather details, a trough aloft will keep a
moist and unstable airmass over the state tonight through Sunday
night. The deepest moisture and most unstable conditions will
reside over the eastern islands of Maui and the Big Island
however, where the threat for locally heavy rainfall and
thunderstorms will continue, mainly over windward areas. The
Flash Flood Watch remains in effect until 6 PM Sunday, but this
may eventually need to be extended through early Monday morning.
Across the remainder of the state, a wet trade wind pattern is
expected with showers favoring windward and mauka areas and a few
showers spreading leeward from time to time.

Due to the moisture depth extending above the high elevation
summits of Mauna Kea and Mauna Loa, a Winter Weather Advisory
remains in effect for a wintry mix of freezing rain/drizzle and
snow through Noon Monday. This may be a bit long and we may be
able to scale this back to run through 6 AM Monday with the
morning forecast package.

The upper trough axis will shift south of the state on Monday,
then linger just south of the islands through mid week. A series
of shortwave troughs may then rotate through the region Thursday
through next weekend. Overall, with a troughing aloft more or less
being maintained over the state through much of next week, we
should see a wetter than normal trade wind pattern prevailing with
showers favoring windward and mauka areas and a few showers
reaching leeward communities from time to time.

&&

.AVIATION...
A slow moving east to west oriented upper trough lies over the
island chain this evening. This trough is causing adverse weather
over the eastern sections of the Big Island, including the chance
for thunderstorms. The widespread MVFR ceilings and showers are
forecast to continue through at least Sunday morning where AIRMET
Sierra is in place for tempo mountain obscuration. AIRMET Sierra
extends from Upolu Point to Cape Kumukahi to South Cape.

Breaks in the cloud deck have allowed for the cancellation of
AIRMET Sierra for the smaller islands this evening, but this may
be needed once again later tonight and into the early morning
hours should clouds and showers increase. Over leeward Big
Island, daytime heating and an unstable air mass will result in
scattered showers and a chance of thunderstorms in the afternoon
and early evening hours. As for the smaller islands, scattered
showers will favor the windward and mountain areas, causing tempo
MVFR conditions.

As strong high pressure remains in place northeast of the state,
easterly trade winds will remain breezy to locally strong tonight
before increasing once again on Sunday as a new, stronger surface
high builds. AIRMET Tango remains in effect to account for low-
level mechanical turbulence over and immediately south through
west of all islands.

&&

.MARINE...
High pressure will be the dominant feature for the next several
days, producing strong trades across the coastal waters. A 1038
mb high far to the northeast of the region will become reinforced
with a 1044-45 mb high currently far to the north-northwest of
the region over the remainder of the weekend. The trade winds are
expected to get a boost over the next 24 hours as the high to the
north-northwest becomes the dominant high impacting our weather.

The persistent trade wind pattern we have been experiencing over
the last several days has already been generating advisory level
surf along east facing shores. The current High Surf Advisory for
east facing shores continues through Tuesday, although its
possible that it may need to be extended.

This supports the Small Craft Advisory (SCA) currently out for
all coastal waters for a combination of winds and seas. As the
winds pick up, gale force winds are possible in the Alenuihaha
Channel. As a result, a Gale Watch is in place for the Alenuihaha
Channel starting Sunday night. The SCA and gales are currently
expected to continue through Tuesday, with some weakening of the
winds possible Tuesday night and Wednesday which would allow for
some areas to be dropped.

A small to moderate northwest swell is filling in, and expected to
peak tomorrow. The next northwest swell is expected to arrive
Wednesday night and currently is expected to run higher than the
current swell. This swell could raise surf to near advisory levels
as it peaks Thursday. A small bump is likely along south facing
shores Monday night into Wednesday, and again at the end of the
week.

&&

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

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 Tuesday 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 Watch from Sunday evening through Tuesday afternoon for
Alenuihaha Channel.

Small Craft Advisory until 6 PM HST Sunday for Alenuihaha
Channel.

&&

$$

DISCUSSION...Jelsema
AVIATION...Vaughan/Lau
MARINE...M Ballard

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