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Hawaiian Islands Synoptic Discussion and Guidance

789
FXHW60 PHFO 071405
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Tue Dec 7 2021

.SYNOPSIS...
A kona low northwest of the islands will continue to bring the
threat of heavy rain and a few thunderstorms with gusty winds to
Kauai County and Oahu today, and potentially tonight. The low
is expected to drift away from the area on Wednesday. A drier
airmass moving in from the east has begun to reach the Big Island
and Maui, but a few heavy showers are possible over Maui
County this morning. A breezy and drier trade wind weather pattern
will spread over all islands by Thursday, continuing into the
weekend.

&&

.DISCUSSION...
A vigorous kona low centered NW of the islands will continue to
bring the potential for impactful rainfall to the islands of Oahu
and Kauai County today, with a nearly stationary band of
moderate to heavy rain persisting over Oahu and surrounding
waters through the night. A few thunderstorms within this band
remain possible, and sufficient vertical wind shear exists for some
storms to become strong, producing locally gusty winds. Latest
guidance indicates little overall movement of the low today,
before it moves W away from the islands tonight and Wednesday.
Therefore, a Flood Watch remains in effect for Oahu and Kauai County
today, and an extension in time through tonight may be required as
the convergence band E of the low lingers near Kauai, and
potentially Oahu.

Impactful weather associated with the kona low has cleared the
Big Island, but a jet streak rotating around the closed low aloft to
the NW has spawned a few heavy showers near Maui County this
morning. These are not expected to persist for very long, but a few
more soaking showers are possible over portions of Maui County
before the potential diminishes later this morning.

Strong deep-layer high pressure nosing in from the NE will bring
locally strong, but mostly dry, trade winds from E to W across the
chain later today and Wednesday, with winds initially favoring
an ESE direction. These may be strong enough to warrant a Wind
Advisory in areas where terrain most accelerates the flow. Trades
will ease only slightly by the end of the week, with breezy ENE
winds anticipated. Trade winds may trend back to strong by the end
of the weekend as strong high pressure builds far N of the islands.

&&

.AVIATION...
Early this morning, a kona low northwest of the island chain
continues to maintain deep moisture streaming across the western
half of the state, particularly over Oahu and its adjacent waters.
Rainfall intensity on Oahu has decreased over the past several
hours, but steady stratiform rain continues to cause impacts to
aviation interests. Prevailing MVFR conditions with pockets of IFR
ceilings and visibilities have been observed. These conditions
are expected to persist on and off throughout the day, though the
axis of greatest moisture is expected to eventually shift towards
Kauai later today or tonight.

Meanwhile, although shower coverage is not currently as
widespread across Maui County, pockets of heavy rain cannot be
ruled out throughout the morning based on latest satellite trends.
Additionally, IFR and LIFR visibilities and ceilings are
impacting Lanai and Molokai this morning and should persist for
the next several hours. On the Big Island, conditions have begun
to gradually improve.

Numerous AIRMETS remain in effect across the island chain. AIRMET
Sierra for mountain obscuration and/or IFR is in effect for all
islands but will need some adjustment later this morning based on
latest satellite and radar trends. This AIRMET will likely
continue throughout the day for all of Kauai and Oahu, and at
least most of the day for portions of Maui County. Additionally,
AIRMET Tango remains in effect for for moderate low-level
turbulence for all islands. In the upper levels between FL290/450,
SIGMET Foxtrot remains in effect for severe turbulence early this
morning from Kauai to the Big Island, though models show this
weakening throughout the morning. Finally, light icing continues
to be a possibility around Kauai and Oahu, which will continue
into this afternoon.

&&

.MARINE...
A slow west moving kona low far northwest of Kauai is still
producing convergence bands of locally moderate rain and isolated
thunderstorms along its eastern flank. These bands are forecast to
better consolidate over the waters from Oahu westward through
Thursday morning. As the kona low exits further west, high
pressure to the northeast of the state will build in and initiate
moderate to fresh trades for the remainder of the week.

The Small Craft Advisory (SCA) for all waters has been extended
through Wednesday morning due to the combination of strong
primarily east southeast winds and the resultant high seas.
Strengthening trade winds will likely maintain SCA conditions,
especially for the typically windier waters around Maui and Big
Island, tonight through mid week. The SCA that is in effect
Wednesday through early Thursday morning covers all waters east of
the Kaiwi Channel sans the Maui leeward waters.

The fading north swell will keep north and some west facing shores
elevated through the day. The arrival of a couple of small, medium
period north northwest swells will maintain very small north and
west
facing shore surf into the weekend. A small south wind wave swell
recently generated by the passage of south-to-north moving rain bands
will taper off to the west today. Above seasonal south facing shore
surf will mainly impact the Oahu and Kauai southern coastlines today
before falling back to winter norms for the rest of the week. East
facing shore surf will maintain a rough chop the next several days.
This will be the result of the diminishing north swell wrap along
with a restrengthened mid week upstream trade fetch.

The Coastal Flood Statement continues through this morning due to
abnormally high ocean levels coinciding with seasonally high
tides. This could cause some elevated coastal run up during high
tides around sunrise. Even in those areas not directly affected
by swell, elevated ocean levels may indirectly induce some minor
coastal water inundation, especially during periods of heavy rainfall
within urbanized areas.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this afternoon for Niihau Kauai and Oahu.

Small Craft Advisory until 6 AM HST Thursday for Maui County
Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 AM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Leeward Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Vaughan
MARINE...Blood

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



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