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Hawaiian Islands Synoptic Discussion and Guidance

391
FXHW60 PHFO 191410 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
410 AM HST Tue Jan 19 2021

.SYNOPSIS...
An unstable cloud band associated with a low level trough begins
to drift westward over Oahu and Kauai today with shower trends
increasing over the western islands through tonight. Strong
northeasterly winds will begin to ease over the eastern islands
later this morning, becoming moderate to breezy trade winds for
Maui and Hawaii Counties. Windy and wet weather conditions will
continue over Oahu and Kauai through this afternoon. Improving
weather trends are forecast from Wednesday through the end of the
week with moderate to breezy easterly trade winds forecast with
periods of showers favoring windward and mountain areas. Trade
winds are expected to strengthen through the weekend with more
scattered shower activity.

&&

.DISCUSSION...
Abundant unstable moisture associated with a low level trough
currently over Maui County this morning will move westward today
spreading widespread rain and locally heavy showers back into Oahu
and Kauai. This large band of showers will combine forces with
strong and gusty northeasterly trade winds to produce several
inches of rainfall along the windward and mountain slopes from
Kauai to Maui today. The Flash Flood Watch to cover the elevated
flooding threat was expanded westward to include Kauai County
today. Enough dynamic forcing and instability over the region may
produce a few thunderstorms along this unstable cloud band.
Stronger winds are expected to shift back into Kauai today, and
the Wind Advisory was expanded to include Kauai County.

Meanwhile out east, the Big Island will see improving weather
trends through the day, with gradually decreasing rainfall
amounts over Maui by this afternoon. A Winter Weather Advisory
remains in effect for the highest peaks on the Big Island above
12,000 feet elevation, where an additional 1 to 2 inches of snow
accumulations are forecast with patchy freezing fog until this
evening. Wind speeds will be weaker today over these eastern
islands. The Flash Flood Watch was cancelled for the Big Island.
The Wind Advisory for Maui County was also cancelled due to these
more moderate trade wind speeds.

Wednesday through Friday, trade winds will become moderate to
breezy with periods of showers favoring windward and mountain
areas, less showers are forecast over leeward areas.

Saturday and Sunday, trade winds will increase through the
weekend, becoming breezy to windy easterlies on Sunday. Isolated
to scattered showers remain in the forecast for both days favoring
typical windward and mountain areas.

&&

.AVIATION...
The front in the area has weakened to a trough and is moving
slowly west, nearing Molokai as of this writing. Areas of MVFR
cigs/vsbys can be expected along and near the trough,
particularly in windward areas. Moderate to strong north to
northeast winds will persist, mainly to the the west of the
trough axis, as robust high pressure remains far north of the
islands.

AIRMET Sierra remains in effect for mountain obscuration for
windward sections of Kauai and Oahu, and for all of Molokai,
Lanai and Maui.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain from Kauai eastward to Maui. AIRMET Tango
is also posted for high level turbulence over the western portion
of the island chain.

AIRMET Zulu is in effect for TEMPO moderate rime icing from Kauai
eastward to Maui.

&&

.MARINE...
A surface trough, which appears to be near Maui early this
morning, is moving slowly westward. There is also a broad surface
high pressure system far north of the region. The tight pressure
gradient between the trough and the surface high is producing
gusty northeasterly winds and elevated, rough seas over most of
the waters along and west of the trough. There are also elevated
seas near the Big Island. A Small Craft Advisory (SCA) is now in
effect for all waters, except Maalaea Bay. In addition to the
strong winds and rough seas, widespread cloud cover, locally heavy
downpours and isolated thunderstorms will likely continue across
most of the state into tonight. Once the surface trough moves west
of the state, expect a wet trade wind weather pattern to begin to
set up later this week as a surface high builds far north-
northeast of the region.

A large north-northwest swell combined with the locally generated
wind waves is producing very large and disorganized surf along
north and west facing shores of most of the islands early this
morning. As a result, a High Surf Warning remains in effect
through this morning for most north and west facing shores of the
smaller islands. There is also a High Surf Advisory (HSA) in
effect for north and west facing shores of the Big Island. The
forecast guidance shows the swell direction will become more
northerly later today and tonight. Therefore, we expect a HSA
will be needed for the north and west facing shores of the
smaller islands from this afternoon into tonight. Expect the HSA
to eventually be cancelled by early Wednesday as the local winds
decrease and the north swell gradually subsides.

Expect modest surf along most east facing shores due to the
locally strong winds during the next couple of days. The surf
will decrease somewhat by Thursday, but will increase again along
east facing shores this weekend as the trade winds strengthen.
Small background surf will persist along south facing shores
through next weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui.

Wind Advisory until 6 PM HST this evening for Niihau-Kauai-Oahu.

High Surf Warning until noon HST today for Niihau-Kauai Windward-
Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory until noon HST today for Kona-Big Island
North and East-Kohala.

Winter Weather Advisory until 6 PM HST this evening for Big
Island Summits.

Flash Flood Watch through this afternoon for Maalaea Bay.

Small Craft Advisory until 6 PM HST this evening for all
Hawaiian waters except Maalaea Bay.

&&

$$

DISCUSSION...Bohlin
AVIATION...Kinel
MARINE...Houston

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



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