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Hawaiian Islands Synoptic Discussion and Guidance

226
FXHW60 PHFO 131333
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
333 AM HST Fri Dec 13 2019

.SYNOPSIS...
Trade winds will increase today as strong high pressure moves
north of the islands and a stalled front near Kauai falls apart.
Rainfall will favor windward slopes across the state. An
approaching upper trough will lead to increased high clouds today.
Trade winds will decrease during the weekend as an increasingly
stable pattern of mainly windward rainfall develops. Stronger
trades are expected early next week.

&&

.DISCUSSION...
A stalled front just northwest of Kauai separates a pair of
surface highs far northeast and northwest of the state. The
resulting pressure gradient remains rather slack, driving gentle
to moderate easterly trade winds across local waters this morning.
Stronger trade winds are noted around Kauai. Trade flow statewide
continues to push low clouds and showers across mainly windward
and mauka areas.

Trade winds will increase today as the front just northwest of
Kauai weakens and the surface high to our northwest begins to pass
by to the north. A typical pattern of windward rainfall will
prevail, with a few showers blowing over to leeward areas. An
approaching upper trough will tap high level moisture and spread
thickening high clouds over the islands today. Breezy trades will
slowly subside during the weekend. The surface high will move off
to the northeast and its trailing ridge north of the islands will
be weakened. This will allow trades to back off to moderate
strength by Sunday. The upper trough over the state will creep
eastward and slowly fill as a mid level ridge strengthens. As a
result, expect an increasingly stable pattern of mainly windward
showers and decreasing high clouds.

Stronger trades are due Monday and Tuesday as another high passes
north of the state, followed by another bout of strong trades late
next week. A ridge aloft will strengthen on Monday, maintaining a
rather stable weather pattern, with showers remaining focused on
windward slopes.

&&

.AVIATION...
Moderate northeast trade winds will continue through this forecast
period. A stationary front is still parked just northwest of
Kauai. Scattered showers with a few embedded heavier showers will
persist over Kauai and Oahu as well as the windward slopes of the
rest of the island chain. Kauai still has low ceilings therefore
will keep the AIRMET for mountain obscuration above 025. Also
added Oahu to AIRMET Sierra for mountain obscuration. Winds will
push some showers to leeward sections of Oahu, Molokai, and Maui.
Right now the other islands have just passing MVFR conditions.
Will issue AIRMET for those areas if warranted.

AIRMET Sierra in effect for Kauai and Oahu for mountain
obscuration.

&&

.MARINE...
The current large northwest swell will gradually subside over the
next few days. As a matter of fact, surf has recently lowered to
just below warning levels for exposed north and west facing
shores and is now at high-end advisory levels. The next large
swell, with forerunners expected to arrive late Sunday night, will
steadily build throughout the day Monday out of the west-
northwest to northwest direction. Surf will likely reach advisory
levels by Monday afternoon and possibly near warning levels Monday
night. This swell should slowly decline Tuesday into Wednesday,
but will likely be reinforced by another large northwest swell
Wednesday night into Thursday.

A high building north of the state will bring the return of
moderate to locally strong trades through the weekend. A slight
decline in trades is possible Sunday, but a new high building
north of the state will boost trades once again to strong speeds
by early next week. Strong trades are expected to persist through
the first half of next week.

Surf along east facing shores is currently below the seasonal
average. A boost in the trades today will lead to higher surf
along east facing shores of all islands, but especially for Kauai
where surf may near advisory levels for east facing shores. Some
increase in surf is also expected from a northeast swell entering
Hawaiian waters today and Saturday.

Due to the large northwest swell and an increase in trades, a
Small Craft Advisory has been posted for all Hawaiian Waters
except Leeward Maui County through early Saturday morning. (Seas
are expected to drop below advisory levels sometime Saturday.)

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

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

&&

$$

DISCUSSION...Powell
AVIATION...Chevalier
MARINE...Kinel

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



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