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Hawaiian Islands Synoptic Discussion and Guidance

596
FXHW60 PHFO 130627
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
827 PM HST Thu Dec 12 2019

.SYNOPSIS...
Trade winds will increase tonight and Friday as strong high
pressure moves north of the islands and a stalled front near Kauai
falls apart. Moisture along the front will fuel enhanced showers
around Kauai tonight, while rainfall favors windward slopes across
the rest of the state. An approaching upper trough will lead to a
period increased high clouds late tonight and Friday. Trades 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 east-northeast and northwest of the state. The
resulting pressure gradient over the islands remains rather weak
and is maintaining gentle to locally moderate easterly trade winds
this evening. However, as the high to the northwest approaches,
northeasterly winds have pushed through the front and reached
western Kauai. This increased moisture around Kauai, and an
approaching upper trough is producing some instability, leading
to active showers flaring over and around Kauai. Modest shower
activity from Oahu to the Big Island is confined largely to
windward terrain and the interior.

Trades will increase tonight and Friday as high clouds briefly
thicken. The somewhat strong surface high to our northwest will
pass by to the north as the front begins to fall apart. Expect
stronger trades to continue to build around Kauai tonight, then
state-wide on Friday as the front further erodes. Enhanced
moisture will continue to pool just north of Kauai while the
upper trough moves overhead. The greatest instability and
potential for heavy showers and thunderstorms will remain north
and west of Kauai, but we have included a slight chance of
thunderstorms for the Garden Isle tonight. Elsewhere, a typical
pattern of windward rainfall will prevail, with a few showers
blowing over to leeward areas. The approaching upper trough will
also tap high level moisture and spread thickening high clouds
over the islands on Friday.

Breezy trades will slowly subside during the weekend as high
clouds thin. The surface high will move off to the northeast, and
its trailing ridge north of the islands will be weakened,
allowing 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 into Friday. A
stationary front is parked just northwest of Kauai. Scattered
showers with a few embedded heavier showers will persist over
Kauai and on the windward slopes of the rest of the island chain.
Kauai has low ceilings therefore will issue an AIRMET for mountain
obscuration above 025. 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 for mountain obscuration.

&&

.MARINE...
The current large northwest swell will gradually subside over the
next few days. Surf will remain at warning levels through tonight
for exposed north and west facing shores and is expected to drop
to advisory levels Friday. 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 stationary front just NW of Kauai will bring some unsettled
weather around Kauai tonight. Meanwhile, 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 tonight and Friday 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 Friday 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 Warning until 6 AM HST Friday 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 6 AM HST Friday for Kona.

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