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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 132226 AAA

Area Forecast Discussion
National Weather Service Honolulu HI
1226 PM HST Thu Dec 13 2018

Winds have been increased slightly based on surface observations,
necessitating a Wind Advisory for pats of the Big Island, Maui,
and Lanai. The strongest wind gusts will occur over and downwind
of terrain. The gusty winds supporting the advisory will persist
through the first half of tonight. A Gale Warning has also been
posted for the Alenuihaha Channel through midnight.

Note that the radar returns currently observed over Kauai and near
Oahu are not due to rainfall.


Another round of breezy and gusty trade winds is in store today
and tonight as strong high pressure passes north of the islands.
Winds will gradually decrease Friday through Monday. A stable
pattern of mainly windward and mauka showers will prevail through
Monday, providing dry conditions for most leeward areas. A front
is expected to move down the island chain during the middle of
next week, bringing an increase in showers and a brief period of
breezy northeasterly trade winds.


.PREV DISCUSSION.../ISSUED 950 AM HST Thu Dec 13 2018/

Another round of breezy and gusty trades is developing. A 1030 mb
surface high is currently located about 800 miles north of the
state, and winds at most island stations are similar to or
slightly stronger than reports from this time yesterday. As the
high drifts closer to the state today, expect a further increase
in the trades. A strong mid-level ridge will continue to build
overhead, and the inversion, currently observed between 3,500 to
7,500 ft, will strengthen and possibly lower. As the inversion
settles near mountainous terrain of the islands, strong wind gusts
may develop this afternoon and into tonight over and downwind of
the terrain on all islands. We will have to monitor closely, as a
Wind Advisory could be needed later today.

A stable pattern of mainly windward rainfall is due today. Small
pockets of moisture are currently affecting windward portions of
most islands. Expect upwards of a half inch of rainfall over
parts of these islands today, mainly on the Big Island, followed
by a downward trend tonight. Leeward areas will be mainly dry

A gradual decrease in trade winds is due for Friday into Monday,
while stable conditions and precipitable water values well below
an inch lead to modest rainfall over windward slopes. The surface
high will weaken and settle around 300 miles north of Kauai during
the weekend. Trades will drop to moderate to breezy levels on
Friday, then slowly decline further through the weekend. The
strong mid-level ridge will remain overhead, maintaining very
stable conditions, and with precipitable water values dropping to
around 3/4 of an inch, light shower activity is expected to be
confined to windward slopes, while dry conditions dominate over
leeward areas.

A shallow front is expected to move across the state during the
middle of next week, bringing an increase in showers and a brief
period of breezy northeasterly trade winds. At this time, the GFS
and ECMWF remain in remarkably good agreement in showing the front
reaching Kauai late Tuesday, then moving down the island chain
through Wednesday. Since the front will be shallow and moving
somewhat quickly, excessive rainfall is not expected.


A cell of high pressure north of Hawaii will maintain a tight
pressure gradient across the region today which should continue
the breezy trade wind pattern. AIRMET Tango remains in effect
through this afternoon all areas below 9000 feet MSL south and
west of the mountain ranges due to expected TEMPO moderate

Low clouds with bases around 2500 ft MSL and tops 7000-9000 ft
will push towards the islands today from the northeast and bring
light to moderate showers. This activity may bring brief MVFR
conditions to these areas due to lowered vis and mountain
obscuration. AIRMET Sierra will be if effect though at least 22Z
for mountain obscuration on the windward slopes of the Big Island.


Strong to near gale strength trade winds will return today and
tonight as a building high passes north of the area. The high
will weaken as it settles slowly south Friday through early next
week, gradually weakening the trades. A Small Craft Advisory (SCA)
remains in effect for all Hawaiian coastal waters through Friday
due to a combination of strong winds and rough seas. However, we
will closely monitor for possible gale conditions across the
typically windy waters around the Big Island and Maui.

The current northwest swell will gradually subside today, followed
by a smaller northwest swell tonight through Saturday. A large
northwest swell will build late Saturday and peak late Saturday
night and Sunday, likely producing warning level surf along north
and west facing shores. Another, even larger, northwest swell is
expected to build late Monday or Monday night, bringing another
round of warning level surf to north and west facing shores
Tuesday through the middle of next week.

An HSA is in effect through Friday for east facing shores of
Kauai, Oahu, Molokai, Maui, and the Big Island due to short-
period, rough surf produced by the strong trades. Surf should
lower below advisory levels by Friday as the trades ease.


Wind Advisory until midnight HST tonight for the Big Island, Maui
and Lanai.

High Surf Advisory until 6 PM HST Friday for east facing shores
of Kauai, Oahu, Molokai, Maui, and the Big Island.

Gale Warning until midnight HST tonight for Alenuihaha Channel.

Small Craft Advisory until 6 PM HST Friday for all remaining




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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