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Hawaiian Islands Synoptic Discussion and Guidance

282
FXHW60 PHFO 110626
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
826 PM HST Mon Dec 10 2018

.SYNOPSIS...
A series of high pressure systems passing north of the islands
will maintain breezy, and strong trade winds through most of the
week. The high will begin weakening on Friday and into the weekend,
leading to calmer trades. A band of clouds and showers is expected
to fall apart in the next couple of days with less trade wind
showers on tap for most of the second half of the week. A change
in the weather is expected early next week as a front approaches
the islands from the northwest.

&&

.DISCUSSION...
Satellite and radar imagery this evening shows a wet band of
unstable cumulus clouds moving across Kauai, Oahu and Molokai.
Short range models continue to predict a wet weather pattern for
much of the Hawaiian Islands tonight and tomorrow. The highest
chances for measurable rainfall will trend towards the windward
and mountain slopes with the stronger shower bands producing
scattered showers over leeward areas.

In the large scale weather pattern a strong west to east oriented
jet stream will drive a several high pressure systems across 30
degrees north latitude this week just to the north of Hawaii.
These high centers will reinforce the strong pressure gradient
over the islands through Thursday before weakening a bit with
more moderate trade winds on Friday. A wind advisory remains in
effect for the windier areas of Maui and Hawaii Counties until
Tuesday evening.

A combination of good low level forcing due to strong winds mixing
through the boundary layer with convergence along mountain ranges
and upper level divergence, will help elevate the trade wind
subsidence inversion into the 7000 to 9000 foot range through much
of the work week. This higher inversion layer will allow deeper
clouds to form producing a wet weather pattern across the Hawaiian
Islands through Friday. These showers will tend to favor windward
and mountain slopes of all islands with some of the stronger
showers passing over leeward sections especially in the overnight
hours. Smaller scale changes in coverage and intensity of the
shower bands will vary from day to day, however the overall wet
trend will remain in the forecast.

The large scale weather pattern changes a bit by this weekend. The
high pressure ridge to the north of Hawaii will weaken as a cold
front dives towards the islands from the northwest. Trade winds
will diminish as the ridge breaks down and pressure gradients
weaken. By Sunday the winds at the 850 MB level will turn
southeasterly creating a blocking effect on precipitation. This
blocking effect occurs when the smaller islands fall in the lee,
or downwind, of the Big Island. The Big Island creates a bow wave
effect (imagine a large rock in a stream with water flowing
around it) pushing clouds and showers away from the islands while
enhancing stable downward motions aloft. Expect the drier period
to last through next Monday as lighter winds also allow a land
and sea breeze wind regime to develop across all islands. Clouds
will trend towards interior sections of each island due to sea
breeze convergence with clearing at night due to land breeze
divergence.

On Tuesday the pattern changes back towards trade winds, as a
strong high pressure center tracks across the Central Pacific
north of the islands with a shallow cold front dropping down out
of the northwest. Looks like more wet weather for the middle part
of next week in the extended weather outlook. Stay tuned.

&&

.AVIATION...
Moderate to strong winds will continue through twentyfour hours
and possibly beyond. High topped stratocumulus and scattered showers
will continue to favor windward and mauka areas. Isolated MVFR
conditions may become more widespread towards morning. An AIRMET
for mountain obscuration could become necessary for winward
facing slopes, especially on Kauai, Oahu, and Molokai.

AIRMET Tango for low-level turbulence below 8,000ft will likely
remain necessary through at least Tuesday afternoon. Easing of
the moderate to strong winds is not expected until later in the
workweek. AIRMET Tango also addresses strong surface winds greater
than 30kts.

&&

.MARINE...
A series of highs building to the north of the state will keep
strong trade winds blowing across the coastal waters through the
week. The first in the series of highs will shift steadily
eastward well to the north of the islands over the next couple of
days as a cold front approaches from the west. This cold front
will then pass by north of the state on Wednesday, bringing a
brief and subtle easing to the trade winds across the marine
area. The next high will then build north of the islands Wednesday
night through Friday, resulting in strengthening trade winds once
again. This high will then slowly settle southward and closer to
the state over the weekend into early next week, and this will
result in a gradual weakening of the trade winds across the marine
area.

A Gale Warning remains in effect for the Alenuihaha Channel
through Tuesday and a Small Craft Advisory (SCA) remains in effect
for the rest of the coastal waters through Tuesday night. The
Gale Warning may need to be extended through Tuesday night for the
Alenuihaha Channel. Meanwhile, the SCA will likely need to be
extended for the entire marine area through the end of the work
week due to winds, seas, or a combination of the two.
Additionally, another round of Gales could affect the Alenuihaha
Channel Thursday and Thursday night as the trade winds associated
with a new strong high get cranking once again.

The current north-northwest will continue to decline through
Tuesday night. The first forerunners of a new long-period
northwest swell are expected to begin arriving late tonight or
early Tuesday, with this swell slowly building through the
remainder of the day and Tuesday night, before peaking on
Wednesday. Surf is expected to near High Surf Advisory (HSA)
levels on Wednesday along exposed north and west facing shores. A
smaller northwest swell is then expected on Friday, followed by a
new and larger northwest swell late in the weekend into early next
week, which could bring another round of advisory level surf to
north and west facing shores.

Meanwhile along east facing shores, strong trades are producing
rough choppy surf. The Mokapu buoy is showing an increase of
short-period energy over the last 24 hours and we should continue
to see an increase in surf along east facing shores on Tuesday.
The HSA remains in effect for east facing shores of Kauai, Oahu,
Molokai, Maui, and the Big Island through Wednesday night but will
likely need to be extended due to the strong trade winds
persisting through the end of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST Tuesday for Lanai-Kahoolawe-Maui
Central Valley-South Big Island-Big Island North and East-Kohala-
Big Island Interior.

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 Windward Waters-Maui County Leeward Waters-
Maalaea Bay-Pailolo Channel-Big Island Windward Waters-Big
Island Leeward Waters-Big Island Southeast Waters.

Gale Warning until 6 PM HST Tuesday for Alenuihaha Channel.

&&

$$

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Jelsema

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