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Hawaiian Islands Synoptic Discussion and Guidance

204
FXHW60 PHFO 250633
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Tue Apr 24 2018

.SYNOPSIS...
Strong high pressure far north-northeast of the state will
maintain breezy trades, with windward and mauka showers through
early Wednesday morning. The trades will gradually weaken from
Wednesday through early Thursday, then shift out of the southeast
by late Thursday ahead of a front. The front will affect the
western end of the state Friday and Friday night, then move to
the eastern end of the island chain on Saturday. A band of clouds
and showers assoicated with the front will likely bring wet and
unsettled weather to portions of the state starting late Thursday
or Friday, and continuing into this weekend.

&&

.DISCUSSION...
A 1035 mb surface high located about 1,450 miles north-northeast
of Honolulu is maintaining a tight pressure gradient across the
state early this evening. As a result, the breezy and gusty low-
level trade wind flow persists across the island chain. Aloft,
a mid-level ridge west of the state is providing northwesterly
flow high above the island chain, with significant upper-level
subsidence. This is maintaining stable atmospheric conditions
over the region. The early afternoon soundings from Kauai and the
Big Island indicated the low-level trade wind inversion was
between 4,500 and 6,500 feet. The latest MIMIC precipitable water
(PW) values continue to show widespread areas with near April
normals of 1.0-1.1 inches, with pockets of 1.3-1.4 inches spread
around the state. Note that the PW values just upstream of the
state were 1 inch or less.

Satellite imagery and radar reflectivity data continue to show a
band of low clouds and trade showers moving slowly westward over
the islands. These showers have primarily been affecting windward
sections, but the strong trades continue to transport brief misty
showers over to leeward sections of some of the smaller islands.

Based on the latest forecast guidance, the surface high will
remain far north-northeast of the area. However, since it will be
gradually weakening, the pressure gradient will relax slightly.
This will bring trade wind speeds down a notch on Wednesday. The
mid-level ridge will also remain in place, which will keep the
stable atmospheric conditions across the region. Once the low
clouds and showers push west of the state later tonight, the
slightly drier air upstream of the islands will begin the move in.
This will likely reduce low clouds and shower coverage on
Wednesday.

Beyond Wednesday, expect some changes to our present trade wind
weather pattern to begin taking shape. The GFS and ECMWF forecast
models continue to show the mid-level ridge will weaken due to
an upper-level trough digging down from the northwest toward the
state. This will result in some destabilization of the atmosphere,
especially by late Thursday. The trough aloft will also likely
cause the development of a deep surface low about 800 miles north
of the islands. This feature will drag a cold front down about 250
miles north and northwest of Kauai by Thursday night, which will
cause the trade winds to weaken, and eventually shift to a
southeast direction.

The models also depict an area of increasing low-level moisture
moving in from the east by Thursday. Assuming the enhanced
moisture arrives in the islands, expect an increase in low clouds
and showers, especially over windward areas. A surface trough may
also form somewhere near the western islands. We will have to
monitor this situation, since there is a potential for heavy
downpours, mainly over windward sections of the western half of
the island chain. In addition to the low clouds and precipitation,
expect increasing high clouds later this week.

On Friday, the winds will continue to shift to a southerly
direction ahead of the approaching front. The front is forecast to
reach Kauai some time on Friday, and eventually push down across
Oahu to Maui County from Friday night into Saturday. The upper-
level trough will be passing over the area, while the islands will
be near the entrance region of a developing jet stream aloft.
This indicates the potential for localized heavy rainfall along
the front as it moves down the island chain, mainly from Kauai to
Molokai.

There is high potential for wet and unsettled weather over
portions of the state this weekend. The forecast models continue
to depict differing scenarios for the front once it reaches Maui
and the Big Island. The front may stall near the Big Island, or
eventually push east of the state. If it stalls, there is a good
chance enhanced moisture will be drawn up over the eastern
islands. This could lead to flooding rainfall for the eastern end
of the state starting some time this weekend. To the west of the
front, a drier and more stable northerly flow will fill in.
Therefore, while there is still a great deal of uncertainty in
where the frontal will move to, Kauai and Oahu have the greatest
chance of experiencing drier conditions starting as early as
Saturday.

Heading into early next week, a return to typical trade wind
weather appears to be unlikely. An upper-level trough will remain
over the region, while a surface low is forecast to meander
several hundred miles north of the state. As a result, surface
winds will likely remain relatively light.

&&

.AVIATION...
Breezy trade winds will persist through the nighttime hours and
only gradually weaken Wednesday as high pressure remains far
north-northeast of the islands. VFR will predominate, but MVFR
conditions will mainly affect windward and mauka sections from
time to time as the trades carry showery low clouds through the
state. An AIRMET for mountain obscuration continues in effect over
Oahu and Kauai.

With the fresh to occasionally strong trades, the AIRMET for
TEMPO low level turbulence to the lee of the mountains also
remains posted.

&&

.MARINE...
A 1035 mb surface high far NNE of the islands is producing
moderate to strong trade winds over the area, and a Small Craft
Advisory (SCA) is posted for all Hawaiian waters through tonight.
The high will weaken as it moves southwestward Wednesday through
Thursday. This will cause the trades to gradually diminish,
causing the reduction to the area of coverage by the SCA to the
typically windy waters around Maui and the Big Island.

A low is forecast to form rapidly far N of the islands Thursday
night and Friday, with a front reaching Kauai early Friday
evening, then stalling over the eastern part of the island chain
Saturday afternoon. The front is expected to linger near the Big
Island through the rest of the weekend. Initially, right behind
the front, northerly winds may briefly reach SCA criteria with
some of the marine zones.

This low N of the islands will be generating a sloppy swell toward
the islands this weekend into next week, with associated seas
rising above 10 feet in most zones. But before this swell arrives,
a moderate 5 foot NW swell with a period of around 15 seconds, is
due to arrive late Thursday, and peak early Friday.

This swell is slated to be reinforced by a series of potential
large NNW swells, generated by the low N of the islands late in
the week. The amount of swell/surf that arrives in the islands
will depend on the evolution of the low and associated fetch, and
current indications are that a fairly significant swell will
produce high- end advisory-level surf along exposed N and W facing
shores, with the peak of the swell around Sunday. There is some
uncertainty with this swell, and the forecast will likely need
adjustments as details become clearer.

The weather pattern in the S hemisphere has become conducive for
S swells to arrive in the islands, and a series of relatively
small SW to S swells is expected over the next week or so. Lastly,
the rough and choppy surf along E facing shores brought on by the
breezy trades, will start easing off after Wednesday, with the
backing off of the trades.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

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-
Big Island Windward Waters.

&&

$$

Discussion...Houston
Aviation...Kinel
Marine...Lau

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