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Hawaiian Islands Synoptic Discussion and Guidance

955
FXHW60 PHFO 251402
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
402 AM HST Wed Apr 25 2018

.SYNOPSIS...
Strong high pressure far north-northeast of the state will
maintain breezy trades, with windward and mauka showers. The
trades will gradually weaken from tonight 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 associated with the
front will likely bring wet and unsettled weather to portions of
the state starting late Thursday or Friday, and continuing this
weekend.

&&

.DISCUSSION...
A 1034 mb surface high located about 1,330 miles north-northeast
of Honolulu is maintaining a tight pressure gradient across the
state early this morning. 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 morning soundings from Kauai and the Big
Island indicate the low-level trade wind inversion is between
7 and 8 thousand feet. The latest MIMIC precipitable water (PW)
values are 1.0-1.1 inches, with isolated pockets of up to 1.3
inches, in the vicinity of the state. Note that the PW values just
upstream of the state are 1 inch or less.

Satellite imagery and radar reflectivity data show broken low
clouds with scattered trade showers streaming into windward Maui
and the Big Island. Elsewhere, the coverage of low clouds and
showers has decreased dramatically overnight compared with
Tuesday evening.

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
starting later today or tonight. This will likely bring trade
wind speeds down a notch by this evening. The mid-level ridge will
also remain in place, which will keep the stable atmospheric
conditions across the region. The slightly drier conditions
upstream of the islands will continue to spread westward. This
will likely reduce the coverage of low clouds and showers over
many areas later today compared with Tuesday.

Expect some changes to our present trade wind weather pattern to
begin taking shape by Thursday. 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 on
Thursday. 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 toward Kauai late Thursday night, which will cause the trade
winds to weaken, and eventually shift to a southeast direction.

The models also depict an area of slightly enhanced low-level
moisture moving in from the east by Thursday. Assuming this pocket
of increased moisture arrives in the islands, expect an increase
in low clouds and showers, especially over windward areas. The
models also continue to hint that a surface trough may develop
near the western islands. This feature may become the focus for
prefrontal showers Thursday night and Friday morning, especially
over Kauai and Oahu. We will have to monitor this situation, since
there is a potential for heavy downpours, mainly over the western
half of the island chain.

On Friday, the winds will continue to shift to a south to
southwest direction ahead of the approaching front. The front is
forecast to reach Kauai Friday afternoon, and eventually push down
across Oahu to Maui County from Friday evening through early
Saturday. At the same, the upper-level trough is forecast to pass
over the area, while the islands will be near the entrance region
of a developing jet stream aloft. Assuming these features evolve
in this manner, it would greatly increase the likelihood of heavy
rainfall, and possibly localized flooding, ahead of and along the
front as it moves down the western end of the island chain,
especially from Kauai to Molokai.

The potential for wet and unsettled weather shifts to the central
and eastern islands as the front continues to push eastward 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. The latest guidance is leaning
toward the front eventually pushing just east of the Big Island.
We will keep conditions wet in the forecast through Sunday over
the eastern end of the state, but there is a chance things could
improve, especially over Maui County, later this weekend. To the
west of the front, a drier and more stable northerly flow will
fill in. Therefore, while there is still some uncertainty in
where the front will move to, Kauai and Oahu have the greatest
chance of experiencing drier conditions starting as early as mid-
day Saturday.

Heading into early next week, a return to typical trade wind
weather appears to be unlikely. The 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. In addition, the models
are hinting that the remnant moisture from the frontal band may
push back westward over the state some time early next week. If
so, expect a wet weather pattern to return to portions of the
state.

&&

.AVIATION...
Breezy trade winds will persist and only gradually weaken later in
the day 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
low clouds, some with embedded showers, through the state. An
AIRMET for mountain obscuration continues in effect over Kauai,
though will likely be cancelled this morning.

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 1034 mb surface high far NNE of the islands is producing
moderate to locally strong trade winds over the area. A Small
Craft Advisory (SCA) is posted for the windier marine zones
around Maui and the Big Island for today and tonight. The high
will weaken further as it moves southwestward through Thursday,
causing the trades to become light and variable on Thursday
night, and the cancellation of the SCA.

A low is forecast to form rapidly far N of the islands Thursday
night and Friday, with a front reaching Kauai early Friday
evening. The front stalls near the Big Island Saturday afternoon,
and lingers there for the rest of the weekend. Initially, right
behind the front, northerly winds may briefly reach SCA criteria
with some of the marine zones.

The 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, peaking 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 today, along with the
diminishing 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.


&&

$$

Houston

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