Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 281412

Area Forecast Discussion
National Weather Service Honolulu HI
412 AM HST Tue Mar 28 2017

A surface ridge north of the state will maintain light to
moderate east to east-southeast surface winds across the islands
through mid-week. An upper level trough, which will dig east of
the island chain later tonight and Wednesday, is expected to bring
an increase in showers over windward facing slopes of the state
during the second half of the work week. In addition, a band of
clouds and showers ahead of a front may impact parts of the state
Thursday night and Friday, but the front is not expected to reach
the islands. A surface high building far northeast of the area
starting late Friday will produce breezy trades this weekend.


A nearly stationary 1031 mb surface high is centered more than
1800 miles east-northeast of Honolulu early this morning. A
surface ridge extends west-southwest from this high through a
point about 290 miles north of Lihue. The pressure gradient south
and southwest of these features is maintaining light to moderate
east to east-southeast winds across the state. The weaker
background surface winds, which are mainly over the western end of
the state, allowed local land breezes to become established over
some of the islands. especially parts of Kauai and Oahu.

An upper level short-wave trough moving by just north of the
islands has provided some enhancement to the low clouds and
showers over parts of the state. For example, the higher terrain
and interior sections of Oahu, as well as portions of the south
shore of that island, saw measurable rainfall overnight.
Additional low clouds and showers are along the windward sections
of the Big Island, Maui and Molokai. Low clouds and showers may
also affect parts of Kauai later this morning.

The surface ridge is expected to remain north of the islands
through Thursday night, which will continue to drive the light to
moderate east to east-southeast low-level flow across the state.
After the upper level short-wave trough passes by later today, a
mid-tropospheric ridge will keep the atmosphere relatively stable
through tonight. However, the forecast models continue to show an
increase in precipitation starting early Wednesday as an upper
level trough digs in to the east of the state later tonight and
Wednesday. The presence of this trough aloft east of the area will
potentially enhance the low clouds and precipitation tracking in
from upstream. Therefore, expect slightly wetter trade wind
conditions during mid-week. The low-level flow will focus most of
these low clouds and showers over windward and southeast facing
slopes. Some leeward areas may also have brief showers.

The western end of the surface ridge to our north is expected to
erode some time on Thursday, as a cold front approaches from the
northwest. This front will likely stall and weaken two to three
hundred miles north-northwest of Kauai by late Friday, so it is
not expected to ever reach the state. However, the forecast models
continue to suggest a band of clouds and showers will form near
the western end of the state to the southeast of the front. These
low clouds and showers might produce additional wet weather over
parts of the state, particularly the western islands, late
Thursday and Friday.

The middle tropospheric ridge will likely strengthen above the
state from late Friday through this weekend. This will result in
more stable atmospheric conditions. Also, a surface high is
expected to build far northeast of the region this weekend, which
will likely produce breezy conditions across the Hawaiian Islands.
We also expect a slightly drier air mass over most of the state
this weekend. There will be some low clouds and brief showers
transported mainly into windward and mauka areas by the trades.


High pressure northeast of the state will keep a light to
moderate trade wind flow in place today and tonight. Clouds and
showers will favor windward and mauka areas through the period,
with localized sea breezes leading to the development of
convective showers in leeward areas this afternoon. Occasional
MVFR cigs/vsbys in SHRA will affect windward areas through mid
morning, with PHTO and PHLI the most likely TAF sites to see these
conditions. Elsewhere, predominantly VFR conditions are expected
over the next 24 hours.

AIRMET Sierra is in effect for mountain obscuration for windward
sections of the Big Island, Maui, Molokai and Oahu. This AIRMET
may need to be expanded to include Kauai later this morning.
Conditions should improve in most areas by mid morning.


The west-northwest (290-310 deg) swell that peaked Sunday into
Monday will hold through mid week due to a slight reinforcement
tonight into Wednesday. A combination of this swell and moderate
to fresh breezes over the channels and waters around the Big
Island will continue to generate rough boating conditions.
Although the winds have dropped slightly below Small Craft
Advisory levels (25 kt), seas will hold around 10 ft through the
day today across windward waters between the Big Island and Oahu.
As a result, the Small Craft Advisory will remain up across these
areas for seas today. Surf associated with the west-northwest
swell should remain just advisory levels along exposed north and
west facing shores through mid week.

Winds will likely remain below advisory levels through Friday as
the ridge of high pressure north of the state slightly weakens in
response to a passing cold front well north of the area. Another
cold front is forecast to approach the region later in the week,
then stall and weaken north and northwest of the area into the
upcoming weekend. Advisory-level winds will likely return across
portions of the waters over the weekend as high pressure builds
north of the state behind this front.

Surf along east facing shores will remain small and choppy through
the week due to moderate to fresh onshore winds persisting. Surf
will begin to build over the weekend as the winds strengthen
locally and upstream of the islands.

A slight increase in surf along south facing shores will be
possible Thursday through Saturday due to a long-period swell (190
deg) associated with recent activity across the southern Pacific.
In addition to this southerly swell, another large west-northwest
swell (discussed in more detail below) expected by Friday could
impact the southern shores once again at select spots due to
wrapping around the islands.

Later in the week, the main forecast challenge will be centered
on another, slightly larger, west-northwest (290-310 deg) swell
that will impact the islands Thursday night through the weekend. A
powerful storm-force low that may have briefly reached hurricane
force earlier yesterday morning off the coast of Japan is
currently tracking east-northeastward across the northwest
Pacific. The latest surface analysis showed this storm-force low
centered just under 1200 nm east of Tokyo with a central pressure
of 980 mb. An ASCAT pass earlier this morning reflected this
system and showed a large batch of gale to storm force winds
(34-50 kt) out of the west focused toward the region. An altimeter
pass over the this batch of storm-force winds showed maximum seas
up to 38 ft, which was in good agreement with all wave model
guidance at that time. Model consensus remains in decent agreement
over the upcoming few days and depicts this system continuing to
deepen to around 970 mb as it approaches and moves across the Date
Line tonight into Wednesday. A large west-northwest swell
associated with this system will result across the region, that
could reach the islands as early as Thursday night, peak through
the day Friday and Friday night, then slowly ease over the
upcoming weekend. Warning-level surf and advisory-level seas will
be expected Friday into the weekend before trending down.


Small Craft Advisory until 6 PM HST this evening for Oahu
Windward Waters-Maui County Windward Waters-Big Island Windward



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