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Hawaiian Islands Synoptic Discussion and Guidance

693
FXHW60 PHFO 281945
AFDHFO

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

.SYNOPSIS...
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.

&&

.DISCUSSION...
A nearly stationary 1031 mb surface high is centered more than
1800 miles east-northeast of Honolulu late this morning. A surface
ridge extends west-southwest from this high through a point about
300 miles north of Lihue. The pressure gradient south and
southwest of these features is maintaining light to moderate east
to east-southeast wind flow across the state.

Water vapor imagery shows a shortwave trough is currently moving
by just north of the islands. The shortwave is providing some
enhancement to the low clouds and showers over and around the
state this morning. Patches of broken clouds and scattered showers
are observed on visible satellite and radar this morning. The light
east southeast winds are focusing most of the clouds and showers
over windward and mauka areas. For this afternoon, anticipating some
afternoon clouds and showers over interior sections of the islands
due to sea breezes building in.

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 flow. As the shortwave quickly
digs towards the southeast 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 late tonight as the shortwave digs in to the east of the
state tonight and Wednesday. The presence of this trough aloft
east of the area will continue enhance the low clouds and
precipitation tracking in from upstream through the rest of
the work week and possibly into Saturday.

Therefore, expect slightly wetter trade wind conditions after
tonight with the relatively weak low level flow focusing most of
these low clouds and showers over windward and southeast facing
slopes. Sea breezes will continue to dominate during the
afternoons and produce cloud build ups and 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 moderate rains 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.


&&

.AVIATION...
VFR conditions are expected to prevail. Some lingering clouds and
showers over the windward sides prompted AIRMET SIERRA for
mountain obscuration. Conditions are beginning to improve over
most of the area, so expect the AIRMET to be dropped shortly.
Some additional clouds are possible again overnight, which may
once again prompt an AIRMET for mountain obscuration. A north to
south oriented jet stream moving over this islands today may
likely require a SIGMET shortly for upper level clear air
turbulence.


&&

.MARINE...
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 below advisory levels along exposed
north and west facing shores through mid week.

Winds should remain below advisory levels through Friday as the
ridge of high pressure north of the state slightly weakens in
response to a two weak cold fronts passing north of the area.
Advisory-level winds will likely return across portions of the
waters over the weekend as high pressure builds north of the state
behind the second front.

Surf along east facing shores will remain small and choppy through
the week due to persistent moderate to fresh onshore winds, but
increase over the weekend as winds strengthen again locally. 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.

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. This is due to a powerful
storm-force low currently tracking east-northeastward across the
northwest Pacific. Recent ASCAT and altimeter obs support latest model
model runs with regard to wind speeds and resulting sea heights.
The large west-northwest swell associated with this system 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 are expected
Friday into the weekend before trending down through the beginning
of next week. The southern shores could see impacts from this
swell again at select spots due to wrapping around the islands.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Oahu
Windward Waters-Maui County Windward Waters-Big Island Windward
Waters.


&&

$$

Discussion...Foster
Aviation...M Ballard
Marine...TS

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