High Surf in Hawaii, March 31, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
358 AM HST Fri Mar 31 2017

.SYNOPSIS...
High pressure remains the dominate feature, with moderate trades
over the eastern half of the state, and lighter winds veering out
of the east southeast across the western half. Showers will be
focused over the windward and mauka areas this morning. Clouds are
expected to form over the leeward sections  Showers will increase
as a band of clouds moves in from the east through Friday
morning. Trade winds will become breezy beginning Friday night
through the weekend with increasing windward rainfall expected.

&&

.DISCUSSION...
General forecast philosophy remains the same, so no significant
changes to the gridded forecast with the morning package.

The cloud band that was just beginning to move over the islands
last evening has brought some showers with reduced visibility to
portions of the Big Island and Maui. The southern portion of the
band continues to impact those areas in the pre-dawn hours. The
portion of the band north of Maui has started to break apart, and
that trend is expected to continue with day time heating. There
remains an increase in low level moisture, and some showers are
being carried in, so have left the windward POPs along for Kauai
and Oahu for this morning.

High pressure remains far to the northeast, with a ridge north of
the islands. Trade winds remain lighter over the western end of the
island chain. Similar to yesterday, expect some clouds and
showers to form over the leeward areas of Kauai and Oahu this
afternoon due to the lighter winds over that end of the state. A
frontal system far north of the islands is expected to remain
north of the islands as it lifts to the north. As it does so, the
high far to the northeast is expected to strengthen, bringing a
return to a moderate to locally breezy trade winds to the islands
this weekend.

Clouds and showers will then remain focused over windward and
mauka areas, with the highest chance for rain being during the
overnight and early morning hours. As the trades increase, some of
the clouds and showers could reach leeward areas. Ridging aloft
over the state will keep the atmosphere stable and limit vertical
height of the clouds. So we do not anticipate heavy rain or
flooding concerns with the increased shower activity.

The surface high to our northeast will begin weakening Sunday
night in response to a front passing to our north. The trade winds
will maintain through the weekend but begin to decrease to
gentle to moderate levels breezes by Tuesday. A drier airmass
will also usher in from the east on Monday resulting in fewer
trade wind showers. Lighter trade winds and less showers carrying
in will continue into midweek.

&&

.AVIATION...
Easterly trade winds will continue over the next 24 hours with
mountain ranges blocking wind flow along western slopes of all
islands allowing some sea breeze cloud development and wind
direction variability during the late afternoon to early evening
time period. Light showers with brief MVFR ceilings with vicinity
showers are also possibly mainly along eastern slopes of all
islands on Thursday after sunset.

There are no AIRMET's currently in effect.

&&

.MARINE...
A large west-northwest swell (310 deg) that was initiated by a
strong weather system, now far north of the state, has begun
affecting the islands, will peak later today into tonight, then
slowly ease over the weekend. Warning and/or advisory-level surf
for exposed north- and west-facing beaches is expected during this
time. Seas have been ramping up quickly to Small Craft Advisory
(SCA) levels and will peak in the 10- to 14-ft range, before
slowly easing through the weekend. If the swell comes in larger than
predicted, warning-level surf could continue into Saturday before
dropping to advisory levels Sunday. A High Surf Warning has been
issued for north and west shores of most of smaller islands
through tonight, with the west shores of the Big Island being
included by noon today. An SCA has also been issued for hazardous
seas for most waters through Saturday.

Local winds and wind-generated waves will likely remain below
advisory levels through today as the ridge of high pressure north
of the state slightly weakens in response to a stalling front
northwest of the islands. Advisory-level trade winds will 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 due to
moderate to fresh onshore winds. Surf will begin to build over the
weekend as trade winds strengthen locally and upstream of the
islands, and may near or reach advisory levels along east-facing
shores by the end of the weekend and into early next week.

A slight increase in surf along south-facing shores is possible
through Saturday due to a long-period swell (190 deg) from recent
activity across the southern Pacific. In addition to this south
swell, wrap from the previously discussed large west-northwest
swell will add some to the mix today at select spots along
exposed southern facing beaches.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Warning from noon today to 6 AM HST Saturday for Kona-
Kohala.

Small Craft Advisory until 6 PM HST Saturday for all Hawaiian
waters except Maalaea Bay-

&&

DISCUSSION...M Ballard
AVIATION...Bohlin
MARINE...Kinel