High Surf Advisory 27, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
320 AM HST Sat Mar 25 2017

.SYNOPSIS...
A persistent ridge of high pressure north of the islands will keep
trade winds blowing across local waters through the week, with a
slight increase into the locally breezy range expected through
this weekend. Clouds and showers will favor windward slopes,
especially at night and in the mornings. Slightly weaker trade
winds are expected by midweek as upper troughing develops across
the area.

&&

.DISCUSSION...
A surface ridge extending east to west along 27N, to the north of
the main Hawaiian islands, keeps moderate to locally strong
easterly trade winds blowing across local waters this morning.
Remnant moisture from an old frontal boundary passed west of the
islands overnight, leaving a typical trade wind weather pattern
of clouds and showers locally.

Satellite loop shows scant low cloud cover statewide, with
scattered clouds lingering mainly across windward shores and
slopes of the smaller islands. Earlier cloud cover across the Big
Island has decreased dramatically, leaving mostly clear skies
everywhere except for along windward shores and slopes, where
scattered low clouds persist. Radar shows very little shower
activity, mainly as isolated light showers embedded within trade
flow upwind from Oahu and the Big Island. A single notable shower
has formed over land near Kailua-Kona on the Big Island.

Models show the ridge will remain north of the islands through the
forecast period, with no areas of enhanced moisture or disturbed
weather expected through the weekend and into early next week.
High pressure along the ridge will consolidate north and
northwest of the islands, tightening the local pressure gradient
and causing trades to increase today across the eastern half of
the main island chain. Clouds and showers will favor windward and
mauka areas, with leeward areas of the Big Island expected to
cloud up each afternoon and evening, then clear out each night.
Deep troughing forecast during the middle of next week west of
170W will weaken the pressure gradient across the islands, causing
trade winds to decrease slightly from Tuesday onwards. An uptick
in windward rainfall may occur Tuesday or Wednesday as ridging
aloft is eroded by a trough skirting by to the north.

&&

.AVIATION...
Some clouds and showers are lingering over the windward side of
Kauai early this morning but are expected to thin as the day wears
on. VFR conditions are expected to prevail. Trade winds will
increase today, which may require AIRMET TANGO for moderate
turbulence to the lee of the mountains. No AIRMETs are in effect
at this time.

&&

.MARINE...
Forerunners of the large incoming west-northwest swell have
been arriving on schedule at buoy 51101 to the northwest of Kauai
during the last few hours. A Friday evening satellite altimetry
pass showed that this swell appears to be running close to the
Wavewatch guidance levels, and the latest ECMWF wave model has
also come into much better agreement with the Wavewatch on the
timing of this swell. Surf on Kauai is expected to build to
warning levels (25 ft north/20 ft west) during the day, then peak
tonight into Sunday morning. Warning level surf should continue
through the day Sunday, before gradually subsiding early next
week. The more westerly direction of the swell will likely cause
Kauai to shadow the other smaller islands. The current forecast
continues to hold surf for exposed north and west facing shores of
Oahu and Maui County just under warning levels. This will be
monitored as more observations come in from the northwest buoys,
and a slightly larger or more northwest swell could bring warning
level surf to Oahu/Maui County as well. Surf along west facing
shores of the Big Island is also forecast to reach low-end warning
level (12 ft) tonight through Sunday night. A high surf warning
has been posted for north/west facing shores of Niihau and Kauai,
and for the west facing shores of the Big Island. A high surf
advisory has been posted for north/west facing shores of Oahu and
Molokai, and for north facing shores of Maui.

Otherwise, small long period south swells will continue through
this weekend and into next week, with a slightly larger long
period south swell forecast for Wednesday night through Friday.
Another large west-northwest swell will be possible from next
Friday into the following weekend.

The Small Craft Advisory (SCA) remains posted for the typically
windier waters around the Big Island and Maui County through
Sunday night. High pressure strengthening northeast of the state
will provide locally breezy conditions through the weekend. The
SCA has been expanded to cover the most of the other coastal
waters late this morning, and the remaining waters early this
evening, as seas build above 10 ft due to the large incoming
west-northwest swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from noon today to 6 AM HST Monday for
Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai-Maui Windward
West-Maui Central Valley-Windward Haleakala.

High Surf Warning from 6 AM this morning to 6 AM HST Monday for
Niihau-Kauai Windward-Kauai Leeward.

High Surf Warning from 6 PM this evening to 6 AM HST Monday for
Kona-South Big Island-Kohala.

Small Craft Advisory until 6 AM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 AM HST Monday
for Oahu Leeward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

Small Craft Advisory from 10 AM this morning to 6 AM HST Monday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui
County Windward Waters.

&&

DISCUSSION...Powell
AVIATION...M Ballard
MARINE...Jacobson