High Surf in Hawaii, January 18 to 24, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
333 AM HST Thu Jan 18 2018

.SYNOPSIS...
Strong and gusty trade winds will persist through today as robust
high pressure passes north of the state. Clouds and showers will
favor windward and mauka areas, though a few showers will
spread leeward on the gusty trades from time to time. Winds will
gradually decrease from Friday into the weekend as the surface
high is pushed far northeast of the islands.

&&

.DISCUSSION...
A 1036 mb high about 1000 miles north of Kauai continues to boost
trade winds across the islands this morning, while clouds and
showers associated with a decaying front move across Kauai.
Ridging aloft has maintained a solid inversion based at about 6700
feet, according to overnight soundings. Winds have been strong
and gusty through the night, with numerous stations on Maui County
and the Big Island reporting advisory level winds. Wind gusts at
isolated locations in leeward Kohala on the Big Island have
reached High Wind Warning threshold.

Since the high is forecast to strengthen as it moves east, we
expect winds will remain strong and gusty through today. The
inversion will likely cause wind acceleration over and around
island terrain and, as a result, the Wind Advisory will remain in
effect for all islands through this afternoon. The High Wind
Warning will remain in effect for leeward Kohala, also through
this afternoon. One or both products may need to be extended into
tonight or possibly Friday, especially for portions of the Big
Island and Maui.

Showers will focus across windward slopes during the next couple
of days. Clouds and showers along the decaying frontal band are
expected to slowly lift northward late today and Friday, while a
separate area of low level moisture extending east from the Big
Island spreads up the island chain on Friday.

Trade winds will begin to decrease on Friday and continue
decreasing into the weekend. During this time, the high will
weaken and move to the northeast of the state, leaving an
associated surface ridge about 300 miles north of Kauai by Friday
night. A North Pacific trough will erode the surface ridge during
the weekend. During this time, trade winds will veer out of the
east to east-southeast and may become light enough for daytime
sea breezes to dominate across the western end of the island
chain.

Models show moderate easterly trades hanging on over the Big
Island with east southeasterly flow across the smaller islands
for early next week. The mid level ridge currently over the state
will erode significantly and be pushed off to the east, leading
to some instability and the potential for heavier showers.

&&

.AVIATION...
Robust high pressure passing N of the area will support strong
trade winds through tonight while a ridge aloft will maintain a
stable island atmosphere. This will lead to strong low-level
winds, with AIRMET TANGO in effect for moderate to severe
turbulence near and downwind of the terrain, and for surface wind
speeds.

Two bands of low clouds embedded within the trade flow will
continue to make for locally cloudy conditions. One band is a
stalled frontal band moving over Kauai from the E, with a similar
cloud band near and upwind of the Big Island. In between, there are
fewer clouds over Maui county and Oahu. Periods of MVFR VIS/CIG
in brief showers are expected at PHLI and PHTO, with radar
detecting scattered light showers moving rapidly E statewide.
Thus far AIRMET SIERRA for mountain obscuration is not anticipated,
but observations will be monitored for its possible issuance.

&&

.MARINE...
Strong- to gale-force winds are forecast to hold through tonight,
then gradually trend down Friday through the weekend as strong
high pressure to the north shifts eastward. The only change in
the morning package is to extend the small craft advisory through
Friday for most waters due to a combination of winds and seas.

For the upcoming weekend, forecast confidence begins to lower due to
significant differences shown between the ECMWF and GFS solutions.
The GFS advertises light and variable winds, while the ECMWF depicts
moderate to fresh trades holding. The current forecast is more in
line with the GFS solution, but is subject to changes over the
coming days. Despite these differences, winds will likely trend down
over the weekend. Surf, however, will likely hold around advisory
levels (8 ft faces) over the weekend along east facing shores.
Guidance reflects this and suggests a large area of fresh to
strong trades holding well east of the state through this time as
the winds trend down locally.

Surf along north and west facing shores continues to drop this
morning and is expected to remain below advisory levels through
the weekend. A small northwest swell is expected Sunday, that will
be reinforced with a moderate northwest swell by Monday,
potentially reaching advisory levels. This reinforcement will peak
by late Monday, then gradually lower through midweek.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala-Kona-
South Big Island-Big Island North and East-Big Island Interior.

High Surf Advisory until 6 PM HST Friday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

High Wind Warning until 6 PM HST this evening for Kohala.

Gale Warning until 6 AM HST Friday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Kaiwi Channel-Maui County Windward Waters-Big Island Windward
Waters.

Small Craft Advisory until 6 AM HST Friday for Kauai Leeward
Waters-Oahu Leeward Waters-Maui County Leeward Waters.

&&

DISCUSSION...Powell
AVIATION...Birchard
MARINE...Gibbs