High Surf Advisory 5, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
324 PM HST Wed Jan 17 2018

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

&&

.DISCUSSION...
Strong and gusty trade winds continue to develop. Currently, a
building 1033 mb surface high is located about 1,100 miles
northwest of Kauai, while a decaying front is stalling about 50
miles north of Kauai. A rather strong ridge aloft over the region
is maintaining a solid inversion based at 3,500 to 5,500 ft
according to the afternoon soundings and recent aircraft data.
Winds have been steadily ramping up during the past 24 hours, with
numerous stations on Maui County and the Big Island reporting
advisory level winds.

A further increase in winds is expected into Thursday as the
surface high passes north of the state and builds to 1037 mb. The
inversion will remain unchanged in most areas, causing the trade
winds to be accelerated over and around island terrain. As a
result, the Wind Advisory has been expanded to cover the lower
elevations of all islands, and with the GFS and ECMWF models
showing winds near the inversion level increasing another 5 kt or
so, we will have to closely monitor locations such as the leeward
Kohala districts for the potential for High Wind Warning
conditions. The Wind Advisory currently runs through Thursday
afternoon, though it will likely need to be extended through
Thursday night for the state and possibly into Friday for portions
of the Big Island and Maui.

Showers will be focused over windward slopes during the next couple
of days. The atmosphere will remain stable, as the inversion
holds firm. The above mentioned decaying frontal band will sag
southward and likely reach Kauai tonight, making windward portions
of the Garden Isle wetter than most areas. This band of moisture
is expected to slowly lift northward late Thursday and Friday. A
separate area of poorly-defined low level moisture is expected to
fill in across windward Big Island tonight then spread up the
island chain on Friday.

A gradual decrease in trade winds will begin on Friday and
continue into the weekend. During this time, the surface high
will be weakened and pushed off 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.

Beyond the weekend, the GFS and ECMWF are not in good agreement
on the details of a developing low near Midway Atoll. However, the
models do show moderate easterly trades hanging on over the Big
Island with southeasterly flow across the smaller islands. The mid
level ridge currently over the state will likely erode
significantly, leading to some instability and the potential for a
few heavier showers.

&&

.AVIATION...
With high pressure building over the state from the northwest and
north, trade winds will be strengthening. Therefore, low cloud
ceilings and accompanying showers will be mainly affecting
windward and mauka sections with brief to TEMPO MVFR conditions.
With the stronger winds, low ceilings may also pass over some
leeward sections from time to time as well. Lastly, a front
currently north of Kauai and drifting south may affect the isle
tonight with additional clouds and showers.

An AIRMET for low level turbulence to the lee of the mountains has
been posted for all islands due to the breezy trades.

&&

.MARINE...
The bump in the northwest swell is already on the decline at buoy
51101, and we anticipate the surf along north and west facing
shores to decline overnight. A High Surf Warning continues
overnight for those areas. Meanwhile, the increasing trade winds
will bring an increase in surf along east facing shores.
Anticipating advisory level surf arriving on Thursday, and
persisting through the end of the week. The High Surf Advisory for
east facing shores will likely be posted overnight.

The current northwest swell will continue to subside through the
end of the week. New west-northwest and northwest swells are
expected to arrive this weekend, and will again boost surf heights
along north and west facing shores of most of the islands from
Saturday night and Sunday into early next week.

The strong winds and the northwest swell will maintain Small
Craft Advisory (SCA) for all waters through at least tomorrow
night. A Gale Warning is in effect for the typical windier areas
around Maui County and the Big Island through tomorrow night. The
SCA will likely need to be extended for some areas as winds and
seas begin to lower this weekend.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST Thursday for all islands except the
summits of the Big Island and Haleakala.

High Surf Warning until 6 AM HST Thursday for north and west
facing shores of Niihau-Kauai-Oahu-Molokai and for north facing
shores of Maui.

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 AM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

&&

DISCUSSION...Wroe
AVIATION...Kinel
MARINE...M Ballard