High Surf in Hawaii, August 21 to 27, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
851 PM HST Tue Aug 21 2018

.SYNOPSIS...
A strong high pressure ridge far to the north of the Hawaiian
Islands will produce breezy winds over the local area ahead of
Hurricane Lane. Deep tropical moisture will move up from the south
with Lane, spreading increasing rain shower bands and possible
thunderstorms over the Big Island by Wednesday morning and
reaching Maui by Wednesday afternoon and evening. These shower
bands will continue to spread northward across the rest of the
state as Lane moves northward. Hurricane Lane will bring damaging
winds, dangerous surf, and intense flooding rains to areas within
the state of Hawaii from Wednesday through Saturday.

&&

.DISCUSSION...
Please note that tropical cyclone operations are in effect. To
incorporate the latest forecast information from the Central
Pacific Hurricane Center (CPHC), public forecast updates from the
Honolulu Forecast Office will be delayed to immediately follow
the latest forecast package from CPHC.

Tranquil weather conditions across the islands today will become
more unsettled and threatening over the next several days as Lane
a category 5 Hurricane roughly 370 miles south-southeast of Kona moves
northwestward into the Hawaiian Islands. Lane is one of the
strongest storms to ever move into the Hawaiian Islands and the
current forecast track will bring local impacts of damaging winds
and life threatening flooding rain across the state from Wednesday
through Saturday. Do not wait until the storm reaches your
island, the time to prepare for hurricane impacts is now.

As Hurricane Lane moves further northward, deep unstable tropical
moisture will spread across the region. Precipitable Water (PW)
values will range from 2.0 to 2.5 inches, producing hot and humid
weather conditions and widespread convective heavy rain shower
bands. This extremely deep moisture will reach the Big Island by
Wed morning, Maui county by Wed night, Oahu by Thu, and Kauai by
later Thu or Thu night. Once this extreme moisture arrives the
heavy rainfall will start to develop quickly with the orographic
effects from mountain ranges alone causing flooding threats across
the islands with additional forcing from an upper level trough
just to the northwest of the islands.

Strong winds will develop, based on the latest CPHC forecast,
starting over the Big Island as soon as Wednesday afternoon with
hurricane force winds developing by Wednesday night. The highest
wind threat for the Big Island will start along the western side
as strong and gusty down slope winds interact with mountainous
terrain. On Maui county, damaging tropical storm force winds
could begin as early as late Wednesday night, with dangerous
hurricane force winds possible starting on Thursday. Oahu could
see damaging winds by Thursday morning, and Kauai by Thursday
night. Hurricane Watches could be needed for other areas as Lane
draws closer.

It's not possible to know which islands will see the worst storm
effects right now. The CPHC forecasts continue to show that all
islands are at risk from direct effects from the core of Hurricane
Lane. Everyone should take this hurricane very seriously and
should prepare accordingly. It's also important to expect that
there will need to be future adjustments to the track and
intensity forecast, particularly with the challenging recurvature
forecast that Lane is presenting. Prepare for the worst and hope
for the best.

The combination of a deep moist unstable layer and strong low
level wind shear along and to the east of the track of Lane will
make tornadoes and large waterspouts a distinct possibility in the
rainbands across the right semicircle of the hurricane. These
continue to be included in the TCV and HLS products as an elevated
threat from Oahu to the Big Island. Will be able to better refine
the threat as Lane approaches.

There is a considerable amount of uncertainty in the models as to
when the deep moisture in the wake of Lane departs. The ECMWF
shows some modest drying out over the weekend, while the GFS and
to a lesser extent the NAVGEM maintain high PW over the islands
into next week, which could be troublesome on any areas that get
excessive rainfall. There may also be some lingering support from
upper troughing to the NW.

Please consult the latest advisories from CPHC. weather.gov/cphc

&&

.AVIATION...
The pressure gradient will tighten across the islands as Hurricane
Lane approaching from the south while high pressures resides far
north. This results in increasing wind speeds over the islands.
AIRMET TURB for TEMPO moderate turbulence below eight thousand
feet on the leeward side of the mountains will continue through
rest of the evening into early Wednesday, with the strongest
turbulence on the Big Island.

Mainly VFR conditions prevail across the area through rest of the
evening into early Wednesday morning, though windward areas will
see MVFR ceilings due to passing low clouds. Weather conditions
will gradually deteriorate on the Big Island and Maui by Wednesday
as Hurricane Lane approaches from the southeast. The best chance
for MVFR or even IFR ceilings and VIS conditions will be on these
islands, with these spreading to the rest of the islands as
rainbands associated with Hurricane Lane reach the area. Otherwise
expect VFR conditions for the rest of the state.

&&

.MARINE...
Marine interests on the Big Island (leeward/southeast coasts)
should finalize all preparations for deteriorating conditions by
noon Wednesday as Hurricane Lane begins to approach from the
south. The earliest potential onset of tropical storm force winds
(34 knots or greater) across the waters around South Point and off
the coast of Kona will be through the day Wednesday. Once these
conditions are realized, it will be too late to prepare. Interests
in Maui County and on Oahu should finalize all preparations come
Wednesday evening and by noon Thursday for Kauai.

The Hurricane Watch currently in place for the waters surrounding
Oahu and Maui County will likely become updated to a warning later
tonight into Wednesday and expanded across the Kauai waters.

Impacts associated with Lane through the rest of the week for the
islands will include: hazardous seas, life-threatening surf
(warning level), beach erosion, overwash onto vulnerable coastal
roadways, and storm surge along exposed coasts/harbors of all
islands as Lane passes through.

Improving conditions are expected by the end of the upcoming
weekend as Lane slowly begins to track away from the island
waters.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell from other sources.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

Hurricane Watch for Oahu-Molokai-Lanai-Kahoolawe-Maui.

Hurricane Watch for Oahu Windward Waters-Oahu Leeward Waters-
Kaiwi Channel-Maui County Windward Waters-Maui County Leeward
Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel.

Hurricane Warning for Big Island.

Hurricane Warning for Big Island Leeward Waters-Big Island
Southeast Waters.

Tropical Storm Warning for Big Island Windward Waters.

Flash Flood Watch from 6 AM HST Wednesday through Friday
afternoon for all Hawaii islands.

High Surf Advisory from noon Wednesday to 6 AM HST Thursday for
Niihau-Kauai Windward-Kauai Leeward-Oahu South Shore-Waianae
Coast-Molokai Leeward-Lanai Makai-Kahoolawe-Maui Leeward West-
Maui Central Valley-Leeward Haleakala.

High Surf Warning until 6 AM HST Friday for Kona-South Big
Island.

&&

DISCUSSION...Bohlin/Ballard
AVIATION...Hui
MARINE...Gibbs