Hurricane and Flash Flooding in Hawaii, August 22 to 26, 2018

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
508 PM HST Wed Aug 22 2018

.UPDATE...
A Hurricane Warning is now in effect for Oahu, Kauai Channel,
Kaiwi Channel replacing the Hurricane Watch. A Tropical Storm
Warning is in effect for Oahu windward waters.

&&

.SYNOPSIS...
Outer rain bands of Hurricane Lane have moved onto the Big Island
today. These shower bands will continue to spread across the rest
of the state tonight and Thursday as Lane moves northwestward.
Hurricane Lane will likely bring damaging winds, dangerous surf,
and intense flooding rains to areas within the state of Hawaii
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.

Increasing winds and heavy rain is expected as Hurricane Lane, a
category 4 hurricane, approaches the island chain. Lane is
currently 225 miles south of south point on the Big Island as of
2 pm HST. The forecast track for Lane continues on a northwest
path tonight before turning more north northwest on Thursday. See
the latest advisory and forecast track from the Central Pacific
Hurricane Center for more information about Lane. There is a
Hurricane Warning for the Big Island and Maui County and a
Hurricane Watch for Oahu and Kauai County, along with a
combination of watches and warnings over the coastal waters (see
advisory list below for specific coastal waters).

The onset of tropical storm-force winds on the Big Island could
start later tonight. Over Maui County, damaging winds could begin
as early as Thursday afternoon, with dangerous hurricane force
winds possible starting on Thursday night. On Oahu, damaging winds
could begin as early as Thursday evening, with dangerous
hurricane force winds possible by Friday morning. Hurricane
Warnings may need to be expanded to include other islands as Lane
draws closer. The highest wind threats for the Big Island will
develop along the western slopes of the island as strong and gusty
down slope winds interact with mountainous terrain. The location
of these damaging wind impacts will depend heavily upon the final
track of Hurricane Lane.

Deep tropical moisture surrounding Lane will also drift northward
ahead of the hurricane through the weekend. Extreme moisture
levels surrounding Lane will create an unstable environment with
muggy and humid weather conditions along with frequent showers.
Heavier rain shower bands will develop with some thunderstorm
activity as the extreme moisture interacts with mountainous
terrain on each island. These conditions will likely lead to
periods of hazardous flash flooding threatening all islands.
Hence, a Flash Flood Watch remains in effect for the entire state
of Hawaii through Saturday.

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.

It's not possible to know which islands will see the worst storm
effects right now due to the close proximity of the hurricane
track to the Hawaiian Islands and uncertainties in Lane's track
caused by increasing wind shear through the atmosphere as it
moves northward. 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 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.

Sunday through Tuesday, there remains a considerable amount of
uncertainty in the model guidance as to when the deep moisture in
the wake of Lane departs the region. The American GFS model
continues to show a deep moisture plume lingering over the entire
state through the weekend and into the first half of next week.
While the European (ECMWF) model shows a large area of drier air
moving in with the easterly trade wind flow. We will need to wait
to see how fast Lane transits the Hawaiian Islands and watch for
the models trending towards a more consistent solution over this
time period.

&&

.AVIATION...
Deep layered clouds and numerous rain showers, the outer bands of
Hurricane Lane, have already moved into Windward Big Island. An
AIRMET for mountain obscuration has been posted for most of
Windward Big Island and will likely be expanded within the next
few hours. Conditions are expected to spread across Maui and the
remainder of the Big Island by late this evening, and Molokai and
Windward Oahu by morning.

Isolated IFR ceilings and visibilities are to be expected in
locally heavy downpours along east and southeast facing slopes and
coasts of the Big Island tonight. IFR conditions may become
widespread on Thursday throughout the eastern half of the state.

Low topped stratocumulus and light showers embedded in the trade
wind flow will favor Windward slopes and coasts of the smaller
islands tonight, but will give way on Thursday to the deep
moisture and strong instability associated with Hurricane Lane.

Thunderstorm activity is likely across waters south and west of
the Big Island on Thursday...and possibly Leeward Big Island as
well.

The sharp contrast of strong high pressure to the north and
falling pressures to the south will result in increasingly strong
straight-line winds across most of the state. AIRMET Tango is
currently in effect for mechanical low level turbulence to the
leeward side of all mountains. Conditions are expected to
gradually intensify over the next twenty-four hours.

&&

.MARINE...
Latest radar imagery is showing the outer rain bands moving
across the Alenuihaha channel and is moving towards the waters
around Maui County. Heavy rain is expected to expand to all waters
around Maui County tonight. Marine interests on the waters
surrounding Maui County should finalize all preparations now 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 Maui County will be early
Thursday morning. Once these conditions are realized, it will be
too late to prepare. Interests on Oahu should finalize all
preparations by Thursday morning and by the end of the day
Thursday for Kauai.

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. See the latest coastal hazard
message and local statement for additional details with regards to
expected/potential impacts.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Hurricane Warning for Oahu-Molokai-Lanai-Kahoolawe-Maui-Big
Island.

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

Flash Flood Watch through late Friday night for all Hawaii
islands.

High Surf Advisory until 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.

Hurricane Watch for Niihau-Kauai.

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

Hurricane Warning for Kauai Channel-Oahu Leeward Waters-Kaiwi
Channel-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Hurricane Watch for Kauai Northwest Waters-Kauai Windward Waters-
Kauai Leeward Waters.

Tropical Storm Warning for Oahu Windward Waters-Maui County
Windward Waters-Big Island Windward Waters.

&&

UPDATE/DISCUSSION...Foster
AVIATION...Bedal
MARINE...Kino