Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 231550

Area Forecast Discussion
National Weather Service Honolulu HI
550 AM HST SAT JUL 23 2016

Tropical Storm Darby is forecast to move over the Big Island
later today, and is expected to affect weather statewide into
early next week. A tropical storm warning remains in effect for
the Big Island and Maui County, and a tropical storm warning
has been issued for Oahu as well. A tropical storm watch has been
issued for Kauai. Locally damaging winds are expected for the Big
Island today and tonight, and for Maui County tonight and Sunday.
Strong winds will be possible on Oahu late tonight through Sunday
evening. Large surf along east facing shores and heavy rainfall
are also expected during the weekend. After Darby moves away from
the islands early next week, a typical trade wind pattern should
resume with passing windward showers.


At 5 AM HST, the Central Pacific Hurricane Center (CPHC)
estimated the center of Tropical Storm Darby to be near 18.8N
153.9W. This position is about 100 miles southeast of Hilo, and
310 miles east-southeast of Honolulu. Darby is moving west near
9 mph, with maximum sustained winds near 50 mph. Tropical storm
force winds extend out up to 125 miles from the center. Forecast
grids and text products have been updated to reflect the 5 AM
CPHC advisory issuance.

Outer rain bands on the western side of approaching Tropical
Storm Darby continue to move across the Big Island early this
morning, with additional rain bands rotating over the coastal
waters around the broad center of the tropical cyclone.
Scattered light to moderate showers are present over some
windward areas elsewhere around the state, with dry conditions
leeward. A tight pressure gradient between a surface high centered
far NNW of the islands and approaching Darby is causing locally
gusty trade winds. The strongest winds observed thus far have
occurred at Kealakomo in Hawaii Volcanoes National Park, where
north winds of 25 to 35 mph with gusts up to 54 mph are being
caused by a combination of the approaching tropical storm and
favorable terrain funneling. Winds have also recently increased
above 50 mph on the Big Island summits.

Weather conditions will continue to deteriorate across the Big
Island through today as the core of the storm approaches from
the east. Rain bands with periods of heavy downpours and strong
winds will become more frequent and intense during the day. It
should be noted, as we saw with Tropical Storm Iselle in 2014,
damaging wind gusts could also occur well away from the core of
the system due to unusual local terrain effects like gap winds and
channeling between the islands. Heavy rainfall and strong/gusty
winds associated with Darby will continue to spread northwest into
Maui County late this afternoon and tonight, and into Oahu late
tonight and Sunday. Isolated thunderstorms will also be possible
as Darby moves through the islands. A Flash Flood Watch remains in
effect through Sunday afternoon for Maui County and the Big
Island, and for this evening through Sunday night for Oahu.

Once Darby passes, expect muggy southeasterly flow to develop for a
brief period early next week. Thereafter, warm and slightly less
humid trade wind weather should return beginning on Tuesday as
Darby moves farther away from the state and high pressure builds
far to the northeast.


TC SIGMET Whiskey series for Darby will cover all or portions of
the Big Island with the next issuance shortly after 15Z. This
implies the potential for severe turbulence, icing and low level
wind shear. AIRMETs are in effect for much of the Big Island not
included in the TC SIGMET for IFR conditions, moderate turbulence
and icing. These hazards could spread towards the smaller islands
later today and Sunday.

For the smaller islands, AIRMET sierra is in effect for tempo
mountain obscurations for north through east sections of Maui and
Molokai. Visibility and ceilings will likely worsen through the

AIRMET tango continues for mechanical low level turbulence south
through west of the mountains across all islands. This is due to
the breezy trades and increasing gradient winds ahead of Darby.

Elsewhere around the state today we anticipate VFR conditions to
prevail with isolated mountain obscurations for windward locations
due to passing clouds and showers in the trade wind flow.


As Tropical Storm Darby moves across Hawaiian coastal waters and
approaches the Big Island today, strong winds and building seas
can be expected. A tropical storm warning remains in effect for
marine zones from the Kaiwi channel eastward, and has now been
expanded to include the waters around Oahu as well. A tropical
storm watch has been issued for the waters around Kauai. Boaters
throughout the islands should seek safe harbor and secure their

Near shore windward buoys show building seas, ranging from 10 to
15 feet as of 4 AM. Increasing winds and high seas will continue
spreading westward through the remaining coastal waters today and
tonight, then continue to affect the waters through Sunday. As
Darby departs the area early next week, seas will diminish and the
weather will trend back toward summertime normals, with a typical
trade wind weather pattern becoming established by Tuesday.

Swells associated with the captured fetch from Darby are already
affecting the islands, and a high surf warning is in place for
east facing shores of Maui and the Big Island, with a high surf
advisory in place for exposed east facing shores of the remaining
islands. There is a possibility that we may have to expand the
High Surf Warning to include east facing shores of islands
currently in the advisory, but this is expected to hinge on the
eventual track that Darby takes.


Tropical Storm Warning for Oahu-Molokai-Lanai-Kahoolawe-Maui-Big

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

Tropical Storm Watch for Niihau-Kauai.

Tropical Storm Watch for Kauai Northwest Waters-Kauai Windward
Waters-Kauai Leeward Waters.

Flash Flood Watch through Sunday afternoon for Molokai-Lanai-
Kahoolawe-Maui-Big Island.

Flash Flood Watch from this evening through late Sunday night
for Oahu.

High Surf Warning until 6 AM HST Sunday for Maui Windward West-
Windward Haleakala-South Big Island-Big Island North and East.

High Surf Advisory until 6 AM HST Sunday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward.



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