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Hawaiian Islands Synoptic Discussion and Guidance

898
FXHW60 PHFO 232021
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1021 AM HST SAT JUL 23 2016

.SYNOPSIS...
Tropical Storm Darby will bring stormy weather at times this
weekend as it tracks northwest close to the Aloha State. More
settled weather will return Monday as Darby departs, leaving us
in very warm and humid southeast flow for a couple of days. Much
more pleasant tradewind weather is expected to return for the
latter half of the week, as high pressure strengthens far
northeast of the islands.

&&

.DISCUSSION...
Forecast details at this point are highly dependent on what Darby
does. Unfortunately, there is still a lot of uncertainty due to
the possibility of interactions or disruption of the circulation
from island terrain. See the latest CPHC advisory for details on
Darby's forecast track and intensity. As far as local effects go:

Flash flooding is perhaps the greatest concern at this point.
Abundant moisture and convergence around Darby will continue to
cause deep convection to blow up from time to time, probably for
the next couple of days, along Darby's track. The greatest risk
starts out for the Big Island today, then shifts to Maui county,
before reaching Oahu on Sunday and Sunday night. Any deep
convective bursts around Darby would bring torrential rain above
and beyond what is occurring in the showery rainbands rotating
around the center. These rainbands are already helping to
saturate ground conditions and cause stream rises and localized
flooding. So, if one of these convective bursts happens over
land, very significant flash flooding could occur.

Regarding the wind threat: Over land, the strongest winds have
been in areas where the winds are blowing downslope from higher
terrain. A couple of examples would be the wind gust to 54 mph
earlier at Hawaii Volcanoes National Park, 53 mph in the Humuula
Saddle, and 35 mph at the Kona airport. Would also expect strong
gusty winds in any deep convective bursts that develop. Not
everyone is going to see tropical-storm-force winds, but there
will at least be some pockets of wind damage associated with
Darby.

Once Darby passes, expect muggy southeasterly flow on Monday.
Thereafter, warm but gradually less humid trade wind weather
should return beginning midweek as Darby moves farther away from
the state and high pressure builds far to the northeast.

&&

.MARINE...
Hazardous conditions for mariners this weekend as Darby tracks
over or close to the island chain. Significant wave heights at
Pauwela buoy on Maui are up over 11 feet/period 11 seconds, and
still 14 feet/10 seconds at Hilo. The seas will be climbing
elsewhere as Darby starts tracking northwest, though the island
terrain will block the highest seas from reaching shadowed leeward
waters. The high surf headlines for east facing shores will very
likely need to be extended beyond today and will be evaluating
that based on the latest data. There will also be pockets of
intense rainfall, very low visibilities, and embedded thunderstorms
associated with Darby this weekend as it continues toward the
west-northwest.

&&

.AVIATION...
TC SIGMET Whiskey series for Tropical Cyclone Darby covers most of
the Big Island. A TC SIGMET implies the potential for severe
turbulence, icing, and low level wind shear. Additionally, AIRMET
SIERRA for IFR conditions are posted along the windward areas on
the Big Island and all of Maui. This will likely need to be
expanded to Molokai at some point today. Prolonged IFR conditions
are expected throughout most of the day for these islands.
However, as the outer bands of Darby move through there will be
brief periods of improving conditions, but are not expected to
last long enough to warrant any change to the IFR airmets.

AIRMET SIERRA for mountain obscurations are posted over the the
north and east slopes of Molokai and Oahu. This may need to be
expanded over the Kauai later today.

AIRMET ZULU for moderate icing is also posted for the Big Island
and Maui. Most significant icing however will be associated with
the deep convection wrapping around the southern portion of Darby.
This threat will increase as Darby moves closer to the islands.

A tight gradient between Darby and high pressure north of the
state will keep the breezy trade winds in place and AIRMET TANGO
remains posted of low level turbulence over and south through
west of the mountains.

&&

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

Flash Flood Watch from 6 PM HST this evening through late Sunday
night for Oahu.

Tropical Storm Watch for Niihau-Kauai.

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

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

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

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 Kauai Northwest Waters-Kauai Windward
Waters-Kauai Leeward Waters.

&&

$$
DISCUSSION/MARINE...R BALLARD
AVIATION...EATON

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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