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Hawaiian Islands Synoptic Discussion and Guidance

502
FXHW60 PHFO 181956
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
956 AM HST Wed Oct 18 2017

.SYNOPSIS...
High pressure north of Hawaii will keep a windy trade wind
pattern in place through the rest of the work week, with the
trades trending down slightly Friday and weakening further over
the weekend. Clouds and showers will favor windward and mauka
areas through the period, but showers will reach leeward areas
from time to time as well due to the strength of the trades.

&&

.DISCUSSION...
Windy conditions continue due to high pressure north of the state.
Overnight the surface analysis depicted at 1034 mb high about 1500
miles north northwest of Oahu. There is good model agreement with
the high slowing moving towards 30N over the next couple of days,
and weakening to around 1030 mb. The resultant pressure gradient
over the islands is producing locally strong gusts which exceed
our Wind Advisory criteria. The Wind Advisory continues today for
areas around Maui County and the Big Island. This advisory will
likely be extended through tomorrow, but will be examining the
wind observations today to make the final determination.

Showers riding in on the trade will continue to be focused over
windward and mauka areas, however the winds will be strong enough
to carry a few showers to leeward areas. Rain totals are expected
to remain on the light side.

As we head into the weekend, the high moves to the east, and the
pressure gradient over the islands begins to weaken. By Sunday, a
trough pushes in to the northwest of the island, which should turn
the winds to a southeasterly direction. By Monday as the trough
moves closer, the winds could turn more southerly. Additional
moisture is expected to spread over the islands in this southeast
and southerly flow. As the winds shift, they will also weaken,
allowing for some afternoon sea breezes to become established. An
upper level trough is expected to pass just north of Hawaii
Monday through Tuesday night resulting in some possibly heavier
showers.

&&

.AVIATION...
Expect another day of strong and gusty trade winds. The
combination of the elevated trade winds and strong low level
inversions (sloping from 8 kft at PHLI to 10 kft at PHTO) will
continue to support at least tempo moderate low level turbulence.
AIRMET TANGO is posted across the state for low level turbulence.
This is expected to stay posted into tomorrow.

Scattered showers embedded in the trade winds will stream over the
state. Most clouds and showers will favor the windward and
mountain areas, but some showers will make it to the leeward areas
under the strong trades. Expect primarily VFR conditions, with
brief MVFR CIGS/VIS is passing showers. Showers are not expected
to be widespread enough to warrant the need for a mountain
obscuration today.

AIRMET TANGO is also posted for upper level turbulence between
FL250 and FL400 due to a jet stream in the vicinity of the
islands.

&&

.MARINE...
Hazardous marine conditions will continue through Friday due to
strong-to gale-force trade winds and rough seas. Latest buoy
observations are holding within the 8 to 11 ft range, with
heights up to 15 ft likely over the windier locations, especially
the Pailolo and Alenuihaha Channels and waters south of the Big
Island. Satellite altimetry data within the past 12 hrs even
showed wave heights up to 14 ft west of Kauai. Gale conditions
will likely hold over the Pailolo and Alenuihaha Channels through
the day Thursday, with Small Craft Advisory conditions continuing
over most waters through Friday.

The latest guidance depicts a weakness developing within the
ridge north of the islands over the upcoming weekend in response
to a frontal boundary approaching from the northwest and a weak
surface trough approaching from the east. Trade winds are forecast
to respond and gradually trend down late Saturday through Sunday.
Seas will drop below advisory levels (10 ft) through this time.

Surf along east facing shores will remain rough through the rest of
the week due to strong trades locally and upstream of the islands.
The high surf advisory for east facing shores has been extended
through Thursday. A downward trend is expected over the upcoming
weekend due to the weakening ridge and a weak trough moving into
the area from the east.

Surf along south facing shores will continue to trend down through
Thursday as the current south swell eases. Mostly shorter-period
and small southeast energy is expected through Friday. An upward
trend is expected once again this weekend with WAVEWATCH III and
ECMWF-Wave solutions showing long-period energy arriving locally
by Saturday (200 deg), peaking Sunday, then slowly trending down
early next week.

Mainly trade wind energy wrapping into the typical locations is
expected for surf along north facing shores into Thursday. A long-
period north-northwest (340 deg) swell is expected to fill in and
peak late Thursday through Friday below advisory level before
slowly easing into the upcoming weekend. A larger, long-period
northwest swell (320 deg) is expected to build Sunday,
potentially producing advisory level surf for north and west
facing shores early next week.

&&

.FIRE WEATHER...
The KBDI at Honolulu is at 576 as of this morning, which is still
below the 600 critical threshold. Gusty winds are expected to
continue with low afternoon relative humidity. This will keep us
below levels for a Red Flag Warning. That being said, some
leeward areas across the state may already be experiencing
critically dry conditions. The combination of dry and windy
conditions in the afternoons is expected to remain through the
work week, before winds ease and humidities rise somewhat over the
weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST this evening for Lanai-Kahoolawe-
Big Island North and East-Kohala.

Small Craft Advisory until 6 PM HST Thursday 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-Maalaea Bay-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

Gale Warning until 6 PM HST Thursday for Pailolo Channel-
Alenuihaha Channel.

&&

$$


DISCUSSION...M Ballard
AVIATION...EATON
MARINE...TS

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



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