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Hawaiian Islands Synoptic Discussion and Guidance

047
FXHW60 PHFO 190643
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
843 PM HST Wed Jul 18 2018

.SYNOPSIS...
High pressure north of the state will drive breezy trade winds
into Thursday. A few windward showers will linger through the
night, but drier weather will spread across the islands through
Thursday. Winds will taper off to moderate breezes Thursday night
and continue through the weekend.

&&

.DISCUSSION...
Radar shows isolated to scattered showers lingering across
windward and mauka areas of the islands this evening, while
satellite loop shows a pall of middle and high clouds passing by
from west to east. A pair of surface highs, far northeast and
northwest of the main island chain, anchors the persistent
subtropical ridge between them.

The tight pressure gradient between the subtropical ridge and a
weak trough passing westward to the south of the islands is
driving breezy trade winds across local waters this evening. This
will continue through Thursday before the trough weakens and moves
away to the west. Water vapor imagery shows a strong midlevel
ridge sitting over the state, maintaining stable atmospheric
conditions. An upper level trough to the northwest will drift
closer to Kauai and keep high clouds streaming overhead tonight.

Expect passing showers, pushed through by decreased trade winds
Thursday through the weekend, mainly across windward areas.
Models keep deep moisture associated with the next tropical
disturbance suppressed mainly to the south of the state. Another
wave of tropical moisture may approach the state from the
southeast by the middle of next week.

&&

.AVIATION...
A high pressure cell will continue to generate moderate to locally
strong lo level trade winds across the area through 200400 UTC.

Expect predominantly VFR conditions during the overnight hours,
although there will be pockets of MVFR ceilings over the windward
and mountain areas of all islands. Weather radar is not picking
up a whole of shower activity, for the exception of windward
Kauai, where an area of scattered showers is moving through.
This area should clear Kauai by midnight, with fewer trade showers
for the rest of the evening. Tops of these clouds and showers are
between 8 and 10k ft. Latest surface weather reports statewide,
indicate ceilings around 25 hundred feet over the windward areas
of most islands. Forecast models point to these clouds as a stable
low level stratocumulus, and for these clouds to continue affecting
the windward and mountain areas of all islands into Thursday. Not
a whole lot of showers, just mostly clouds. There is also a layer
of broken to overcast high level cirrus that will persist across
the island skies into Thursday.

The moderately strong low level trades will maintain AIRMET Tango
below 8k feet through 200400 UTC south through west of mountains
of all islands.

&&

.MARINE...
A surface trough, currently just south of the Big Island, will
continue to move west through the night. The passing of this
trough has helped to tighten the pressure gradient over the state,
causing locally fresh to strong trade winds throughout most of the
coastal waters. A Small Craft Advisory (SCA) is in effect for the
typical windy waters around the Big Island and Maui County and
the waters around Oahu. Winds will decline a notch Thursday and
Friday, and the SCA will likely be scaled back to only include the
typically windy waters through the weekend.

The High Surf Advisory remains in effect for east facing shores
through tonight. The Hilo PacIOOS buoy (51206) continues to show
an upward trend in the seas, with the latest observations
registering 11-12 ft. Seas are expected to level off in the next
few hours, then begin to subside. This will be monitored through
night for any possible extension of the HSA.

A series of reinforcing south and southwest swells will maintain
small to moderate surf along south facing shores across the state
through Friday. Slightly larger and longer period south-southwest
swells spreading across the state this weekend may cause surf to
approach the HSA (8 ft) threshold along south facing shores from
Saturday into Monday.

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.

&&

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

Small Craft Advisory until 6 PM HST Thursday for Kaiwi Channel-
Maui County Windward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 AM HST Thursday for Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Maui County Leeward
Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Lau
MARINE...Eaton

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



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