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Hawaiian Islands Synoptic Discussion and Guidance

750
FXHW60 PHFO 191353
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 AM HST Sun Aug 19 2018

.SYNOPSIS...
Humid and unsettled conditions associated with a trough of low
pressure moving through the state will keep the rainfall and
thunderstorm chances in the forecast today across the western end
of the state. Although the bulk of this activity will favor
windward and mauka locations with moderate trades in place, some
showers will manage to develop across leeward areas periodically.
Breezy and drier trade wind conditions are expected to fill in
tonight through the early portion of the week. Although it remains
too early to determine how the islands will be impacted,
Hurricane Lane is forecast to track through the region near the
islands through the middle part of the week.

&&

.DISCUSSION...
Short-term (through tonight) guidance remains in good agreement
and is initializing well with the current pattern and trends.
Consensus supports humid (mid 70 dewpoints) and unsettled weather
conditions continuing today as a surface trough tracks westward
across the state. The latest surface analysis and satellite
imagery showed this feature near Kauai early this morning slowly
tracking westward. Plenty of deep tropical moisture associated
with it combined with modest mid-level lapse rates led to an
active overnight period for the western end of the state
(Oahu/Kauai). Heavy showers and thunderstorms developed over
portions of Oahu through the night with peak rainfall totals
ranging from 1-2".

The latest hi-res model reflectivities support the potential for
heavy showers remaining in place over Oahu/Kauai through the day
today. Localized flooding can't be ruled out if and where any
heavy activity develops and becomes focused for any given
duration. Over the eastern end of state, a return of moderate to
breezy trades along with more stable conditions are expected to
gradually fill in from east to west.

The extended (Monday through Friday) guidance remains in
decent agreement and supports a drier and more stable trade wind
pattern filling in from east to west through Tuesday as the
aforementioned trough tracks away from the state. Trades will
likely climb into the breezy category due to a strengthening
pressure gradient between high pressure to the north and Hurricane
Lane approaching the region from the east-southeast. Forecast
uncertainty ramps up heading into the middle/latter half of the
week as Hurricane Lane passes near or just south of the islands.
It remains too early to forecast the exact location of Lane and
whether it will directly impact portions the state. Interests are
encouraged to monitor the latest advisories for Lane from the
Central Pacific Hurricane Center.

&&

.AVIATION...
High pressure far northeast of the state will keep light to
moderate trade winds in place through tonight. A weak trough of
low pressure will move from westward through Kauai this morning,
then continue west of of the island chain this afternoon and
tonight. Deep tropical moisture associated with this trough will
bring some enhanced showers and isolated thunderstorms along with
MVFR cigs/vsbys to mainly windward sections of Kauai and Oahu this
morning. Drier conditions will overspread the entire state this
afternoon, although an isolated thunderstorm or two can't be ruled
out over leeward sections of the Big Island and over the island of
Kauai.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai and Oahu. Conditions should improve for
both islands later this morning.

&&

.MARINE...
A weak trough of low pressure will continue to move through the
western coastal waters today producing some locally heavy
showers and isolated thunderstorms. Light to moderate trade winds
are expected to continue through today then strengthen to fresh
to strong speeds tonight. A Small Craft Advisory (SCA) will
likely be issued with the afternoon package today for the
typically windy waters around Maui county and the Big Island. The
latest forecast for Hurricane Lane has it passing by to the south
of the Big Island Tuesday night through Wednesday. As a result,
trade winds will likely increase further and the SCA may have to
be expanded to additional zones during the middle of the week. It
is still too early to determine the exact impacts that Hurricane
Lane may have on the coastal waters, so mariners are urged to keep
up to date with the latest forecasts.

Long period energy from Hurricane Lane is expected to begin
filling in along east and southeast facing shores of the Big
Island and Maui starting tonight and Monday. As Hurricane Lane
moves closer to the islands middle of this week, swell is expected
to increase and will likely result in large surf. We will
continue to monitor the track and intensity of Hurricane Lane, as
any changes in the track or intensity will significantly affect
surf heights. A small, north- northwest swell is expected to
arrive on Monday and continue through Tuesday. A larger north
swell is expected to arrive late Tuesday, peak Tuesday night and
early Wednesday, then slowly fade through Thursday.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Gibbs
AVIATION...Jelsema
MARINE...Kino

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

Current Conditions
Temp		1.7 C
RH		65 %
Wind		E 7 mph
Road		Open (4x4)