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Hawaiian Islands Synoptic Discussion and Guidance

279
FXHW60 PHFO 180655
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
855 PM HST Fri Aug 17 2018

.SYNOPSIS...
Light to moderate trade winds will continue through Saturday, then
begin to increase into the moderate to breezy category by the end of
the weekend as a weak trough of low pressure moves through from east
to west. Humid and unsettled conditions associated with this trough
will translate to increasing rainfall chances and potentially a few
thunderstorms. Breezy and drier trade wind conditions are expected
to fill in Sunday night through the early portion of next week.
Although it is 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 next week.

&&

.DISCUSSION...
Short-term (through Sunday night) guidance remains in good
agreement and is initializing well with the current pattern and
trends. Consensus supports increasing moisture and rain chances
across the state as a weak surface trough moves through from east
to west. The latest surface analysis and satellite imagery showed
this feature located within a couple of hundred miles east of the
Big Island tracking toward the west within the trades. Satellite-
derived precipitable water (PW) imagery depicted a large area of
higher moisture associated with it with values at and above 2".
Increasing moisture combined with mid 70 dewpoints will translate
more humid conditions.

Provided light to moderate trades in place, the bulk of the shower
activity will focus over windward and mauka locations, with the
exception being over the typical leeward locations such as the Big
Island through the afternoon hours (due to peak heating/sea
breezes). In addition to the increasing moisture and rainfall
chances expected, a few thunderstorms will remain possible,
especially through the afternoon hours. The afternoon upper air
sounding at Hilo supported this potential with decent mid-level
lapse rates/no inversion. Confidence, however, remains low, mainly
due a mid- to upper-level ridge that guidance is depicting nosing
westward over the state through the day Saturday.

The extended (Monday through Thursday) guidance remains in decent
agreement and supports a drier and more stable trade wind pattern
returning Monday through Tuesday. 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 of next 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.
Forecast confidence will steadily rise through the weekend into
early next week.

&&

.AVIATION...
High pressure far northeast of the islands will maintain light to
locally moderate trade winds across the state tonight into
Saturday. A deep band of moisture, embedded in the trade wind
flow, is anticipated to reach the Big Island towards morning and
spread across the smaller island by afternoon.

Clouds and showers will favor windward coasts and slopes through
the period. However, both shower coverage and intensity will
increase statewide on Saturday. Locally heavy downpours, possibly
in thunderstorms, are expected across Windward Big Island and the
east Maui uplands beginning Saturday afternoon. An AIRMET for
mountain obscuration will likely become necessary across east Maui
and Windward Big Island Saturday morning. It may need to be
expanded in the afternoon to included Windward Molokai and Oahu.

&&

.MARINE...
Light to moderate trade winds are expected to continue through
Saturday, keeping winds below Small Craft Advisory (SCA)
thresholds. A weak trough of low pressure will pass through the
islands Saturday and Saturday night, bringing the potential for
some heavier showers, particularly to the windward waters.
Following the passage of the weak trough, high pressure will
build well northeast of the state through the middle of next
week, while Hurricane Hector is forecast to pass by to the south
of the Big Island on Wednesday. As a result, we should see a
strengthening of the pressure gradient across the islands
Saturday night and Sunday, with SCA conditions likely returning
to the typically windy waters around Maui and the Big Island.
Moderate to strong trade winds are then expected to continue
Sunday night through the middle of next week, and SCA conditions
may expand to other marine zones. 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.

No significant swells are expected through the weekend, so expect
surf to remain small on all shores. 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. We will continue to monitor the track and intensity of
Hurricane Lane as it moves closer to the islands, as an elevated
easterly swell is expected to move into the coastal waters
beginning Sunday night and continuing through the middle of next
week. This easterly swell will likely result in large surf for
east and southeast facing shores of the Big Island and
potentially produce large surf for east facing shores of Maui.

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

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Bedal
MARINE...Jelsema

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



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