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Hawaiian Islands Synoptic Discussion and Guidance

911
FXHW60 PHFO 171927
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
927 AM HST Fri Aug 17 2018

.SYNOPSIS...
A moderate and somewhat unstable trade wind flow will continue.
Showers will be focused over windward slopes, with afternoon
clouds and showers, some briefly heavy, developing over leeward
areas. An increase in humidity and enhanced shower activity is
due this weekend, as area of tropical moisture passes overhead in
the trade wind flow. Breezy and drier trade winds will return on
Monday. Hurricane Lane will be crossing the region near the
islands during the middle of next week, though it remains too
early to determine how it will affect island weather.

&&

.DISCUSSION...
A moderate and somewhat unstable trade wind flow remains in place.
The trades are being driven by surface high pressure centered far
northeast of the state. Weak troughs passing north of the islands
are disrupting the local pressure gradient over the state and
keeping the trades slightly weakened. A persistent upper level
trough parked over the region is making the atmosphere somewhat
unstable, with the overnight soundings and recent aircraft data
showing no inversion. Small pockets of moisture moving along the
trade winds have been periodically enhanced, mostly recently over
windward East Maui where a brief thunderstorm flared this
morning. Otherwise, modest showers have been confined to windward
slopes this morning, mainly across the eastern end of the state.

The somewhat unstable trade wind flow will continue today. Low
level moisture will be slightly higher from Oahu to the Big
Island, resulting in a greater amount of windward shower activity
on those islands. Expect another round of afternoon convection
across leeward and interior Big Island, where a few thunderstorms
are expected. On the other islands, spotty showers, some briefly
heavy, could develop across leeward terrain during the afternoon.
Conditions will also be somewhat humid as dew points creep up
into the low to mid 70s.

Humidity and enhanced shower activity will increase during the
weekend. A pocket of tropical moisture, currently located more
than 300 miles east of the Big Island, will be move overhead in
the moderate trade wind flow. This should drive dew points up into
the mid 70s, leading to a noticeable increase in humidity.
Showers will continue to favor windward slopes, while day time
sea breezes over sheltered leeward areas produce afternoon clouds
and spotty showers. With precipitable water values reaching as
high as 2 inches and the lingering upper level trough providing
instability, some showers could be heavy, both over windward
slopes and across leeward terrain during the afternoon hours.

Drier and more stable trade wind weather is expected on Monday.
The area of moisture will move off to the west a ridge builds
overhead. The result will be a rather dry and stable trade wind
flow with modest showers confined to windward slopes.

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.

&&

.AVIATION...
High pressure far northeast of the islands will maintain light to
moderate trade winds across the state into Saturday. Winds are
not strong enough to warrant an AIRMET for low level moderate
turbulence although there may be light turbulence for Molokai,
Lanai and Maui especially in south and west sections downstream of
the mountains.

Most of the area will have VFR conditions with the exception of
windward areas, especially on Maui and the Big Island where there
will be a better chance for shower activity and periods of MVFR
conditions. During the early hours of Saturday and into Saturday
morning expect an even better chance for heavier rain along the
windward slopes of the Big Island which may lead to sustained MVFR
conditions requiring an AIRMET for mountain obscuration.

There are currently no AIRMETs in effect.

&&

.MARINE...
Light to moderate trade winds are expected to continue through
this weekend, but remain below Small Craft Advisory levels.
Periods of heavier showers are expected this weekend, particularly
over the windward waters, as remnant tropical moisture moves in
from the east. Hurricane Lane, currently about 1330 nm east-
southeast of Hilo, Hawaii, may bring tropical storm conditions to
waters south of the Big Island next week.

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 next week Tuesday. We will continue to
monitor the development of Hurricane Lane, currently located in
the eastern Pacific, which may bring an easterly swell to the
coastal waters as early as Sunday night of next week.

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

&&

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

&&

$$

DISCUSSION...Wroe
AVIATION...Chevalier
MARINE...TS

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



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