Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 281330
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Sun May 28 2017

.SYNOPSIS...
Light winds will prevail through the holiday weekend, allowing for a
few windward showers during nights and mornings, with clouds and
showers shifting to leeward and interior areas in the afternoons.
The island atmosphere will be a little unstable today, which will
allow a few showers to become briefly heavy. Light to moderate trade
winds on Tuesday and Wednesday are expected to become breezy by the
end of the week. The trade winds will bring a few brief windward
showers, with mostly dry conditions leeward, as a more seasonable
weather pattern develops.

&&

.DISCUSSION...
A complex weather pattern continues to prevail across the tropical
central north Pacific, with surface troughs NW and SE of the islands
helping to support a light wind regime, as is a break in the surface
ridge N of the islands. The large-scale flow is generally out of the
E and SE, and this flow has been keeping windward portions of Oahu
rather wet overnight, where up to a quarter of an inch of rain has
fallen. Meanwhile windward portions of the other islands saw the
expected isolated shower coverage, except Kauai, where land breezes
helped skies completely clear. Radar depicts some heavier shower
cores where convergence is maximized, and highlights the instability
being provided by a low aloft that water vapor imagery depicts as
being directly over the island chain. Overnight soundings show that
this feature is supplying a fair amount of instability, especially
in the mid- to upper-levels.

Still a good chance the low aloft promotes the development of a few
heavy showers today, mainly over interior and leeward areas in the
afternoon. While the profile is not conducive for thunderstorm
development in most areas, the elevated heat source of the Big
Island slopes may be able to tap into the instability aloft, and a
slight chance for thunderstorms remains in the forecast. With the
low moving away from Kauai, heavy showers are not expected there.

The continuation of light winds through Monday will allow the
current pattern to persist, with a few windward showers during the
night and morning, with the focus for clouds and showers shifting to
the interior and leeward portions of the islands in the afternoon.
The chance for heavy showers will diminish on Memorial Day however
as the low aloft moves steadily E.

By Tuesday the nearby surface troughs will dissipate and high
pressure far NE of the area will gradually strengthen, and light to
moderate trade winds will gradually strengthen to breezy levels by
the end of the week. A relatively stable atmosphere will allow for a
few brief windward showers, and mostly dry conditions leeward as a
more seasonable weather pattern develops.

&&

.AVIATION...
An upper level low currently over Maui, has brought on a surge of
enhanced showers embedded in a low level easterly wind flow. This
low level flow is strong enough to carry some of these showers into
the windward and mountain areas of all islands but Kauai and Lanai
where an offshore breeze is better established. Windward Oahu is
mostly affected by these showers, nearly bringing on AIRMET Sierra.

Light trades on tap for today will allow sea breezes to develop
later this morning across the lee and interior areas of all islands.
This will lead to some scattered showers over these areas this
afternoon. The upper trough should be over the Big Island by this
afternoon, and this is where the greatest impact will be with
scattered afternoon showers and isolated thunderstorms. The upper
trough may affect Maui County, and as far west as Oahu, with locally
heavy showers this afternoon. The potential is high for AIRMET
Sierra and widespread mountain obscuration for the Big Island slopes
this afternoon. We do not anticipate widespread mountain obscuration
for the other islands but we will monitor the situation closely.

The tops of the showers are currently between 10 and 15k feet. Due
to the upper trough, the tops of the showers may top out at 20k feet
this afternoon, especially across Maui and the Big Island. The tops
of the CB and thunderstorms may max out at 33k feet. Brief MVFR cig
and vis are expected in these showers and thunderstorms.

Otherwise, VFR is the predominant flying condition. AIRMET Tango for
high level turb will be lowered with the 16z aviation package as
this area of strong winds will be north of the island.

&&

.MARINE...
A High Surf Advisory remains in effect through this afternoon for
all south facing shores. The current large south swell will
gradually diminish through Tuesday. Please see the latest Coastal
Hazard Message (CFWHFO) on our website for additional information
about the hazardous surf conditions.

The south swell is rolling in on top of higher than normal sea
levels and king tides. Please see the Special Weather Statement
(SPSHFO) product on our website for more information about the
potential impacts from the high tides combined with the large surf.

A northeast swell building today will produce moderate surf through
Memorial Day along east facing shores. North swells will be very
small though Memorial Day. A moderate north northwest swell is
forecast to arrive Monday night, peak Tuesday night, then subside
through Thursday.

Winds will remain below the Small Craft Advisory (SCA) threshold
through Memorial Day. Trade winds are forecast to gradually
strengthen starting Sunday night. Winds are expected to reach SCA
speeds around Maui County and the Big Island starting Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Today for South facing shores
of all Hawaiian Islands.

&&

$$

SYNOPSIS/DISCUSSION...Birchard
AVIATION...Lau
MARINE...Donaldson



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