High Surf in Hawaii, June 11 to 15, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
413 AM HST SAT JUN 11 2016

.SYNOPSIS...
Moderate trade winds will continue through the weekend, become
stronger Sunday night and Monday, and remain gusty for most of the
next week. A fairly wet trade wind weather pattern is expected over
the next couple of days as a disturbance aloft approaches from the
east, and passes over the island chain. Expect bouts of clouds and
showers to pass over the islands in the trade wind flow, with
windward showers occasionally spreading leeward on the smaller
islands. The atmosphere may become sufficiently unstable to support
the development of locally heavy downpours, and possibly a
thunderstorm, mainly Sunday and Monday.

&&

.DISCUSSION...
Currently, a nearly stationary 1032 mb surface high is centered
around 1250 miles to the NNE of PHNL. Meanwhile, a surface trough is
located around 375 miles E of PHTO, associated with nearby closed
low aloft, with both features tracking toward the W at 10 to 15 mph.
Latest satellite images show enhanced moisture associated with these
features, with clouds tops as high as 35 kft, in an area 350 to 550
miles E of the Big Island. A few lightning strikes were detected
overnight in this area as well. Closer to the islands, an area of
broken to overcast showery low clouds noted over windward waters
Friday night is now over most islands, but the back edge is
beginning to clear the islands from E to W. Latest sounding data
indicates that the subsidence inversion remains in place over the
islands, but is weaker than normal at a height around 8-10 kft.

Today and tonight...
High pressure to the NNE will support moderate to locally breezy
trade winds as the surface trough/low aloft to the E track toward
the islands. The expectation is that the area of clouds and showers
now traversing the islands will clear to the W by this afternoon on
Kauai, and a little sooner over the other islands. So a cloudy and
showery morning should turn drier and sunnier as the day progresses.
Latest GFS guidance keeps much of the moisture associated with the
approaching trough over windward waters tonight, and does not
indicate organized moisture sources moving over the islands.
Meanwhile, the latest ECMWF guidance brings more of this moisture
into the islands, starting with windward portions of Maui and the
Big Island late tonight. Latest official forecast is more in line
with ECMWF, and depicts a wet trade wind pattern.

Sunday through Wednesday...
The subtle surface trough will push W across the island chain
Sunday, leading to a slight weakening of the pressure gradient, and
slightly diminished trade winds. The trough will pass W of the
islands Sunday night and Monday, as the high to the distant NNE
strengthens to near 1039 mb. This strong high will move slowly W
through Wednesday, and will support strong trade winds beginning
Sunday night.

The low aloft will be gradually weakening as it moves W over the
island chain from the E, but latest guidance continues to indicate
moisture and instability will be great enough to support locally
heavy showers and possibly even thunderstorms. Peak instability and
moisture is expected Sunday and Monday, and phase especially well
over windward waters. The expectation is that a wet trade wind flow
will focus numerous showers over windward areas, occasionally
spreading to leeward areas, especially as trade winds strengthen.
Current forecast indicates a chance of thunderstorms on Maui and the
Big Island Sunday afternoon, and now also contains a slight chance
of thunderstorms over windward waters Sunday night, and over Kauai
and Oahu Monday morning.

The low aloft will weaken to a trough as it gradually moves W of the
area Tuesday and Wednesday. While the atmosphere will gradually
become more stable, inversion heights and PWAT will remain elevated
through the period, and strong trade winds will maximize orographic
forcing as well. Thus the forecast calls for a wet trade wind
regime.

Thursday and next Friday...
Breezy trade winds will continue, supported by strong high pressure
to the N. Although confidence is low, a typical trade wind weather
pattern on Thursday may lead to a wetter pattern on Friday. Latest
GFS guidance indicates that moisture associated with a dissipated
front will get caught up in the trade wind flow, leading to
increased moisture. A trough aloft extending over the area from the
NE may add to atmospheric instability.

&&

.AVIATION...
A band of showery, BKN to OVC low clouds over the islands will
continue moving WSW over the islands through mid-morning.
Widespread MVFR ceilings and ISOL MVFR VIS will continue over NE
sections of the Big Island, Maui, Molokai, Oahu and Kauai, so AIRMET
SIERRA is in effect for mountain obscuration on those islands.

&&

.MARINE...

A high surf advisory is in effect for S facing shores of all
islands, as swells generated in the S Pacific last week continue to
build in island waters. Latest spectral data from PACIOOS buoys at
Barbers Point and Lanai continue to indicate values near those that
are typically associated with S shore high surf events historically.
Elevated surf is expected to continue through Tuesday, and an
extension in time may be required for the advisory. As trade winds
strengthen early next week, expect an increase in the choppy wind-
driven waves along E facing shores. Peak surf heights are expected
to approach the high surf advisory threshold along the east facing
shores.

Trade winds are locally breezy, but are currently below Small Craft
Advisory (SCA) speeds, confirmed by a late-night ASCAT pass. Strong
high pressure building far N of the area Sunday night and Monday
will require a SCA for at least windy areas around Maui and the Big
Island, and potentially other zones as well. This will remain the
case for most of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Sunday for South facing shores of
Niihau Kauai Oahu Molokai Lanai Maui and the Big Island.

&&

DISCUSSION/MARINE...Birchard
AVIATION...Donaldson