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Hawaiian Islands Synoptic Discussion and Guidance

857
FXHW60 PHFO 020202
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST FRI JUL 1 2016

.SYNOPSIS...
Strong and gusty trade winds will affect the state through
Saturday, with a slight drop in winds expected for the rest of
the holiday weekend. A dry and stable air mass will result in
rather dry conditions with very little rainfall through Saturday.
The airmass will gradually moisten from the east through the
remainder of the holiday weekend, leading to increased showers
that will periodically spread leeward on the smaller islands.

&&

.DISCUSSION...
The main concern in the short term remains the strong and gusty
trade winds. The trades are being driven by a relatively strong
1034 mb surface high centered about 1200 miles northeast of the
islands. Wind gusts over and downwind of terrain are being
enhanced by an exceptionally low inversion being produced by a mid
level ridge building in from the east. The afternoon soundings and
day time aircraft data continue to show a strong inversion based
between 2500 and 5000 ft, and a handful of sites have recorded
advisory level wind gusts of around 50 mph, mainly on the Big
Island but also on Lanai and Oahu.

The window of strongest winds will be this evening through early
Saturday. During this time, a broad and weak mid level trough
approaching from the southeast is expected to tighten the pressure
gradient above the surface, producing conditions conducive for an
increase wind gusts over and downwind of island terrain. A wind
advisory remains in place through Saturday afternoon for the
entire state, including the recently added Big Island summits,
and there is a small chance of experiencing high wind warning
conditions on the Haleakala summit overnight.

Rather dry conditions will hold into Saturday. The above mentioned
mid level ridge and exceptionally low inversion will keep low
level moisture minimal. Windward terrain may squeeze out some
light showers, but dry conditions will dominate.

Trade winds will back off slightly by Sunday, and shower activity
is expected to return to more normal levels. The above-mentioned,
broad, mid level trough will pass over the islands from east to
west. While the surface high to the north will change little, the
easing of the mid level pressure gradient and rising of the
inversion is expected to cause winds to decrease enough to drop
the wind advisory. The mid level trough will also bring an
increase in low level moisture that will boost shower activity
back to normal levels. Expect mainly windward rainfall, with
breezy trade winds carrying a few showers over leeward portions of
the smaller islands at times.

The high clouds that have been producing periods of interesting
sky conditions during the past several days will thin over at
least the eastern half of the state on Saturday as an upper level
low to our west is displaced. Weak upper level ridging will edge
westward and push off the persistent upper level low sitting
about 750 miles west of Kauai over the next 24 hours or so.
Thicker high clouds will likely return by Tuesday.

&&

.AVIATION...
Strong, gusty trade winds and stable atmospheric conditions will
persist across the island chain through Saturday. Therefore,
AIRMET Tango remains in effect for moderate to isolated severe low
level turbulence over and immediately leeward of the higher
terrain on all islands. In addition, sustained surface wind
speeds of 30 knots or greater is included in AIRMET Tango.

A few low clouds may impact windward facing slopes and mountains
tonight resulting in brief and localized MVFR ceilings.
Otherwise, expect VFR conditions to prevail.

&&

.MARINE...
Elevated trade winds will hold through the holiday weekend. An
ASCAT pass earlier today detected winds of at least 30 kt in the
Alenuihaha Channel and near South Point, and with a further
increase in winds expected for tonight and Saturday, a gale
warning remains in effect for these areas as well as the Pailolo
Channel. A Small Craft Advisory (SCA) remains posted for the rest
of the coastal waters and channels through Sunday, and the SCA
will likely remain in place for most waters through the holiday
weekend.

Rough, advisory-level surf will be possible along the east facing
shores over the weekend, beginning as early as late Saturday.
Although east shore surf is expected to peak around the 8 foot
advisory level, rather high afternoon tides could produce run up
issues, such as along Kamehameha Hwy on windward Oahu. Elsewhere, no
significant surf is anticipated through the middle of next week, and
a 2 to 3 foot south swell is due next Thursday and Friday. Please
refer to the recently issued Collaborative Nearshore Swell and Wind
Forecast (SRDHFO) for more details.

&&

.FIRE WEATHER...
Strong and gusty trade winds and a rather dry air mass will
remain over the islands through Saturday. Rainfall received in
late May slightly eased drought conditions across the islands,
which rules out the possibility of Red Flag Warning criteria
being met. However, fire danger will be elevated due to the
strong winds and low afternoon humidities.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST Saturday for all Hawaii islands.

Gale Warning until 6 PM HST Saturday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Sunday for all other waters
and channels.

&&

$$

Public/Marine/Fire Weather...Wroe
Aviation...Foster

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



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