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Hawaiian Islands Synoptic Discussion and Guidance

429
FXHW60 PHFO 280641
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Tue Jun 27 2017

.SYNOPSIS...
Breezy trade winds associated with high pressure north of the state
will continue through Thursday, before slightly trending down over
the upcoming weekend. Dry and stable conditions will limit shower
activity over the state through Thursday, with the best chances
favoring windward and mauka areas. A slight increase in windward and
mauka shower coverage will be possible Friday through the weekend as
a weak upper disturbance moves into the area from the east.

&&

.DISCUSSION...
Recently animated water vapor loop showed plenty of middle- to upper-
level moisture pooling northward across the state due to a weak
upper low centered several hundred miles west-southwest of Kauai.
This moisture was supporting mostly transparent high clouds lifting
northward over the state this evening, with the best coverage over
the northwest islands. At the surface, the latest analysis showed a
ridge of high pressure positioned north of the area that was
supporting breezy trade winds currently in place. The afternoon
upper air soundings exhibited strong subsidence inversions with very
dry profiles (Lihue inversion down to around 5 KFT). Precipitable
water (PW) values came in just under an inch at Lihue and only up to
1.25" at Hilo. The latest satellite-derived PW imagery reflected
this dry air and didn't indicate any potential near-term changes or
moisture surges upstream of the islands within the trades.

The latest short-term model guidance has initialized well with the
current pattern and continues to depict dry and stable conditions
persisting through Thursday. The previously discussed high clouds
will likely remain over portions of the state into Wednesday
before clearing out as the weak upper low shifts westward and away
from the state. Model PW values will remain around and slightly
below average for this time of year through Thursday. This will
support below average rainfall chances, with the best chances
favoring windward and mauka areas through the late night and early
morning periods. Trade winds will remain breezy each day with
strong gusts, especially through the afternoon periods.

Friday through the upcoming weekend, the latest guidance remains in
decent agreement and supports an upper low, currently centered
several hundred miles northeast of the Big Island, cutting off and
drifting back toward the state. Guidance also depicts a weak surface
and mid-level reflection (subtle trough) approaching and moving over
the islands from east to west. Model PWs are forecast to increase to
above average due to a rise in low- to mid-level moisture moving
into the area associated with these features. Lowering upper heights
combined with a weak surface trough and higher moisture will support
increasing trade wind shower activity. Trade winds are forecast to
weaken but hold in the moderate to locally breezy range.

Early next week, guidance supports dry and stable conditions
returning as the previously discussed upper low shifts west of the
islands an fills. Trade winds are forecast to strengthen and become
breezy with locally strong gusts as high pressure remains north of
the state.

&&

.AVIATION...
Expect a VFR evening ahead as a trade wind regime continues
across the main Hawaiian Islands. Low cloud coverage diminishes
from the Big Island westward, with both Oahu and Kauai having the
least coverage thus far. A slight increase in cloud coverage is
expected overnight across the windward waters, and the moderate to
strong trade winds will help carry some of the these clouds
ashore, favoring the windward and mountain areas. Brief MVFR cigs
are expected in these parts of the larger islands, primarily
windward Big Island and parts of windward Maui. Due to a strong
inversion, which ranges from 5k feet at Lihue to 75 hundred feet
at Hilo, only light brief showers are expected with no restriction
to visibility. These showers will be accompanied by MVFR cigs
with bases around 25 hundred feet. Except for a pop up showers
along the Kona coast overnight, leeward sectors of all islands
will be VFR.

A broken to overcast layer of cirrus, above fl200, is affecting
primarily Oahu and Kauai. This cirrus is expected to thin out
Wednesday morning.

As noted above, the low level trades are quite breezy with a few
of the terminals. Coupled with a strong inversion, AIRMET Tango
for low level turb will continue beyond 16z.

There are no other AIRMETS in effect, and none is expected
overnight.

&&

.MARINE...
The Small Craft Advisory (SCA) now includes the Kaiwi Channel,
Maui windward waters, Oahu leeward waters, the typical windy
zones around Maui and the Big Island. The SCA goes through
Thursday, and may continue for some zones into Friday, followed
by slightly weaker trade winds this weekend.

The locally strong trades combined with a small northeast swell
will produce choppy surf along east facing shores through the
week. A small short-period northwest swell is also forecast from
Friday into this weekend.

Small pulses of swell energy from the southeast through southwest
will continue bringing near normal summertime surf along south
facing shores into mid-week. Reinforcing long-period south swells
will generate a slight bump in surf heights from Thursday into
this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Gibbs
AVIATION...Lau
MARINE...Morrison

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



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