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Hawaiian Islands Synoptic Discussion and Guidance

641
FXHW60 PHFO 172150 AAA
AFDHFO

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
1150 AM HST Thu Aug 17 2017

UPDATED AVIATION SECTION

.SYNOPSIS...
Moderate to locally breezy trade winds will continue through the
remainder of the work week, then ease to light and moderate
levels this weekend. The trade winds will deliver periods of
clouds and showers to windward areas, with showers most active
during nights and mornings. While leeward areas of the smaller
islands will remain mostly dry, the leeward side of the Big Island
will see clouds and a few showers during the afternoon and
evening hours each day. An increase in trade wind showers is
possible late in the weekend through early next week.

&&

.DISCUSSION...
A relatively stable trade wind weather regime will continue into the
weekend, with passing clouds and showers favoring windward slopes
and coasts, especially during nights and mornings. Visible imagery
shows a smattering of showery low clouds riding in on a moderate to
locally breezy trade wind flow, with water vapor imagery depicting a
dry NW flow aloft over the region. Thin cirrus in island skies is
riding over the top a ridge aloft to the W of the islands and
dissipating as it moves overhead.

Trade wind speeds are expected to gradually and slightly diminish
the next couple of days, as a surface high far NE of the islands
moves NE, and a weak trough (the remnants of former east Pacific
Tropical Storm Jova, which was spawned by the remnants of Atlantic
Hurricane Franklin after it crossed Mexico) passes between the
islands and the high center. Meanwhile, the development of a weaker
high to the NW of the islands is expected to help support a light to
moderate trade wind flow through the weekend. A weak mid-level low
to the NNE of the islands will very move slowly W the next couple of
days, but is not expected to provide significant instability, at
least in the short term. It will however ensure that trade winds
drop a few windward showers as the subsidence inversion will remain
sufficiently high to support shower development, but sufficiently
low to suppress cloud tops to below 8000 feet or so.

An area of disturbed weather about 700 miles SE of the Big Island
continues to be monitored for potential development of a tropical
depression. Although associated thunderstorms flared overnight, they
have since dissipated and there is a medium chance that a weak
tropical cyclone will develop over the next several days. See
Tropical Weather Outlooks issued by the Central Pacific Hurricane
Center for updates and details.

Forecast models differ in their handling of the pressure and
moisture fields associated with the area of disturbed weather to the
SE, especially early next week. An increase in moisture riding in on
the trades during the Sunday-Tuesday time frame is the general
expectation, with latest model runs trending toward a somewhat later
arrival time. The low aloft and its associated destabilizing cool
pocket of mid-level air is expected to be closer to the islands at
the same time, and there is a chance this combination leads to the
development of heavy showers. Confidence is low as to 1) the timing
and the amount of low-level moisture that arrives on the trade wind
flow, and 2) how weak the trade wind flow will become this weekend
and early next week as the remnants of Jova pass N of the islands.
ECMWF guidance has been indicating that winds will become
significantly reduced Sunday and Monday while GFS guidance indicates
a continued light to moderate trade wind flow.

&&

.AVIATION...
A high pressure ridge north of the islands will maintain moderate
to locally breezy trade winds for another day or so. Winds are
sufficiently strong enough to maintain AIRMET Tango for mechanical
low level turbulence to the lee of the mountains on all islands.
Winds are expected to start to decline tonight and Friday, so
expect conditions to improve by this weekend.

VFR conditions are expected to persist across the area, though
there will be some clouds and showers focused over the windward
sides of the islands.

&&

.MARINE...
Surface high pressure far northeast of the islands will maintain
moderate to locally strong trades winds. This will continue to
result in Small Craft Advisory (SCA) conditions for the typically
windy waters adjacent to the islands of Maui County and the Big
Island. The SCA for these areas is in effect through Friday
afternoon. The high is expected to continue drifting eastward
during the next few days, and a slight downward trend in trade
wind speeds is expected from Friday night through this weekend.

A small, long period south-southeast swell is expected to peak
late Friday, before gradually subsiding this weekend. A small,
reinforcing long-period southwest swell is forecast to reach the
islands Saturday, and continue into early next week.

Surf is expected to trend down along east facing shores this
weekend as the trades weaken slightly. Surf will likely remain
below the High Surf Advisory criteria along all shorelines
through the middle of next week.

Finally, another period of above normal high tides is expected
through this weekend. The water levels associated with these
tides are forecast to be slightly lower than those observed in
July. In addition, no large swells are expected during the next
several days. There could still be some localized coastal
flooding from these tides. See the Special Weather Statement,
SPSHFO, for more details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...M Ballard
MARINE...Powell

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

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