Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 220640
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Tue Feb 21 2017

.SYNOPSIS...
Gusty trade winds will continue the next couple of days, with some
decrease in wind speed expected from Friday into the weekend.
Clouds and showers will favor windward and mauka areas, with brief
showers occasionally spreading leeward with the strong wind. An
upper-level disturbance passing over the islands tonight and
Wednesday will allow some showers to become briefly heavy, with
another round of afternoon thunderstorms expected over the Big
Island slopes on Wednesday.

&&

.DISCUSSION...
The pressure gradient S of strong high pressure cells centered NE
and NW of islands is supporting gusty trade winds. Showers moving
along with the trades are affecting mainly windward areas, and radar
shows some of the cells contain heavy rainfall, with showers also
seen spreading leeward on the smaller islands. Afternoon heavy
showers and thunderstorms over the leeward slopes of the Big Island
dissipated around sunset, but help to highlight the instability of
the island atmosphere. Water vapor imagery depicts a sharpening mid-
and upper-level trough over the islands which is responsible for
bringing significant mid-level instability.

The short-term forecast challenge is if (and where) surface-based
parcels will be able to tap into this instability, and if the summits
on the Big Island will be impacted with winter weather conditions.
The current forecast anticipates a slight chance of thunderstorm
development over the Big Island slopes on Wednesday afternoon, and
in the low cloud plumes over Hawaiian Offshore waters to the lee of
the islands, but nowhere else. Will likely be increasing the
probability of thunderstorms with the morning forecast update,
depending on the overnight sounding data. The upper trough will
linger through Wednesday before exiting to the E by Thursday, and
the island atmosphere will become more stable.

Global models are showing a change in the weather pattern over the
weekend into next week. A near-gale low is expected to form along a
passing cold front well N of the area, and then linger N of the
islands into next week. The current island forecast has trade winds
weakening over the weekend, with winds becoming ESE. The wind speed
and direction are highly dependent on the low`s location and
strength so the forecast from Sunday into early next week will be
highly fluid. If the global models remain on track with this
forecast, the low will also bring moisture from the tropics up and
over the islands early to middle next week. The light winds,
abundant low level moisture, and instability aloft would bring the
possibility of enhanced convective afternoon showers.

&&

.AVIATION...
High pressure north of the state will keep a breezy trade wind flow
in place through Wednesday. Clouds and showers will focus primarily
across windward and mauka areas, but some showers will also spill
over into leeward areas from time to time due to the strength of the
trades. Brief MVFR cigs/vsbys will be possible in passing SHRA
across windward areas, otherwise predominantly VFR conditions are
expected.

AIRMET Sierra may be needed for mountain obscuration across windward
sections of the islands later tonight, and will issue as conditions
warrant.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain across all islands. Conditions are expected
to continue through Wednesday.

&&

.MARINE...
A strong high building far NW will support strong to near gale force
trade winds, with winds especially strong where winds are most
accelerated between islands and headlands. A Small Craft Advisory
(SCA) is in effect for all waters through Wednesday evening, and a
Gale Warning may be necessary for the Alenuihaha channel and other
windy channels. A slight decrease in wind speed expected Wednesday
night and Thursday may allow some zones to be removed from the SCA,
but winds are likely to remain sufficiently strong to warrant a
continuation in some zones, while combined seas will likely warrant
an extension in others. A trend toward moderate trade winds is
currently expected for the weekend.

A new NW swell will build Wednesday and peak Wednesday night, with
peak surf heights along N and W facing shores expected to be near,
but below, advisory-level. Another long-period WNW swell building
Friday and peaking Saturday will bring peak surf heights that will
likely warrant a High Surf Advisory for exposed N and W facing
shores. A small to moderate N swell around Friday and Saturday will
shift to the NNE early next week before diminishing by Wednesday.
The strong trade winds will continue to supply short-period choppy
surf along E facing shores the next couple of days, but surf heights
are thus far expected to remain below advisory levels.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for all Hawaiian
waters.

&&

$$

Discussion/Marine...Birchard
Aviation...Jelsema



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