Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 222000
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Wed Feb 22 2017

.SYNOPSIS...
Strong and gusty trade winds will continue the next couple of days,
with a gradual 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 today will
allow some of the trade wind showers to become briefly heavy, with
strong thunderstorms possible over the leeward Big Island slopes
this afternoon.

&&

.DISCUSSION...
The pressure gradient south of strong high pressure cells centered
northeast and northwest of islands will continue to support gusty
trade winds today. Showers moving along with the trades will focus
over windward areas today, but strong winds will occasionally
carry showers to leeward sections of the smaller islands. This
weather pattern will likely continue through the day for Molokai
and westward.

Water vapor imagery highlights a slow moving, closed low aloft
centered over the middle part of the island chain that is
responsible for bringing significant instability to the mid and
upper levels of the island atmosphere today. The instability
provided by the upper trough will bring the potential for
thunderstorms today across the Big Island, particularly the
leeward section which updrafts will be less disturbed by the
trade winds and the inversion. A special 18Z sounding was launched
at Hilo this morning showed a 500 mb temperature of -16 degrees
celsius and a 700 mb temperature of 3 degrees celsius. The
soundings showed a high lapse rate between 10 to 30 kft with a low
level inversion below 10 kft. CAPE was at 1341 J/kg.

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 forecast anticipates thunderstorm development over the leeward
Big Island slopes and interior areas this afternoon, and over the
offshore waters, but nowhere else. Given the parameters in the
Hilo sounding described above, the thunderstorms that do develop
will have the potential to be stronger than what is normally seen
in Hawaii. A Special Weather Statement has been issued
highlighting the threats associated with these thunderstorms,
primarily lightning, gusty winds and small hail. With the upper
trough axis about over Maui County this morning, some enhanced
showers could still fire up this afternoon over Maui this
afternoon.

The low aloft is expected to move east and away from the islands
later tonight and Thursday, with more stable conditions developing
for tomorrow. Trade winds will continue to deliver passing
windward showers that will favor nights and mornings as the upper
trough departs, with low level winds backing slightly toward the
northeast Thursday and Friday. GFS indicates potential for a
slight increase in low level moisture Thursday night and Friday
as moisture associated with an old front rides in on the trades,
but a shortwave mid level ridge is expected to limit total
rainfall.

Global models are showing a change in the weather pattern late in
the weekend into next week, with a trend toward lighter winds, and
potentially wetter weather, but there is significant uncertainty due
to model differences. A surface low developing far north northwest of
the area will likely lead to a reduction in the local pressure
gradient after Sunday, but the GFS develops another low to the
east of the islands that leads to very light winds. The ECMWF
does not develop this second low, and maintains a light to
moderate ESE flow that becomes quite wet due to the development of
a nearby mid-level low. The GFS keeps the bulk of the moisture
east of the islands, and with the second surface low, but does
depict another trough aloft near the islands. The current forecast
is a blend of the two solutions, and indicates a light to moderate
ESE flow, with a broad brush approach to the PoP forecast.

&&

.AVIATION...
High pressure north of the state will keep a breezy trade wind
flow in place through tonight. Clouds and showers will be focused
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. Isolated MVFR conditions expected in
passing showers, especially across windward areas. Otherwise, VFR
conditions prevailing across all smaller islands.

On the Big Island, an unstable airmass will likely result in
thunderstorm development with MVFR/IFR vsbys and the potential for
gusty winds and small hail on the Big Island this afternoon.
Mountain obscuration could become a concern along windward
exposures this evening.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain across all islands. Conditions are
expected to persist into the evening.

&&

.MARINE...
Gale Warning posted for the Alenuihaha and Pailolo Channels, with a
Small Craft Advisory (SCA) for all other Hawaiian waters. The Gale
Warning remains in effect until late this afternoon. High pressure
north of the state should begin to weaken tonight and Thursday as
a low far north of the state moves south- southeast and a front to
the west of the state moves east. Model guidance suggests the
winds start to fall below the SCA threshold tonight for the
western waters, with the typically windier area around the Big
Island and Maui county remaining locally strong.

The current SCA configuration is also posted for an incoming
moderate long period west-northwest swell. Buoy 51001 and 51101
started to show signs of the swell earlier this morning, with the
current observations (1900Z, 1840Z) reading 5.9ft 14.8 sec and 5.2 ft
13.8 sec, respectively. The spectral density plot from the
Hanalei buoy (51208) is just now starting to show a small rise in
the longer period. Surf should start to rise along the exposed
north and west shores this evening and hold into Thursday. Surf is
should remain below advisory levels.

Another, slightly larger long period west-northwest swell is
expected to fill in late Thursday night and peak Friday night
before gradually subsiding through the weekend. This swell will
likely generate near advisory to advisory level surf for the
exposed north and west facing shores. A small pulse from the
north-northeast is expected Friday and Saturday.

South facing shores could see a very small bump in surf Friday
into this weekend from a small long period southwest swell.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Gale Warning until 6 PM HST this evening for Pailolo Channel-
Alenuihaha Channel.

Small Craft Advisory until 6 PM HST Thursday for all remaining
Hawaiian coastal waters.


&&

$$

DISCUSSION...Foster
AVIATION...Bedal
MARINE...Eaton



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