High Wind in Hawaii, May 1, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
838 PM HST Mon Apr 30 2018

.SYNOPSIS...
Thick high clouds and residual moisture will linger across Maui
and the Big Island through Tuesday, while the rest of the main
island chain can expect rather dry and stable conditions with
light northerly winds. The residual moisture will drift to the
north Tuesday night through Thursday and the atmosphere will
become unstable as an upper level trough digs toward the state.
Moist, cloudy, and unsettled conditions are expected to develop,
with the highest chances of rainfall expected to focus across the
western half of the state late Wednesday through Friday.

&&

.DISCUSSION...
A 300 mile wide band of thick middle and high clouds extends
northeast to southwest across the eastern third of the main island
chain this evening. Accompanying a front which dissipated east of
the Big island, this band also marks the southeast flank a deep
upper trough parked over the region. A jetstream along this flank
will keep winds above High Wind Warning threshold at the Big
Island summits through tonight and possibly longer. Light
northerly winds persist at low levels, maintaining rather dry
conditions across the smaller islands. Little change is expected
through the day Tuesday.

Unsettled weather will develop Wednesday and persist across
portions of the island chain through at least Friday and possibly
into the weekend. The deep upper trough over the region will be
reinvigorated as another disturbance aloft drops in from the
northwest. The band of moisture currently over the Big Island
will drift northward across the central islands Tuesday night to
the western end of the state on Wednesday. Initially, rainfall
along the diffuse band of low level moisture will likely be
spotty, even though thick mid to high level clouds will spread to
much of the state. As the upper level trough approaches Wednesday
night and Thursday, the atmosphere will become increasing moist
and unstable, mainly across the western half of the island chain.
Gentle to moderate trade winds will focus rainfall along windward
slopes. However, showers could develop anywhere and heavy rainfall
will become possible. The upper level trough will weaken as it
nears the state on Friday and, aside from a brief uptick in trade
winds, little change is expected.

Forecast uncertainty increases heading into the weekend, but it
appears that a moisture axis will persist somewhere between Kauai
and Maui. Unstable conditions could maintain a heavy shower threat
along the band on Saturday. Models show building northeasterly
winds behind the band as it slowly moves eastward on Sunday.

&&

.AVIATION...
A remnant, nearly stationary, surface feature remains east of the
Big Island. Thus, MVFR conditions in low ceilings and light
precipitation continue to affect the eastern portion of the isle.
VFR will predominate elsewhere around the state this evening and
into Tuesday. And light winds, becoming northeast by late morning
and early afternoon, will again hold sway over the area.

An AIRMET for mountain obscuration is posted for the east side of
the Big Island, mainly because of the lower ceilings. Also, TEMPO
light icing is possible in the layer from 140 to 210 from Molokai
eastward in middle and high level cloudiness.

&&

.MARINE...
The remnants of a frontal boundary, currently southeast of the
Big Island, is forecast to begin drifting back over the islands
Wednesday through Friday bringing rain chances back up across the
waters. Light to moderate winds could drop off this coming
weekend as another cold front drops south across the central
Pacific toward the region.

Surf along north facing shores continues to trend down this
evening as the large north-northwest swell that peaked over the
past couple of days eases. This trend is anticipated to persist
through the rest of the week, which will result in minimal surf
through the second half of the week and into the upcoming weekend.

Surf along south facing shores is forecast to hold through the week,
as reinforcing small south-southwest swells from the southern
Pacific move through. The next reinforcement is expected to fill
in through the day Wednesday with two overlapping south-southwest
sources expected over the upcoming weekend.

Looking ahead into early next week, the ECMWF-Wave and WAVEWATCH
III are advertising another large northerly swell event that could
begin as early as Sunday night. The upper-level pattern is
forecast to amplify across the Pacific as a potent shortwave
trough dives south from the Aleutians toward the state,
potentially cutting off into a large upper low to the north of
the islands. If this scenario evolves, guidance suggests another
low developing to the north with northerly gales nosing southward
behind its attendant cold front. Details will follow in later
packages as confidence rises.

See the latest Collaborative Nearshore Swell and Wind Forecast for
more details regarding surf on Oahu at:

www.prh.noaa.gov/hnl/pages/SRF.php#Discussion

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 AM HST Tuesday for Big Island Summits.

&&

DISCUSSION...Powell
AVIATION...Kinel
MARINE...Gibbs