Area Forecast Discussion
Issued by NWS Honolulu, HI

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098
FXHW60 PHFO 261321
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
321 AM HST Sat May 26 2018

.SYNOPSIS...
A breezy trade wind weather pattern can be expected into Sunday,
with lighter trade winds early next week. An area of showery low
clouds moving through today will dampen windward areas. While some
decrease in showers is expected tonight and Sunday, a developing
upper-level low may bring wetter conditions Monday and Tuesday. High
clouds are expected to continue over the islands for the next
several days.

&&

.DISCUSSION...
In summary, persistent high pressure to the NE and N of the islands
will support locally breezy trade winds for the bulk of the forecast
period, but a weak low-level trough near the islands early next week
may lead to a short-lived weakening and veering of the low-level
flow, with winds potentially shifting to the ESE/SE. The trough will
develop in response to a low aloft that is expected to graze the
island chain before drifting away to the W. The proximity of the low
may lead to an increase in shower coverage and intensity - mainly on
Memorial Day and Tuesday, while the somewhat subtle veering of the
low-level winds may affect the movement of the plumes of steam, ash
and gases emanating from the Big Island`s active volcano. High
clouds will pepper island skies for at least the next couple of
days.

Currently, locally breezy trade winds are being supported by a 1030
mb surface high centered to the distant NE, with a trailing ridge
along 32N. A mid-level ridge over the islands continues to support a
subsidence inversion near 8000 feet or so, thereby capping the
incoming low-level moisture, which has increased somewhat overnight.
So, despite the stability, somewhat showery weather for windward
areas is expected today as diffuse remnants of an old front caught
in the trade flow fuel an increase in moisture. Thick high clouds
are streaming over the area within SW flow aloft associated with a
trough aloft about 800 miles NW of Kauai.

The mid-level ridge will remain in place through Sunday, with fewer
windward showers expected tonight and Sunday. On Monday and Tuesday,
the mid-level ridge is expected to break down and slide S as a
trough aloft currently W of the islands gets energized by a short
wave dropping in from the N. Models indicate a closed low aloft
developing near 30N160W on Monday that then tracks SW through
Wednesday to near 25N1654W. With the core of the low passing NW of
the islands, the greatest instability is expected to be over the W
end of the island chain Monday and Tuesday. A concurrent moisture
increase may bring the potential for some heavier rainfall and
deeper convection. Confidence remains low, and heavy rain has not
been added to the forecast at this time. High clouds are expected to
stream over the islands in SW flow aloft for the next several days.

The surface trough near the islands is expected to lead a slight
veering of the low-level flow to the ESE early next week, but more
impactful may be the flow in the lowest 10000 feet of the
atmosphere, which is expected to become SE to S. This could spread
emissions from the volcano to additional areas on the Big Island,
and to other areas in the state.

The forecast for Wednesday and beyond features high pressure
building well N of the area, supporting trade winds. The remainder
of the forecast is dependent on the evolution of the low aloft, and
its potential surface reflection. With model solutions diverging
significantly by that time, forecast confidence is quite low.

A Special Weather Statement highlights the ongoing ashfall (and
potential for more) across portions of the Big Island`s Kau
district. Multiple small eruptions of ash from Kilauea have been
observed on radar overnight. This activity is generally expected to
continue, with the potential for even stronger explosions. We
continue to vigilantly monitor radar data from PHWA, which is one of
the best tools we have for ash detection during cloudy nighttime
conditions.

&&

.AVIATION...
An extensive band of high level cirrus clouds blankets the state due
to an upper level low west of Kauai. Bands of low level clouds
embedded in the trade wind flow will continue to produce brief
periods of MVFR ceilings and scattered showers along north and east
sections of all islands this morning. Windward slopes all islands
will see periods of low level MVFR cloud ceilings through the early
morning hours. AIRMET Sierra for mountain obscuration remains in
effect for windward slopes of Kauai, Oahu, Molokai, Maui and the Big
Island this morning. This AIRMET should be dropped for most islands
by around 11 AM HST.

Moderate trade winds will continue to blow across the Hawaiian
Islands as a strong high pressure center far to the northeast slowly
drifts east. AIRMET TANGO series remains in effect for moderate low
level turbulence over and immediately south and west of mountains
below 8000 feet of all islands. This AIRMET will likely remain
posted for the next 24 hours.

Periodic explosive eruptions at Kilauea/Halemaumau crater continue
to produce billows of volcanic ash at times, with the plume of
emissions predominately rising to approximately 10,000 feet and
spreading to the southwest over Kau district on the Big Island. MVFR
VIS can be expected in this area and SIGMET TANGO series for
volcanic ash remains in effect.

&&

.MARINE...
A surface high far northeast of the area is producing locally strong
trade winds. A Small Craft Advisory (SCA) remains in effect through
early Sunday morning for strong trade winds across the typically
windy waters adjacent to the islands of Maui County and the Big
Island. The trades will gradually weaken from Sunday through
Memorial Day as an upper-level trough approaches from the northwest,
and induces a weak surface trough in the vicinity of the islands.
The trade winds are expected to strengthen again some time after
Memorial Day as the trough moves west and fills, while a new surface
high builds far north of the area. Assuming this scenario occurs, a
SCA may be required for the typically windy waters adjacent to the
islands of Maui County and the Big Island starting Tuesday.

Surf is forecast to remain below the High Surf Advisory criteria
along all shores across the state into the middle of next week. A
series of swells from the Tasman Sea will maintain surf near the
summertime average along south facing shores through Memorial Day.
Surf is expected to build to above-average heights along south
facing shores starting around the middle of next week. Rough surf
along east facing shores will subside early next week as the trade
winds weaken, then increase again by mid-week. The current small
northwest swell will maintain tiny surf along most north and west
facing shores of the smaller islands into early next week.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...Bohlin
MARINE...Houston



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