Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 271331

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Sun May 27 2018

Moderate trade winds along with passing mainly windward showers
will continue through today. Somewhat lighter trade winds are
expected on Memorial Day. A developing upper-level low to the west
of the state will likely bring wetter conditions and mostly
cloudy skies to the islands from Memorial Day through midweek. A
more typical trade wind weather pattern with less rainfall and
more sunshine may return by the later part of the week.


Radar shows scattered light showers across some windward parts of
the state, while rain gauge reports indicate only light amounts
of a quarter inch or less during the past three hours. At the
surface, a 1030 mb high centered about 1600 miles northeast of
Oahu is driving moderate trade winds over the islands. Aloft, a
mid level ridge is over and south of the state, while a mid/upper
level trough to our west is producing areas of high clouds which
are streaming overhead within upper level SW flow. The atmosphere
remains stable, with inversions based around 6000 feet.
Atmospheric moisture is near normal, with PW values as measured
by the 12Z soundings ranging from 1.09 inches at Lihue to 1.33
inches at Hilo. MIMIC-TPW satellite imagery shows similar
estimated PW values around the state.

Little change is expected today, with moderate trade winds
continuing, along with passing mainly windward showers. The
atmosphere will remain stable during this time due to the nearby
mid-level ridge, so only light rainfall amounts are expected. High
clouds will continue to stream overheard at times as the upper
trough remains to our west, so many areas will see less sunshine
than would normally be the case in a trade wind pattern.

As we head into Memorial Day and continuing through the middle of
the upcoming week, conditions will become more unstable over the
islands. The mid/upper trough currently to our west will deepen as
shortwave energy drops into the trough from the North Pacific.
This will shove the mid-level ridge to the south and east of the
state, bringing increasingly cold mid-level temperatures overhead,
thus increasing atmospheric instability. Combined with veering
lower/mid level flow which will bring increased moisture into the
state from the east and southeast, the potential for some heavy
downpours and perhaps even a few thunderstorms will exist during
this time. The highest chance for adverse weather appears to be
over the western end of the state, but confidence remains low as
it remains possible that the best moisture convergence associated
with the trough aloft may remain just west of Kauai. We will
continue to refine the forecast during the next couple of days.
Otherwise, expect areas of high clouds to continue streaming

The developing surface trough near the islands is expected to
lead a slight veering of the low-level flow to the ESE during the
first half of the work week, but more impactful may be the flow
in the lowest 10000 feet of the atmosphere, which is expected to
become SE. This could spread emissions from Kilauea volcano to
additional areas on the Big Island, and possibly to other areas in
the state as well, during this time.

For the Wednesday through Friday time period, surface high
pressure will build back in north of the area with moderate trade
winds returning. However the low aloft is forecast to linger a few
hundred miles west of the area which may keep some instability
around especially over the west end of the state. Moisture
embedded in the trades will likely allow for windward and mauka
shower activity to continue.

A Special Weather Statement continues to highlight the trace
ashfall potential across portions of the Big Island`s Kau
district. With moderate trade winds expected to continue today,
we can continue to expect any ashfall to be focused over this
area. The somewhat lighter and more easterly winds forecast for
at least Monday and Tuesday will have to be monitored, as the
potential exists for ash and other volcanic emissions to spread
over other areas of the Big Island and possibly further up the
island chain.


An extensive band of high level cirrus clouds continues to blanket
the state due to an upper level trough west of the islands. Bands
of low level clouds embedded in the trade wind flow will continue to
produce periods of MVFR ceilings and scattered showers along north
and east sections of all islands today. AIRMET Sierra for
mountain obscuration remains in effect for windward slopes of
Kauai, Oahu, Molokai, Maui and the Big Island and will continue
for most islands through the morning hours.

A strong high pressure center far to the northeast will produce
moderate trade winds over the islands today. 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. AIRMET Tango will likely remain posted through
this afternoon.

Periodic eruptions at Kilauea/Halemaumau crater may continue to
produce plumes of volcanic ash at times, with the plume of
emissions reaching up to 10,000 feet carried southwest over the
Kau district on the Big Island in the wind flow. Expect periods
of MVFR VIS in this area and SIGMET TANGO series for volcanic ash
remains in effect.


A ridge far north of the area is producing locally strong trade
winds. The trades will weaken marginally through Memorial Day as
a trough aloft approaches from the northwest and induces a weak
surface trough over the area. Winds are expected to strengthen
again in the new week as the trough moves west and fills and a
strong new high builds north of the area. A Small Craft Advisory
(SCA) is in effect for strong trade winds around the Big Island
and Maui. The SCA may be dropped as early as this evening,
however. The SCA will likely be re-issued later Monday or Tuesday
as winds again strengthen.

Surf will remain below advisory levels along all shores through
the new week. Only small surf is expected along north facing
shores. A series of swells from the Tasman Sea will keep surf
along south facing shores near the summertime average through
Memorial Day. South-shore surf will build to above-average heights
by mid-week. Rough surf along east facing shores will subside
early in the week as the trade winds weaken, then builds again by
mid-week as the trades strengthen.


Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.



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