Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 270816 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1016 PM HST Sat May 26 2018

Moderate trade winds will continue through Sunday, with lighter
trade winds expected on Memorial Day. An area of showery low
clouds moving through this evening will dampen some windward
areas. While some decrease in showers is expected on Sunday, a
developing upper-level low may bring wetter conditions to much of
the state Memorial Day and Tuesday. Varying amounts of high
clouds are expected to continue over the islands for the next
several days. A more typical trade wind weather pattern may return
by the later part of next week.


Radar shows scattered light showers across some windward parts of
the state early on this Saturday evening, 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 between 7000 and 8000 feet. Atmospheric moisture is slightly
above normal, with PW values ranging from 1.21 to 1.44 inches on
the 00Z soundings, and MIMIC-TPW satellite imagery showing similar
estimated PW values around the state.

Little change is expected overnight and Sunday, 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

As we head into Memorial Day and continuing through the first half
of next 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. 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 overheard.

The developing 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. 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 trade winds expected to continue through the
weekend, we can continue to expect any ashfall to be focused over
this area. The lighter winds forecast for early next week will
have to be monitored for the potential 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 low west of Kauai. 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 tonight. 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 early morning hours.

A strong high pressure center far to the northeast will produce
moderate trade winds over the islands through Sunday. 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
Sunday afternoon.

Periodic eruptions at Kilauea/Halemaumau crater continue to
produce plumes of volcanic ash at times, with the plume of
emissions reaching from 10,000 to 12,000 feet being carried to
the southwest over Kau district on the Big Island in the wind
flow. MVFR VIS can be expected 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 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
during the second half of 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 Sunday
morning, however. The SCA may need to be re-issued Tuesday as
winds again strengthen.

Surf will remain below advisory levels along all shores through
the upcoming 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 AM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.



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