Area Forecast Discussion
Issued by NWS Honolulu, HI

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884
FXHW60 PHFO 260137
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
337 PM HST Fri May 25 2018

.SYNOPSIS...
A breezy trade wind weather pattern can be expected through the
weekend as high pressure dominates far north of the area. Clouds
and passing light showers will favor windward and mauka areas. An
upper level trough along with a weak surface trough will cause our
winds to become lighter Monday and Tuesday. A wetter weather
pattern is also possible due to instability and increased
moisture. High pressure will build back in north of the area on
Wednesday with a trade wind weather pattern returning.

&&

.DISCUSSION...
High pressure is currently centered far north of the state. This
places the us a breezy trade wind environment. Radar imagery is
currently showing some passing light showers embedded in the trade
wind flow. These are primarily falling over some windward and
mauka locations while most leeward locations remain dry.
Meanwhile, an upper level low is seen spinning several hundred
miles west of the area. Southerly flow on the west side of the
system is drawing up some high cirrus clouds over the area..

The high is forecast to move slowly eastward over the next couple
of days and slowly weaken. Models indicate that a band of moisture
associated with an old front will move through the islands tonight
and Saturday. This should cause an uptick in windward and mauka
shower activity. With an upper ridge over the area, most showers
are expected to be on the light side but they may be a bit more
persistent. The trade winds will be sufficiently strong enough to
blow a few light showers over to select windward areas at times.

Trade winds will become a bit lighter on Sunday as the surface
high moves off to a position far northeast of the area. A few
passing showers can be expected over windward and mauka areas.

A bit of a change to this weather pattern is becoming more likely
during the Monday and Tuesday time period. An upper level trough
is forecast to drop down to a position just to the west northwest
of the area. This will cause for the airmass over the area to
become a bit less stable especially over the west end of the
state. Also, a weak surface trough is also forecast to develop
over or near the west end of the island chain. This will cause a
drop off in winds across the area. Precipitable water values are
forecast to increase during this time frame. Thus an increase in
shower activity is expected. The instability from the upper trough
will also increase the chances for some thunderstorms to
possibly form as well especially over the west end of the island
chain as well as over the Kona slopes during the afternoon hours.
Locally heavy rainfall is a possibility with any thunderstorms
that might form.

By Wednesday, the upper trough along with its surface reflection,
are forecast to gradually move westward away from the state. In
the meantime, high pressure will build back in north of the area
with a trade wind weather pattern returning for the remainder of
next week. Clouds and passing showers will tend to favor windward
and mauka areas with most leeward locations being dry.

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.

&&

.AVIATION...
Mainly VFR conditions are expected for all TAF sites through
tonight, though passing low clouds and showers may bring brief
MVFR ceiling and VIS conditions along the windward areas. There
may be enough low cloud coverage at times to warrant AIRMET SIERRA
for mtn obscuration along the windward slopes, but none have been
needed so far.

Brisk trade winds will persist across the area today as a strong
high pressure far north-northeast of the islands continues to
advance east. AIRMET TANGO series will remain posted for low
level turbulence over and immediately south and west of mountains
below 8000 feet on all islands at least through today.

Periodic explosive eruptions at Kilauea/Halemaumau crater continue
to produce billows of volcanic ash at times, with the plume of
emissions predominately rising to near or slightly above the
inversion level 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 high far northeast of the area is producing locally strong
trade winds. The trades will weaken tomorrow 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 next week as the trough moves west
and fills and a strong new high builds north of the area. A Small
Craft Advisory (SCA) will likely remain in effect through
Saturday night for strong trade winds around the Big Island and
Maui.

Surf will remain below advisory levels along all shores through
at least the middle of next week. A series of swells from the
Tasman Sea will keeping 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 next week as trade winds weaken,
then build again by mid-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...Burke
AVIATION...TS
MARINE...Donaldson



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