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Hawaiian Islands Synoptic Discussion and Guidance

621
FXHW60 PHFO 241405
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
405 AM HST Thu May 24 2018

.SYNOPSIS...
Breezy trade winds are expected to persist through Saturday. The
strong trades will continue to transport low clouds and showers
that will focus over windward areas, with a few brief showers
carried over to leeward sections of some of the smaller islands.
The trades are expected to ease somewhat from Sunday through
early next week. Trade showers may increase slightly from Friday
night through Saturday, with more widespread rainfall possible
across parts of the state starting next Tuesday.

&&

.DISCUSSION...
The tight pressure gradient south of a 1030 mb surface high
centered near 35N 157W, or about 925 miles north of Honolulu, is
maintaining strong and gusty trade winds across the Aloha State
early this morning. Elsewhere, the poorly defined leading edge of
a band of broken low clouds and scattered showers along the tail
end of a dissipating front is about 265 miles north of Hilo. This
leading edge is moving slowly southward.

Water vapor imagery indicates a weak mid-tropospheric ridge
remains directly above the islands, which is keeping the
atmosphere relatively stable across the region. As a result, there
is a strong low-level trade wind inversion between 6000 and 7000
feet. Satellite derived estimates of precipitable water (PW)
indicates values are 1.2-1.4 inches near the islands. Loops of
infrared satellite imagery and radar reflectivity data show an
area of broken low clouds and scattered showers sagging slowly
southward across the islands from Oahu to the Big Island. Most of
the low clouds and showers are banking up along the windward
sections of the islands, but the breezy trades are transporting a
few brief showers over to leeward sections of some of the smaller
islands. This area of low clouds and showers will continue to push
slowly south through this afternoon.

The forecast guidance indicates the surface high far north of the
state will continue to move slowly eastward. The pressure gradient
south of this feature will maintain the breezy trade winds across
the islands through Saturday. At the same time, the mid-
tropospheric ridge will produce relatively stable atmospheric
conditions in the vicinity of the state through Saturday. This
will maintain a significant low-level inversion over the islands,
which will continue to keep a lid on the low clouds arriving in
the trade wind flow during the next few days. The band of low
clouds and showers associated with the dissipating front, which
was described above, will continue to sag down toward the state.
The remnants of this band are expected to provide an increase in
low-level moisture and an uptick in windward trade showers across
much of the state from Friday night through Saturday.

The surface high will eventually begin to have less influence on
the local weather once it moves far northeast of the area later
this weekend. This will likely result in a gradual reduction in
trade wind speeds starting Saturday night or Sunday. At the same
time, the mid-tropospheric ridge is forecast to weaken on Sunday
and Monday as an upper-level low develops near the western end of
the island chain by next Tuesday. This feature may act to
destabilize the atmosphere, in addition to causing a surface
trough to develop near the western islands. At the same time,
residual moisture will likely be in the vicinity of the islands.
This may bring an increase in showers from Monday night or Tuesday
into the middle of next week, with a chance the rainfall could be
locally heavy. There is still a great deal of uncertainty in the
long-range forecast at the moment, so the details about the winds
and weather across the state next week remain ambiguous. However,
if this scenario did occur, surface winds passing across the
active lava flows and erupting volcano on the Big Island could
become problematic for some areas of the Big Island outside of the
Puna and Kau Districts.

In the meantime, a Special Weather Statement continues to highlight
the relatively light ashfall that is occurring across portions of
the Big Island's Kau district. Several bursts of ash from
Halemaumau/Kilauea crater have been noted recently in radar data
(extending as high as 6000-8000 feet), and all indications are
that this activity will continue for the foreseeable future.

&&

.AVIATION...
Mainly VFR conditions are expected for all TAF sites today, 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 to warrant AIRMET MTN OBSC along the windward
slopes at times.

Rather brisk trade winds will persist across the area today as a
strong high pressure passes far north of the islands. AIRMET
TANGO series remains posted for low level turbulence over and
immediately south and west of mountains below 8000 feet on all
islands. These conditions are expected to persist through
Thursday.

Periodic explosive eruptions at Kilauea/Halemaumau crater continue
to produce billows of volcanic ash at times, with the plume of
emissions predominately rising to 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. SIGMET TANGO
series for volcanic ash remains in effect.

&&

.MARINE...
High pressure passing by to the north of the state will keep a
breezy trade wind flow in place across the coastal waters through
Friday. The trades are expected to trend down over the weekend
through early next week as an approaching front shifts the high
northeastward away from the island chain. A Small Craft Advisory
(SCA) remains in effect for the waters east of the Kauai Channel
with the exception of the windward Big Island waters through
tonight. SCA conditions are expected to continue through Friday
night across the typically windy waters around Maui and the Big
Island, and may continue into the weekend before lowering below
the SCA threshold.

No significant swells are expected, with surf remaining below
advisory levels through at least the middle of next week. A
series of south-southwest swells will continue through the middle
of next week, keeping surf along south facing shores near to a
notch above the summertime average. A small northwest swell may
give surf a slight bump along north facing shores over the
weekend. Breezy trade winds will deliver choppy short-period
waves to east facing shores into the weekend.

&&

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

Small Craft Advisory until 6 AM HST Friday for Oahu Windward
Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward
Waters-Maui County Leeward Waters.

&&

$$

DISCUSSION...Houston
AVIATION...Hui
MARINE...Jelsema

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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