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Hawaiian Islands Synoptic Discussion and Guidance

977
FXHW60 PHFO 240628
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Wed May 23 2018

.SYNOPSIS...
Breezy trade winds are expected to persist into the start of the
Memorial Day Holiday weekend. 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 into early next week. Trade showers may
increase slightly on Saturday and next Tuesday as areas of
additional moisture moves through.

&&

.DISCUSSION...
The tight pressure gradient south of a 1031 mb surface high
centered near 35N 158W, or slightly more than 900 miles due north
of Honolulu, is maintaining strong and gusty trade winds across
the Aloha State early this evening. Elsewhere, the leading edge of
a band of broken to locally overcast low clouds and showers along
the tail end of a dissipating front is about 330 miles north-
northeast of Hilo. This leading edge is moving slowly southward.

Water vapor imagery indicates a weak mid-tropospheric ridge is
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 near 7 thousand feet.
Satellite derived estimates of precipitable water (PW) indicates
values are 1.2-1.3 inches near the smaller islands, while they are
around 1.1 inches in the vicinity of the Big Island. Loops of
satellite imagery and radar reflectivity data show an area of
broken low clouds and scattered showers sagging slowly toward the
south-southwest across the smaller islands, with the leading edge
just starting to spread across the northern 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. Expect these low clouds and showers to eventually spread
south across the remainder of the windward Big Island later
tonight and early Thursday morning.

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 at least through Saturday. At the same time, the mid-
tropospheric ridge will produce 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 for 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 showers across much of the
state from Friday night through Saturday.

The surface high will eventually begin to have less influence on
the islands once it moves far northeast of the area later this
weekend. This will likely result in a gradual reduction in trade
wind speeds. At the same time, the mid-tropospheric ridge is
forecast to weaken on Sunday and Monday as an upper-level low
develops overhead by next Tuesday. This could bring an increase
in trade showers late Monday and Tuesday, and may induce a surface
trough near or west of the islands by the middle of next week.
There is still a great deal of uncertainty in the long-range
forecast at the moment. 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 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 5000-8000 feet), and all indications are
that this activity will continue for the foreseeable future.

&&

.AVIATION...
Mainly VFR conditions are expected for all TAF sites through early
Thursday, though passing low clouds and showers may bring brief
MVFR ceiling and VIS conditions along the windward areas.

Rather brisk trade winds continue across the area at least through
Thursday as a strong high pressure passing 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 most marine zones tonight, and for
the waters east of the Kauai Channel with the exception of the
windward Big Island waters through Thursday night. SCA conditions
will likely continue into the Saturday for the typically windy
waters around Maui and the Big Island.

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 next week,
keeping surf along south facing shores near to a notch above
the summertime average. The current small north-northwest swell
will slowly diminish through Thursday. Another 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 PM HST Friday 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.

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Channel.

&&

$$

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

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