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Forecast Discussion for Honolulu, HI

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000
FXHW60 PHFO 260142
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
342 PM HST Sat May 25 2019

.SYNOPSIS...
A moderate and stable easterly trade wind flow will focus clouds
and showers across windward slopes through tomorrow, with mainly
dry conditions prevailing over leeward areas. Trades will drop
slightly on Memorial Day into Wednesday, leading to increased
chances for afternoon sea breezes and a few showers across leeward
areas. Trades are expected to gradually strengthen Thursday into
next Saturday.

&&

.DISCUSSION...
A moderate easterly trade wind flow remains in place. Trades are
being driven by a broad area of high pressure far northeast of the
state and its associated surface ridge extending to roughly 300
miles north of Oahu. The trade winds have brought in a slightly
drier air mass, and with dew points down several degrees into the
mid to upper 60s, humidity levels are more comfortable today.
Precipitable water over and upwind of the islands is running near
seasonal average of about 1.25 inches, and as a result, modest
showers have been confined to windward slopes. A mid level ridge
is maintaining stable conditions with an inversion based between
5,000 and 7,500 ft, and an upper level trough digging just north
of the state is producing thin high clouds over mainly Kauai and
Oahu.

The stable, easterly trade wind flow will hold at moderate
strength into tomorrow, then likely decrease again by Memorial
Day. The ridge driving the trades will remain somewhat disrupted
by a dissipating front several hundred miles northwest of the
state, and a north Pacific trough will weaken the ridge late
Sunday into midweek. As a result, trades will ease and potentially
shift out of the east-southeast on Memorial Day and Tuesday.
While modest showers will persist along windward slopes through
the period, this will likely cause widespread leeward sea breezes
each afternoon, triggering leeward clouds and a few showers. Even
though a weak upper level disturbance may drift in from the south,
the atmosphere should remain rather stable, which will minimize
chances for any heavy showers. A wind shift out of the east-
southeast could also lead to higher daytime temperatures.

The trades should strengthen back to moderate and locally breezy
levels late next week. Surface high pressure currently northeast
of the state will build westward, leading to a gradual
strengthening of the trades from Thursday into Saturday. A typical
rainfall pattern is expected, with showers favoring windward
slopes.

&&

.AVIATION...
High pressure far northeast of the islands will continue to provide
moderate, easterly trade winds to the region over the next 24
hours, with clouds and showers focused mainly over the typical
windward areas. VFR conditions are expected to prevail.

Afternoon soundings showed inversion tops somewhat lower, between
6000 and 8000 feet. Despite the increased surface winds, this is
not expected to be enough for moderate mechanical turbulence downwind
of the mountains.

&&

.MARINE...
Fresh to strong easterly trades associated with a ridge north of
the islands will continue into Sunday, then trend down into the
light to moderate category early next week. The Small Craft
Advisory currently in place for the strong trades in the windier
areas surrounding Maui County and the Big Island has been extended
through tonight. As the trades begin to trend down, localized
land and sea breeze conditions near the coasts will become a
possibility Monday through Wednesday. A return of fresh to strong
trades is forecast through the second half of the upcoming week as
the ridge strengthens and the stalled frontal boundary to the
northwest lifts northward and diminishes.

Surf along south facing shores will remain up each day through next
week due to the active pattern across the southern Pacific featuring
a series of low pressure systems passing through Hawaii`s swell
window from southeast of New Zealand to south of French
Polynesia. The largest surf is expected Sunday through Monday,
which may require a High Surf Advisory for all south facing
shores. This long-period energy will steadily fill in and rise
through the overnight period tonight, then peak Sunday through
Monday. As this source begins to ease late Monday through Tuesday,
a new long-period south swell (slightly smaller) will fill in and
hold through midweek. This overlapping trend will continue into
the second half of the week as another small south swell moves
through.

A late season small northwest swell associated with a gale that
crossed the Date Line a few days ago is building down the island
chain today. The nearshore PacIOOS buoys at Hanalei and Waimea
are lining up well with guidance, which depicts a 4-5 ft swell at
12 sec out of the northwest (300-320 deg). Surf has responded at
the exposed north and west facing shores, which should hold into
Sunday before slowly easing Monday through Tuesday.

A similar north-northwest swell will become a possibility by
Thursday in response to a decent sized pocket of strong breezes
focused at the islands tonight through Sunday night as a low near
the western Aleutians tracks east-southeastward toward the Date
Line.

Surf along east facing shores will remain up through the weekend
due to a decent sized fetch of fresh to strong trades upstream of
the islands that has setup. A downward trend is expected early
next week as this source weakens.

&&

.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...Wroe
AVIATION...TS
MARINE...Gibbs

Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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