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Hawaiian Islands Synoptic Discussion and Guidance

173
FXHW60 PHFO 201409
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
409 AM HST Sat Jul 20 2019

.SYNOPSIS...
A high pressure ridge will remain north of the Hawaiian Islands
into the middle of next week. Expect moderate to breezy trade
winds through the period with showers favoring windward and
mountain areas. Additional moisture may move up from the deep
tropics from early Monday morning into Tuesday, possibly
increasing showers across the Big Island and Maui. More typical
trade wind weather returns from Wednesday onward.

&&

.DISCUSSION...
A trade wind weather pattern will continue into the first half of
next week as a high pressure ridge remains locked in place north
of the islands. Regional infra-red satellite imagery shows a
fairly active convergence zone around 10 degrees north latitude
just south of the state. Cloud bands with embedded showers are
slowly drifting in with the trade winds. A seasonably deep upper
level low also appears on water vapor satellite imagery roughly
500 miles north of Kauai.

Large scale lift and instability associated with this upper low
continues to weaken the stable subsidence inversion over the
islands allowing deeper clouds to form and more shower activity
mainly along windward and mountain slopes in the overnight through
early morning hours through Sunday. Brief periods of showers are
possibly over leeward slopes with stronger shower bands,
especially in the overnight to early morning hours.

Global models continue to show a weak trough developing south of
the Big Island this weekend. Both the American (GFS) model and the
European (ECMWF) model are moving this trough in a general
westward direction after it forms. The GFS model keeps a more
northwesterly track, which would result in advecting deeper
tropical moisture into the islands for the first few days of next
week. The latest GFS model run brings this deeper moisture and
enhanced showers into the southern slopes of the Big Island late
Sunday night or early Monday morning, this moisture pushes
northward into East Maui by Monday afternoon. Isolated
thunderstorms are also possible over the Big Island starting early
Monday morning. Meanwhile the ECMWF weather model keeps us in a
drier air mass for most of this time period, with the deeper
tropical moisture passing just south of the state. The latest 00Z
ECMWF is starting to trend more towards the GFS solution.

We are keeping a close watch on these model differences and have
trended the morning forecast grid package towards the wetter GFS
solution for the Big Island and Maui starting early Monday
morning through Tuesday. Some of this deeper moisture may move
into the upstream trade wind flow for windward and mountain slopes
across the western half of the state by Monday afternoon. Our
confidence in these enhanced showers for the eastern half of the
state remains fair at this time. More typical easterly trade wind
weather returns from Wednesday onward with passing shower bands in
the moderate to breezy easterly wind flow.

&&

.AVIATION...
Strong high pressure far northeast of the Main Hawaiian Islands
will persist. Bands of weak trade wind showers will continue to
impact mainly northeast through east facing slopes and coasts.
AIRMET Sierra may become necessary for Windward portions of Kauai
later this morning.

Light to locally strong east winds will continue into Saturday
and likely beyond. AIRMET TANGO will remain in effect for moderate
turbulence below 7,000 ft.

&&

.MARINE...
Fresh to locally strong easterly trades are forecast to continue
through next week as high pressure remains positioned north-northeast
of the area. The stronger winds will remain across the typically
windier locations from Maui County to the Big Island due to
terrain accelerations. The current Small Craft Advisory remains
in place for these areas through Sunday and may need to be
extended into early next week.

Rough surf will continue along east facing shores each day due to
strong onshore winds. Heights are forecast to slightly trend down
through the latter half of the weekend into early next week due to
the upstream trades trending down.

Surf along south facing shores will remain small through the
weekend, then trend up through the first half of the upcoming week
due to a recent system passing through/near the Tasman Sea.
Heights should remain below advisory levels as this south-
southwest swell moves through and peaks Monday through Tuesday. In
addition to this long-period source, a combination of small south
to southeast swells will be enough to keep the surf from going
flat along southern exposures through midweek.

A south-southeast (150-160 deg) long-period swell associated with
a compact gale that has developed southeast of the Tuamotus will
be possible late next week with a peak around Thursday night into
Friday. Surf will rise along exposed shores but should remain
below advisory levels.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Gibbs

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



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