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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 270157

Area Forecast Discussion
National Weather Service Honolulu HI
357 PM HST Fri May 26 2017

High pressure northwest of the state will slowly weaken as it
shifts southward through the Memorial Day weekend, with light
trade winds and localized land and sea breezes prevailing across
the island chain. As a result, showers will favor windward areas
during the overnight and early morning hours, and the interior
and leeward sections of the islands during afternoons and
evenings. Typical trade wind weather is expected to return on
Tuesday and continue through the end of next week, with showers
remaining confined primarily to windward and mauka areas.


Currently at the surface, a 1030 mb high is centered around 1400
miles northwest of Kauai. Meanwhile, a weak 1015 mb low is
located around 1300 miles north-northeast of the state, with a
weakening front extending south and southwest to a location
around 175 miles north of Oahu. Light northeasterly trade winds
prevail across the island chain this afternoon, with localized
sea breezes in leeward areas. Satellite imagery shows partly
to mostly cloudy skies in place, with cloud cover most prevalent
over the island interiors and leeward locales. Radar imagery
shows numerous showers over Kauai and Oahu, with scattered showers
over Maui County and the Big Island. Main short term concerns
revolve around rain chances as well as the building high surf and
elevated tide levels.

Tonight through Memorial Day,
The weakening front to the north of the islands will slowly
dissipated through the Memorial Day weekend, while high pressure
to the northwest of the state weakens and sinks southward. As a
result a weak pressure pattern will keep winds light across the
island chain through the Holiday weekend, with a general light
background trade wind flow, along with daytime sea breezes and
overnight land breezes developing in many areas. Meanwhile aloft,
broad upper level troughing will remain over the state through
Sunday, with the trough axis moving overhead from northwest to
southeast Saturday through early Sunday. Upper level ridging will
then build in from the west for the tail end of the Memorial Day

As for sensible weather details, with the light background trade
winds and localized sea/land breezes expected through the Holiday
weekend, showers are expected to favor windward areas during the
overnight and early morning hours and the interior/leeward
sections of the islands during afternoons and evenings. Shower
coverage is expected to remain highest over Oahu and Kauai early
this evening as lingering deep moisture associated with the
eastern extent of a mid-level trough remains over those areas,
with isolated to scattered showers over the other islands.
Showers are then expected to dissipate by mid evening hours in
most areas, with some isolated showers continuing to drift in on
the light trades over windward areas through the remainder of the
night. On Saturday, the coverage of afternoon/evening showers
will likely be a bit higher across the central and eastern islands
compared to what was seen today, with similar coverage across
Oahu and Kauai. On Sunday the highest afternoon/evening shower
coverage will shift to the Big Island as mid-level ridging begins
to nose in from the west. On Memorial Day, shower coverage should
remain highest over the Big Island and Maui, as the convergence
will be maximized here as the trades begin to return by late in
the day.

Monday night through next Friday,
High pressure northeast of the state will build westward to the
north of the islands through late next week. This will bring
trade winds back to the island chain beginning Monday night, with
speeds reaching moderate levels Tuesday through the end of next
week. Aloft, upper level ridging will remain over the islands
through much of next week, and this will keep the airmass fairly
stable. As a result, showers will be confined primarily to
windward and mauka areas through the period, with rainfall
amounts remaining light.


Light trade wind flow will persist through 48 hours. Weak sea
breezes along leeward coasts in the afternoon and early evening
hours will encourage low topped shower development over island
interiors. Patchy showers will persist overnight across windward
slopes and coasts but partial clearing is expected along leeward
coasts after sunset.

VFR conditions will prevail all areas. No AIRMETS in effect or
anticipated at this time.


Surf along south facing shores has been steadily rising today,
will peak tonight through Saturday, then slowly ease into next
week. Advisory level surf will hold through Saturday night and
possibly Sunday morning, before very slowly lowering through the
first half of next week. A High Surf Advisory remains in effect
for exposed south facing shores through early Sunday. Please see
the Coastal Hazard Message (CFWHFO) on our website for more
information about the surf. A small, long-period south swell
(170-190 deg) is expected to arrive late next week. Otherwise, no
significant sources out of the south are expected Wednesday
through the following weekend.

In addition to the building surf along south facing shores,
coastal flooding impacts associated with record-level King Tides
will continue each day along all shores through the Memorial Day
weekend, especially for Maui County and Oahu. Beaches and
shorelines, boat ramps and docks, and vulnerable low-lying roads
and structures will be impacted during these late afternoon and
early evening high tides. Please see the Special Weather Statement
(SPSHFO) product on our website for more information about the
potential impacts from the high tides and south swell.

Trade winds will continue to weaken over the upcoming holiday
weekend as the ridge weakens in response to an area of low
pressure well northeast of the state. Land/sea breeze conditions
are expected each day over the weekend and early next week, which
will allow the seas to trend down locally.

Surf along east facing shores will trend down through the weekend
as the trades relax. A small north-northeast swell, however, will
support small surf continuing at the exposed spots each day
through Monday.

Models remain persistent in showing a small to moderate northwest
(330 deg) swell filling in Monday night through midweek due to a
low pressure system tracking north of the state Sunday through
Monday. Otherwise, small surf is expected to persist each day,
mainly at the locations exposed to north-northeast swell.


High Surf Advisory until 6 AM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai
Leeward-Lanai Makai-Kahoolawe-Maui Leeward West-Windward
Haleakala-Leeward Haleakala-Kona-South Big Island-Big Island
North and East.




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