Heavy Rain in Hawaii, May 28 to 29, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
947 AM HST Sun May 28 2017

.SYNOPSIS...
Light winds will prevail through the holiday weekend, allowing
for a few windward showers during nights and mornings, with clouds
and showers shifting to leeward and interior areas in the
afternoons. The island atmosphere will be a little unstable today,
which will allow some showers to become briefly heavy. Light to
moderate trade winds on Tuesday and Wednesday are expected to
become breezy by the end of the week. The trade winds will bring a
few brief windward showers, with mostly dry conditions leeward,
as a more seasonable weather pattern develops.

&&

.DISCUSSION...
Surface troughs NW and SE of the state have interrupted island
trade winds, bringing variable winds with daytime sea breezes and
nighttime land breezes over the islands. This trend will continue
through Memorial Day. The synoptic scale wind flow is light from
the ESE, and this pushed some persistent showers into windward
Oahu overnight. Otherwise skies cleared overnight bringing a sunny
start to most places this morning. Without the cooling trades,
sensible weather will be hot and humid during the day with cooling
temperatures at night. Skies over the islands will become
increasingly cloudy this afternoon as sea breezes and land heating
create rising motion over interior areas. This diurnal pattern of
clearing skies overnight with clouds and passing showers during
the day will continue through Memorial Day.

An upper level trough now over the eastern end of the state has
brought some instability to the atmosphere. The early morning
sounding from Hilo shows an increase in atmospheric instability
with ML CAPE increasing from zero yesterday afternoon to 311 J/kg.
This instability has weakened our normally cloud capping
temperature inversion, and will allow for some locally heavy
showers to develop during the peak afternoon convective times, and
last into the evening. The early morning Lihue sounding shows a
decrease in instability with the trough well east of the Garden
Isle. The heavier showers seen in the Lihue area yesterday are not
expected today. With the instability over the Big Island, there
will be a chance of thunderstorms over the higher elevations of
Mauna Kea and Mauna Loa along with the potential of locally heavy
showers.

High pressure will build north of the state Monday night and
Tuesday bringing back our normal trade winds with windward and
mountain showers expected.

&&

.AVIATION...
A weak high pressure ridge north of the Hawaiian Islands will
continue to produce weak easterly trade winds allowing onshore sea
breezes to develop over each island during the day with offshore
land breezes at night. Cloud ceilings and scattered showers will
develop over island interiors by afternoon with brief periods of
MVFR conditions forecast through the early evening hours. Land
breezes overnight will tend to clear out cloud cover over each
island by the early morning hours.

An upper level low currently over the Big Island will continue to
drift eastward away from the state over the next 24 hours.
Unstable air with this low may produce thunderstorm activity over
the Big Island between 22Z and 03Z. Cloud tops will range from
10-15kft across the islands with CB cloud tops up to 30kft. Lower
cloud ceilings may lead to mountain obscuration SIGMET Sierra
later this afternoon for the Big Island and Maui.

No AIRMETs currently in effect.

&&

.MARINE...
A High Surf Advisory remains in effect through this afternoon for
all south facing shores. The current large south swell will
gradually diminish through Tuesday. Please see the latest Coastal
Hazard Message (CFWHFO) for additional information about the
hazardous surf conditions.

Actual tides at Honolulu continue to arrive about 0.5 feet above
predicted. High tides today and tomorrow are predicted to be over
2 feet, and just under 2 feet on Tuesday at Honolulu. A similar
period of tides exceeding 2 feet are expected in mid to late June,
between June 21 and June 27. It is during this period that tide
levels are expected to reach the same heights as during this
current event.

The combination of king tides and the south swell continues to
bring the likelihood of coastal inundation, particularly during
the afternoon high tide period. With the south swell surf on the
decline and the height of the high tides diminishing, impacts will
slowly diminish over the next few days. Please see the Special
Weather Statement (SPSHFO) product for more information about the
potential impacts from the high tides combined with the large
surf.

A northeast swell building today will produce moderate surf through
Memorial Day along east facing shores. North swells will be very
small though Memorial Day. A moderate north northwest swell is
forecast to arrive Monday night, peak below advisory levels Tuesday
night, then subside through Thursday.

Light winds are expected to persist through Memorial Day, with
trade winds forecast to gradually strengthen starting Tuesday.
Winds could reach Small Craft Advisory (SCA) levels in the typical
windier locations around Maui County and the Big Island
Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening 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.

&&

Morrison...Discussion
Bohlin...Aviation
M Ballard...Marine