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Hawaiian Islands Synoptic Discussion and Guidance

948
FXHW60 PHFO 250655
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST TUE MAY 24 2016

.SYNOPSIS...
Trade winds will continue on a gradual decline, with showers, some
locally heavy, remaining focused over windward slopes through early
Wednesday. Widespread afternoon sea breezes will trigger isolated
heavy downpours on Wednesday, mainly over interior and mauka
sections, as the atmosphere becomes increasingly unstable. A more
stable but humid, hybrid trade wind and weak afternoon convective
pattern will prevail on Thursday, followed by light and variable
winds and a land and sea breeze weather regime heading into the
weekend.

&&

.DISCUSSION...
Trade winds continue to decrease this evening, as a surface trough
deepens far north of the state. Latest satellite imagery showed an
area of moisture just east of the islands this evening, and these
clouds will reach the state in the coming hours, bringing more
showers. Meanwhile, a rather strong upper level trough approaches
the state from the northwest, as revealed by the water vapor
imagery. This approaching trough will bring cooler air aloft,
resulting in the air mass becoming more unstable in the island
vicinity. Therefore the incoming showers may be enhanced, increasing
the chance of locally heavy rainfall. Since trade winds are still
the prevailing, much of the clouds and showers will affect windward
areas. Expect passing low clouds and showers to continue across the
area through the rest of the evening into early morning, with
showers focusing mainly along windward areas. The exception will be
Kona area of the Big Island, where evening convection may still
develop before clearing out overnight.

As trades weaken further on Wednesday and the atmospheric
instability is maximized during the passage of the upper level
feature, afternoon sea breezes will trigger convection over the
islands, especially over the interior and lee areas. Given there
will be moisture available in the island vicinity, the threat of
heavy showers continues. In fact, air mass may become unstable
enough for the possible development of thunderstorms during the
afternoon and evening.

As the upper level feature exits east, airmass in the island
vicinity will become more stable. A hybrid trade wind and convective
weather pattern is expected on Thursday. The weak trade winds will
bring some showers to the windward slopes while afternoon sea
breezes will generate clouds and a few afternoon showers over
leeward and interior sections. There may be some clearing overnight
as land breezes develop.

Rather light wind regime will persist into the holiday weekend, as
the aforementioned surface trough stays north of the islands.
While some bursts of southeast winds may occur over windward Maui
and Big Island waters during the period, land and sea breezes weather
regime will continue over the islands, featuring afternoon clouds
and spotty interior showers with partial clearing at night. It may
feel rather humid as well with the light winds. There is also a
chance that volcanic emissions from the Big Island may spread to the
smaller islands during the weekend.

&&

.AVIATION...
An area of increased low-level moisture is approaching from the
E, while a trough aloft is digging toward the islands from the NW.
The two will combine to create an unstable island atmosphere that
will be conducive for the development of locally heavy showers
through Wednesday. While trade winds will be gradually weakening,
they will remain sufficiently strong overnight for the best chance
of showers to remain over windward coasts, slopes and adjacent
waters. Although the greatest moisture is near Maui and the Big
Island, a locally heavy shower is possible statewide as the trough
aloft, and it's associated instability, draws closer. The light
winds will allow the focus for clouds and showers to shift to
interior and leeward areas from late Wednesday morning into
Wednesday evening.

IFR VIS/CIG in +SHRA can be expected in the most intense showers
through Wednesday, with the potential for fairly widespread MVFR
CIG/VIS to develop over the next 12-24 hours. An AIRMET for
mountain obscuration appears inevitable, with the most likely
locations being windward Maui/Big Island overnight. The passage of
the trough aloft also brings the potential for moderate turbulence
aloft on Wednesday.

&&

.MARINE...
No small craft wind and seas conditions are expected through the
rest of the week as winds will stay mainly on the lighter side while
moderate swells, at most, are expected. Winds over the windward
coastal waters of Maui and the Big Island may increase a bit at
times through the rest of the week.

A potentially moderate southwest swell may arrive on Thursday, peak
on Friday, then decline during the weekend. This swell will
originate from the Tasman Sea, and given the large amount of south
Pacific islands along the path of the swells from this direction,
surf locally tends to be inconsistent. See the the Collaborative
Nearshore Swell and Wind Forecast (SRDHFO) for more details.
Otherwise rather small surf is expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Aviation...Birchard
Rest...Hui

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



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